Most of the known oil accumulations of Northern Iraq probably originated by upward migration from earlier, deeper accumulations which were initially housed in stratigraphic or long-established structural traps, and which are now largely depleted. The earlier concentrations had their source in basinal sediments, into which the porous, primary-reservoir limestones pass at modest distances east of the present fields. Development of the region favored lateral migration from different basinal areas of Upper Jurassic and Lower-Middle Cretaceous time into different areas of primary accumulation.
Important factors affecting primary accumulation included: (1) early emergence and porosity improvement of the reservoir limestones, followed by burial under seal-capable sediments; (2) the timely imposition of heavy and increasing depositional loads on the source sediments, and the progressive marginward advance of such loads; (3) progressive steepening of gradients trending upward from source to accumulation area; (4) limitation of the reservoir formations on the up-dip margin by truncation or by porosity trap conditions. In late Tertiary time, large-scale folding caused adjustments within the primary reservoirs, and associated fracturing permitted eventual escape to higher limestone reservoirs, or to dissipation at surface.
The sulfurous, non-commercial crudes of Miocene and Upper Cretaceous reservoirs in the Qaiyarah area are thought to stem from basinal radiolarian Upper Jurassic sediments, which lie down dip, a few tens of miles east of these fields. Upper Cretaceous oils of Ain Zalah and Butmah drained upward from primary accumulations in Middle Cretaceous limestones, which were filled from basinal sediments of Lower Cretaceous age situated in a localized trough a few miles northeast of these structures. The huge Kirkuk accumulation, now housed in Eocene-Oligocene limestones, ascended from a precedent accumulation in porous Middle-Lower Cretaceous limestones, which drew its oil from globigerinal-radiolarian shales and limestones of the contemporaneous basin, a short distance east of the present field limits.
Eocene-Oligocene globigerinal sediments, considered by some the obvious source material for Kirkuk oil, seemingly provided little or no part of the present accumulation. The reservoir formation may have been filled from these sources, to lose its oil by surface dissipation during the erosional episode preceding Lower Fars deposition. Upper Cretaceous basinal sediments probably contributed nothing to known oil field accumulations, though they may have subscribed to the spectacular impregnations of some exposed, Upper Cretaceous reef-type limestones. Neither Miocene nor pre-Upper Jurassic sediments have played any discernible role in providing oil to any producing field. Indigenous oils are thought to be negligible in the limestone-reservoir formations considered.
The exceptionally rich oil accumulations of the Middle East have attracted much comment in recent years. In this paper the sedimentary and tectonic history of the North Iraq sector of the Arabian Gulf-Iraq-Iran geosyncline is reviewed briefly, and inquiry is pursued into the probable origins and modes of accumulation of the oils found in the Kirkuk, Butmah, and Ain Zalah; Bai Hassan and Jambur; and Qaiyarah, Najmah, Jawan, and Qasab fields. The first three named fields, which are producing at present, yielded a total output of 194 million barrels of oil in 1954, mostly from the Kirkuk structure. Before development drilling was suspended in 1939, the four connected fields of the Qaiyarah area had been proved to contain, within their two superimposed limestone reservoirs, very large reserves of heavy, sulfurous, unmerchantable oil. Jambur and Bai Hassan are newly confirmed fields, under development, but not yet in production. It is too early for their importance to be fully assessed, though it is clear already that they will prove to be rather small fields in comparison with the nearby Kirkuk. The region considered is that part of Iraq which lies north of Latitude 33° North. The area embraced is approximately 90,000 square miles.
Several recent papers treat of the Middle East as a whole and touch upon sedimentary history, tectonics, oil geology, or oil-field development in Northern Iraq. History of exploration has been dealt with briefly, by Lees (1950a) and by Barber (1948). Later information on exploration and development appears in the annual reviews of “developments in foreign fields” featured in successive July numbers of the Association Bulletin, and in papers by Baker (1953), Wellings and Daniel (1954). The general geological background is covered in publications by De Boeckh, Lees, and Richardson (1929), Lees and Richardson (1940), Lees (1950a, b, 1951, 1952, 1953), and Henson (1950a, b, 1951a, b). Some of these contributions discuss oil-origin and -migration problems of the region.
Rock unit nomenclature for Northern Iraq is being prepared for publication in the near future. References to rock units by name have been excluded from this paper, except in the cases of the Euphrates limestone, Lower Fars and Upper Fars. Bakhtiari units, for which time-stratigraphic treatment would have been inappropriate.
The isopach and facies maps are based upon all available collections of surface and well samples, and on unpublished records of unsampled measured sections. Stratigraphic limits have been determined, and facies variants differentiated, principally upon the evidence provided by thin sections. [Note: All rock samples from wells and outcrops are studied in thin section as part of routine examinations carried out in geological laboratories of the Iraq Petroleum and associated companies. Over 300,000 thin sections of rock samples from North Iraq are on file at Kirkuk]. Age determinations of surface samples of Paleozoic to Jurassic age have been based largely on extensive macrofossil collections.
Stratigraphic interpretations summarized in map form represent the combined findings of a large number of geologists and paleontologists, including C. Andre, R.C. van Bellen, E.J. Daniel, G.F. Elliott, T.F. Grimsdale, F.R.S. Henson, J.M. Hudson, R.G.S. Hudson, A. Keller, G.M. Lees., J. McGinty, D.M. Morton, K. al Naqib, J. Robinson, A.H. Smout, R. Wetzel, the writer, and others.
THE TECTONIC FRAMEWORK
As introduction to the fundamental architecture of the North Iraq basin it suffices to make the simple division into “nappe zone,” “folded zone,” and “unfolded area” which is illustrated in Figure 1. These units were differentiated during the late-Tertiary (mainly Pliocene) Alpine orogeny, which raised the high Zagros Mountains in the area adjacent to and beyond the northeastern frontiers of Iraq, and cast up the large, elongate, anticlinal folds of the foothills and frontal ranges. All the known oil fields of Northern Iraq are contained in anticlinal traps within the “folded zone.”
An elaborate analysis of the tectonic construction of the Middle East in general, including Northern Iraq, has been made by Henson (1951b), who stresses the important role played by pre-Miocene vertical movements in the “unfolded area” and in the “folded zone.” Henson considers that folding due to late-Tertiary compression was superimposed on and molded against an earlier, deep-seated, block-faulted framework, inherited from the pre-Miocene history of the region. Other accounts, by De Boeckh, Lees, and Richardson (1929), Lees and Richardson (1940), and Lees (1950a, b, 1951, 1953), have accepted uncomplicated compressional folding, with a broad, “normally folded zone” intervening between the “nappe front” and the unfolded “foreland.”
Tripartite division into unfolded, folded, and nappe zones, on the dominantly northwest-southeast trend of the Zagros, is appropriate for post-Miocene time, and also is harmonious with existing concepts of a Cretaceous-Pliocene geosyncline developing on the same Zagros trend, with its axis migrating more or less progressively southwestward, until the paroxysm of orogeny in Pliocene times. Conditions in Jurassic and pre-Jurassic time were somewhat different. The basinal trend ran more nearly north-south than northwest-southeast in the known Jurassic stages, and the Jurassic-Cretaceous transition was probably a time of tectonic disturbance, with widespread uplift and tilting of fault-bounded blocks on the western margins of the basin. Several deduced faults of this age are directed north-south, across the Zagros trend. The axes of thickest basinal deposition for the Jurassic stages underlie thin developments of shallow-water Lower Cretaceous sediments.
Although the “unfolded area” has been referred to by De Boeckh, Lees, and others as a “foreland,” it is now emphasized that a very thick sedimentary sequence underlies much of it (Lees, 1953). The outcrops of the Arabo-Nubian massif or “shield” lie far to the south and southeast of the limits of the region considered.
Gravimetric data and slight structural manifestations in the flat-lying Tertiary cover provide evidence of important buried structural elements throughout the “unfolded area.” Lateral variations in sedimentary thicknesses over small distances are deducible, and it is concluded that most of these variations are controlled by pre-Miocene faulting, of various dates, the relief due to faulting having been erased at surface due to subsequent erosion and deposition. The tectonic pattern suggested by the evidence is a complex one, resulting from interplay of vertical movements, on fault trends which are oriented predominantly north-south, east-west, northwest-southeast, and northeast-southwest, the importance of individual trends showing marked local variations. Some of the prominent detected structural features are continuous for some tens or even hundreds of miles. Some pass beyond the limits of the region considered, into Syria and Southern Iraq. A few transgress directly across the limit of the unfolded area into the folded zone (Figure 1).
Within the unfolded area, a single large broad dome on an east-west alignment occurs in the southwest of the region, in the Ga’ara Depression north of the Wadi Hauran. This dome, which exposes Lower Triassic sediments in its culmination, is on the flank of a very gentle broad uplift which must reflect basement arching. A second prominent east-west-fold, narrow but gentle, is known at Anah, on the Euphrates, in a fault trough setting which argues strongly for fault origin of the anticline itself. Other anticlinal features within the “unfolded area” are small and indefinite, with very low dips and dubious or no closures; most of them bear no directional relationship to the Zagros-trend folds of the folded zone, or to the deduced directions of late-Tertiary tangential pressures.
The boundary between the folded and unfolded zones is abrupt. The southernmost and westernmost anticlines of the folded zone, Jebels Sinjar, Sheikh Ibrahim, Sadid, Makhul, and Hamrin North and Hamrin South, are surprisingly large folds, tens of miles long, and rising in some cases several thousands of feet out of the adjacent synclines. The boundary follows a more or less arcuate line, from northwest-southeast in the southeast to east-west in the northwest, but it is offset sharply in the area west of Qaiyarah. Here its course is picked out by the north-south alignment of the pitching ends of five rather small and feeble anticlines. The line plunges below alluvium in the plains east of Baghdad (or else is offset northward, near Naft Khaneh, to the foot of the exposed Iraq-Iran frontier folds).
The folded zone has an average width of about 100 miles, and the alignments of the individual folds and of the zone itself follow the Zagros swing from east-west in the north to northwest-southeast in the south. There is an over-all tendency for anticlines to increase in amplitude and tightness as the “nappe front” is approached, the most northeasterly folds being themselves thrust and locally overturned. But there are many departures from the superficially natural expectation that the folds should diminish in strength with increasing distance from the thrust front. Thus Hamrin North, Makhul, and Sinjar are much more powerful and much more elevated folds than the anticlines which occur immediately to the north and northeast, whereas flat, low domes appear, between steep, narrow, anticlinal neighbors, in the middle of the zone. Other oddities include the existence of great elevational differences between adjacent anticlines of otherwise similar dimensions, the pitching down of successive anticlines along linear features, and the appearance of marked asymmetries in some folds, and of occasional opposed asymmetries in some pairs of adjacent folds. These and other anomalies provide evidence for the opinion of Henson (1951b) that the late-Tertiary folding in the “folded zone” was intimately affected by a pre-existing complex of faults, which left residual features to buttress or deflect folding, and which predisposed the region to react in irregular and complex fashion to simple tangential pressures.
Where directional characters can be read into the complications, one or other of the four main tectonic trends discerned in the “unfolded area” is generally found to be involved. As some few important tectonic features on these trends pass directly from folded into unfolded terrain it is argued that the “folded zone” was probably traversed, prior to folding, by a tectonic network comparable with that which is now detectable in the unfolded area.
The Precambrian basement is not exposed anywhere in Northern Iraq, and deep boring has not yet penetrated the base of the Triassic.
Paleozoic rocks crop out in the extreme north of the region close to the Turkish frontier, where Cambro-Ordovician shales and quartzites are overlain by a thin, incomplete Lower Carboniferous marine succession, which is covered by thick Upper Permian limestones. There are no discernible angular discordances at the unconformities. Permo-Carboniferous limestones are recorded from Harbol, in southeastern Turkey (Tasman, 1949), and limestones of similar age, overlying bituminous Devonian shales and sandstones, have been described from Hazro, northeast of Diyarbekir (Tolun, 1949). According to Ternek (1953). Upper Permian limestones crop out extensively in the area between Lake Van and the Turco-Iraq frontier north of Amadia-schists, phyllites, and quartzites underlying the limestones are also dated as Permian by this author, but it seems probable, from the nature of the succession known in Iraq, that these clastics are at least as old as Lower Carboniferous.
Graptolite-bearing Lower Paleozoic clastics have been encountered in some deep wells in Syria, where they underlie Permo-Carboniferous limestones and marine shales, siltstones, and quartzites. Thick lower Paleozoic sections of marine clastics occur in Northern Saudi Arabia, and Cambrian limestones, shales, and sands are known in Jordan.
In the Bakhtiari Province of Iran the thick, exposed, Cambrian succession includes bedded salt in its lower parts, and the intrusive salt of the spectacular domes of the southern Arabian Gulf, Laristan, etc., is of Cambrian age (Lees, 1953b, etc.). Lees (1952) and others (e.g. O’Brien, 1950) have suggested tentatively that Cambrian rocks, including important salt components, may underlie much of the “normally folded zone” of the Zagros. Such salt could have permitted decollement-type folding over an unfolded basement (O’Brien), or it could have facilitated the development of markedly different fold patterns in the underlying basement and in the overlying sedimentary cover (Lees, 1952). The absence from Northern Iraq of any apparent salt dome, despite the great thickness of post-Cambrian sediments, argues against the presence of any thick salt series at depth.
Though the positions, dimensions, and directional trends of pre-Triassic basins are unknown, it is probable that thick deposits of marine Paleozoic sediments underlie most of the region.
Triassic sediments are known from many exposures in Kurdistan, from deep well sections on the Qalian, Atshan, and Butmah anticlines and on Syrian anticlines, and from exposures in the Ga’ara Depression in the southwest of the region. The upper part of the Triassic is calcareous or dolomitic; the lower part is dominantly clastic, comprising sandstones and quartzites in the southwest, and shales and marls with subordinate limestones in Kurdistan. Only the calcareous upper division has been reached in wells in Iraq. Subsurface successions include frequent intercalations of anhydrites and calcareous shales, which are absent or inconspicuous at outcrop. The total thickness of the Triassic section at exposure exceeds 5,500 ft (1,676.8 m). The same order of total thickness is estimated for the Quaiyarah area, and some 4,000 ft (1,219.5 m) is present in central Syria. The Ga’ara sections are much thinner, and show sand intercalations in the limestone sequence, and continental features in the lower clastic division. The Triassic basin occupied all Northern Iraq and extended far into Persia, Turkey, and Syria; its geography remains largely unknown, except that one approach to the margin lay in the southwest of the region considered.
The occasional evaporites of the well sections indicate intermittent barred-basin conditions, and the entire sedimentary suite suggests slow subsidence over the whole basinal area. Oil was probably generated from time to time within this environment, and indications of indigenous oils have been noted in some wells, and at outcrop. But the faunas of the time were sparse, and planktonic organisms were rare, so that these rocks may offer only limited potentialities as oil source rocks.
In Kurdistan the Liassic rocks comprise an upper dolomitic limestone unit, about 600 ft (182.9 m) thick, and a lower limestone-shale-anhydrite sequence which is extensively slump bedded. The Liassic of the Ga’ara is partly obscured, but includes evaporitic limestones and dolomites with some dispersed sand and intercalated shales.
The wells show much thicker sections than those of Kurdistan, and a more mixed assemblage of rock types, including bedded anhydrites, argillaceous limestones, shales, dolomites, and oolitic and pseudo-oolitic limestones. The bipartition of the Liassic of the mountain sections is not matched by any simple lithological division of the subsurface successions.
Generation of oil probably occurred in the semi-barred basinal environment indicated by the sediments, but total organic content of the formations was probably never great.
The Liassic basin was more localized than that of the Triassic. The thickest sedimentation and the deepest-water facies, indicating the center of the basin, is found in the wells to the west of Mosul. Facies correlations suggest a trend approximately northwest-southeast in this area, but the basin axis may have swung westward into Syria to cross the Syria-Iraq frontier at about the latitude of Jebel Sinjar.
The Middle Jurassic is the earliest time interval for which any serious paleogeographic reconstruction has been attempted in map form (Figure 2). Isopachs in the northern part of the area are fairly well controlled by surface and subsurface reference sections; the two “trough” features on the line of the Tigris, and the swings of contours in the mountain area are almost unavoidable. There are certainly other irregularities in actual thickness, not suggested by available controls, which may be revealed by eventual drilling. The construction of the southern and western parts of the map hangs entirely upon the small group of exposures in the Wadi Hauran.
The thicknesses and facies of Middle Jurassic sediments remain unknown throughout the large area which lies between the Wadi Hauran, in the southwest, and the Atshan-Ain Zalah wells, close to the Tigris, in the north.
No attempt is made in this or other figured isopach maps to differentiate between contours which are adequately supported by observation and those which are in large measure conjectural.
Isopachs are intended to show existing thicknesses. Original thicknesses were much reduced by early Neocomian erosion in the area west of the Tigris and north of Atshan. The original area of thickest deposition probably lay between the Tigris, north of Mosul, and Jebel Sinjar. The original thickness trend may have been east-west, rather than northwest-southeast in this area.
Similarly, the succession has been planed off by pre-Albian erosion in the southwestern part of the area; the zero-thickness line reflects erosional convergence. The original basin margin lay to the west of Muhaiwir. But the sediments here are neritic limestones, locally oolitic and sandy, indicating approach to the continental limit.
Slight erosional unconformity without visible discordance is found between Middle and Upper Jurassic rocks in the Qalian and Najmah wells, close to the area of maximum depositional thickness, yet in the mountain zone, where the sediments are very thin, deposition was continuous, and the facies are more “basinal” than are found elsewhere in the region. Sedimentation was probably rapid, in the Tigris area, on a rapidly subsiding but always relatively shallow, shelving floor, while slow deposition continued in the deeper mountain zone, which was more remote from the basin margins, and which subsided slowly.
Fauna and lithological facies are generally similar in the Tigris area and Kurdistan. Posidonia shales and limestones, and Radiolaria-rich sediments occur in both areas, as does super-abundant debris of a problematical organism of planktonic habit. [Note: Identified with “filaments d’Algues?,” Pl. VIII, 2; Pl. IX, I, in J. Cuvillier and V. Scal, 1951].
But the initial sediments of the Middle Jurassic in the zone of thick sedimentation are pellety, oolitic limestones with a rich benthonic fauna, including numerous gastropods, rare Haurania spp., and ubiquitous pellets of the encrusting foraminifera Nubecularia. A discontinuous, bedded-anhydrite unit intervenes in some wells between this shallow-water limestone and the main mass of the radiolarian sediments above. The pellety limestone and anhydrite are not developed in Kurdistan.
Rocks of this time interval are markedly bituminous both in the mountain zone and in the thick-sedimentation belt. Semi-euxinic depositional conditions are presumed, and source potentialities are rated high. Indigenous oil has been noted in the upper part of the sequence in some wells, but no oil has been produced from these formations, which are of very low permeability. In the northwestern part of the region, ample opportunity for dissipation of indigenous oil was afforded by long exposure following Berriasian uplift and preceding Aptian transgression.
Upper Jurassic (Callovian-Kimmeridgian)
Figure 3 illustrates the distribution of the principal facies and total thickness isopachs of the Upper Jurassic. The conditions of the Middle Jurassic were largely repeated in this time interval. Again the mountain zone shows very thin, ammonitiferous, radiolaria-rich, euxinic shales and limestones, with a thin terminal anhydritic unit which is dated as Kimmeridgian and tentatively equated with the Hith anyhdrite formation which caps the “Arab zone” fields of the Hasa and Qatar. Again the thickest sedimentation is found in the Tigris area, where very thick oolitic and pseudo-oolitic and chemical limestones culminate in a massive-bedded anhydrite, some 600 ft (182.9 m) thick. The western limits of existing sediments of this age were imposed by erosion of early Neocomian date, which bared the uplifted area northwest of Mosul down to the Middle Jurassic. The planing down of a tilted fault block embracing the Najmah, Qalian, and Atshan wells is illustrated; the existence and positioning of the bounding faults is conjectural.
Upper Jurassic sediments are not exposed in the southwestern part of the region, where the youngest sediments seen, below transgressive Albian sandstones, are of Middle Jurassic age.
Little is known of conditions in the zone between the mountain exposures and the wells of the Tigris area. By analogy with conditions found in the Tithonian-Berriasian stages it is argued that the thick, oolitic limestones of Najmah and Qalian pass eastward into thick, basinal, euxinic sediments, which become progressively thinner as the mountain zone is approached. Source potentialities are seen in these postulated basinal sediments, thickness and nature of which remain to be established by drilling.
Jurassic-Cretaceous Transition: Tithonian-Berriasian
The basinal configuration of the Tithonian-Berriasian stages (Figure 4) reproduces that of the Upper Jurassic, and the facies are similarly distributed. The disparity in thickness between the Kurdistan sections and the Tigris area well sections is smaller. In Kurdistan the sediments comprise about 300–600 ft (91.5–182.9 m) of radiolarian shales and thin-bedded limestones with abundant ammonites (Spath, 1950). The lower beds are heavily impregnated with bitumen.
The rocks of the broad, rapidly subsiding, shallow shelf are chemical limestones and calcareous mudstones, with rare oolitic and pseudo-oolitic limestone intercalations, generally tight, impermeable and unfossiliferous. Oolitic intercalations increase in frequency and importance upward. Observed fossils are gastropods, rare alga, and lituolids. The total thickness of the shelf facies is about 950 ft (289.6 m). Between these two extremes, a recent deep test on the Kirkuk structure has revealed a very thick mudstone-shale-limestone sequence, with an upper, unfossiliferous mudstone unit more than 2,000 ft (609.6 m) thick, overlying about 450 ft (137.2 m) of black, calcareous, Tithonian shales and limestones which contain a superabundant radiolarian fauna. The radiolarian rocks met in this well were partially impregnated with thick glutinous bitumen, but yielded small amounts of very light, asphaltene-free oil from fractures.
The Tithonian sediments are regarded as very important potential source rocks, without any apparent outlet to adjacent potential reservoirs. The thickness and plasticity of the overlying mudstone cover have probably been adequate to prohibit any upward migration of oil at Kirkuk, and the lateral equivalents of the radiolarian sediments toward the southwestern margin of the basin are impervious chemical limestones and mudstones.
Important tectonic movements occurred at or about the end of Berriasian time. These were manifested in:
Tabular elevation of the area lying northwest and west of Mosul, on bounding faults of unknown position (one such fault is deduced along the course of the Tigris, north and east of Ain Zalah; others may be conjectured on gravimetric and other evidence).
Tilting and uplift of a block embracing the Najmah, Qalian, and Atshan areas (Figure 3). (The existence and course of the north-south fault limiting the basinward side of this block are conjectured).
Pronounced eastward shift in the facies and isopach pattern between Tithonian-Berriasian and Valanginian-Aptian time.
Copious local deposits of sedimented bitumen in the basal beds of the ensuing Valanginian testify to breaching of important early oil accumulations at about this time.
Stratigraphic relationships were rather complex during this period. The map (Figure 5) is a schematic construction, rather than a true facies-isopach construction. Seven different formations are recognized within the interval; all have diachronous contacts with adjacent formations, and interdigitation between formations is common.
The thin deposition in the northwest is due in part to absence of sediments of the Valanginian and Hauterivian over the high tabular feature which was raised during the Jurassic-Cretaceous transitional period. This large uplift was not overlapped until late Barremian-Aptian time. The sediments then deposited were Orbitolina marls and marly neritic limestones, with rare local sand and silt concentrations.
In the southwest at Awasil the succession comprises a thin Valanginian oolitic-neritic limestone, thick Barremian-Hauterivian sandstones (correlative with the producing sands of the Basra fields), and a thin, dolomitized limestone of Aptian age. The southwestern limit, not shown on the map, must lie between Awasil and Muhaiwir, and is due to post-Aptian and pre-Cenomanian erosion. Rocks of Valanginian-Aptian age do not crop out in the southwestern part of the region, the pre-Albian outcrop being overlapped by transgressive sandstones of the Albian-Cenomanian interval.
Basinal marls, shales, and limestones, with abundant radiolaria, ammonites, and other planktonic fauna, occupy the entire eastern border of the region, and are also indicated in a narrow trough lying north of Mosul. The lower part of this sequence grades westward into neritic marls (and eventually into sands in the southwest). The upper part passes westward, rather abruptly, into neritic, reef-and-shoal limestones of great thickness.
The northwestern area represents an emergent uplift, later submerged, the southwestern sand-girded area indicates shelving approach to a continental mass, and the basin proper lay close to or beyond the northeastern borders of Iraq. The neritic limestones appear to have developed on an initially shallow, broad shelf, which subsided, in step with sedimentation, much more rapidly than did the basin area proper, or the high shelving areas in the west. The sharp, westward swing in the neritic-basinal facies boundary suggests control of facies by east-west faulting, and similar faulting is suggested by the approximately east-west trend of the basinal trough north of Mosul, and by the east-west course of isopachs west of Rowanduz.
The basinal sediments of the eastern parts of the region are notably bituminous. Depositional conditions were largely euxinic, and these rocks are considered to have been rich and effective source beds. The thick neritic limestones, with which these source beds are laterally and vertically juxtaposed, are locally of high porosity and permeability; they are classed as excellent potential reservoir and carrier formations.
Continuity of the neritic limestone zone southward from Kirkuk is inferred from reappearance of the same facies developments in Southwestern Persia (Kent, Stinger, and Thomas, 1951). But it may be that the sandstone units of the west pass eastward, gradationally and directly, into radiolarian sediments of a subordinate basin which may underlie the Mesopotamian plains.
Thickness and facies distribution for the Albian-Cenomanian units (Figure 6) follow approximately the pattern which is found in the preceding Aptian stage. In the southwest, following an early uplift and peneplanation, an extensive marine sandstone blanket was deposited (correlative with the producing sands of the Burgan field). Basinal sedimentation continued in the east, and neritic limestones spread over the high northwestern area. In the center of the region, west of Kirkuk and south of Qalian, the Albian is represented by a thick sequence of marls, chemical limestones, and anhydrites.
In Cenomanian time the semi-lagoonal environment of the central area was modified, and tabular neritic limestones spread over the southwestern and western margins of the basin. Such limestones are thickly represented in the Awasil wells, and they occur as far to the southwest as the vicinity of Rutbah.
The isopachs represent actual thicknesses. After Cenomanian deposition was complete there were vertical adjustments of structural units of large dimensions, followed or accompanied by regression, and most of the region except the southeastern basinal area was subjected to erosion. Thinning of represented sediments on to the elongate northwest-southeast feature which runs through Mosul and Kirkuk is a result of erosional modification of an uplifted spur rather than of thin original deposition. The Makhul-Qalian area, in which lagoonal sedimentation occurred during Albian times, lost its veneer of Cenomanian neritic limestone during this erosion.
The broad pre-erosional uplift on northwest-southeast trend passing through Kirkuk merits comment, because all the “commercial” oil fields discovered in Northern Iraq lie on its flanks.
As in the Valanginian-Aptian, the basinal sediments are excellent candidate source rocks in some areas. They contain rich radiolarian and globigerinal faunas and are locally characterized by abundance of the problematical planktonic organism Oligostegina (especially near the boundary between the basinal and neritic facies). The neritic limestones provide excellent potential reservoir-carrier formations, especially where diagenetically modified during the terminating emergence. The impermeable Albian lagoonal-anhydritic sediments of the central area provided a limit to the possible migration of fluids toward the southwest within the neritic limestone zone.
Oil is found in porous, neritic, Albian limestone reservoirs in Kirkuk and Ain Zalah.
Turonian rocks are probably restricted to the southern and eastern parts of the region (Figure 7). Sedimentation was continuous from Cenomanian to Senonian times in the southeast, where basinal globigerinal limestones and marls prevail. Neritic limestones are found in wells in the southwest, thinning westward; these units are not seen at outcrop. In Kurdistan, mid-Turonian Oligostegina limestones lie on eroded Albian neritic limestones and dolomites. Westward from Sulaimania through Kirkuk to Qaiyarah and Makhul similar Oligostegina sediments, becoming progressively more marly and plastic, transgress over thin eroded Cenomanian on to eroded Albian lagoonal-anhydritic deposits of the central Tigris area. The Turonian is erosionally terminated, except in the basinal province, so that the original area of deposition was certainly larger than shown on the map. The absence of neritic rudist-bearing Turonian limestones is of interest, as such limestones are recorded from southwestern Persia (Kent, et al., 1951), where their presence is paleontologically established, and from southeastern Turkey (Tasman, 1949), where published evidence for rocks of this age in reef facies is dubious.
The Oligosteginal rocks are locally bituminous and were deposited under locally euxinic conditions. They could be invoked as potential source beds. In the west they are sufficiently marly and plastic to have functioned as cap-rock seals to the underlying porous limestones.
Deposits attributed to the lower Senonian interval are found in two widely separated areas (Figure 7). Basinal globigerinal marls occur, in conformity with similar Turonian sediments, in the southeastern parts of the region, but these are cut out in the pre-upper Senonian unconformity over most of Kurdistan and the central and western areas. In the northwest, Oligostegina limestone with cherts and shales, carrying a restricted fauna of post-Turonian and pre-upper Campanian age, overlie eroded, Albian, neritic limestones. These sediments are restricted to a very small area, which probably represents a small graben-type basin, let down below sea level during the general emergence to which most of the region was subjected from late Turonian to the beginning of upper Campanian time. Precise age of these deposits is unknown. They were laid down under partially anaerobic conditions, and could be regarded as potential source rocks.
The top of the lower Senonian in the northwestern “basin” is taken at a minor break which marks the onset of upper Senonian transgression, and which introduces normal marine globigerinal and benthonic faunas. The limits of the area of existing sediments shown on the map probably correspond closely with the area of original deposition, as erosion at the mid-Campanian break was small.
Oil is produced from fractured cherts and Oligostegina limestones of this age in the Ain Zalah field, but the main producing reservoir lies in the Middle Cretaceous neritic limestones below.
Upper Cretaceous-upper Campanian-Maastrichtian
The onset of full-scale Upper Cretaceous transgression, at some time during the upper Campanian, inaugurated an episode of thick and varied sedimentation (Figure 8). Simultaneously with the commencement of transgression, the western part of the region, was segmented into three east-west aligned troughs, which presumably originated by faulting at depth. Globigerinal “basinal” sediments accumulated thickly in these steadily subsiding troughs, while neritic limestones were developed around their margins, and over the sinking horst-like residual shallows between them. A high feature of some nature was raised, to stand above the early strand line, in a zone coursing west-northwest/east-southeast through Shiranish, Aqra, and Rania. Meanwhile, globigerinal, “basinal” sediments transgressed from the west over the large area lying south of this high.
Shortly after commencement of transgression, uplifts of great magnitude occurred in the region to the east of the frontier, and a great bulk of Flysch-type clastic detritus, derived from this uplifted mass, was poured into a more or less linear, northwest-southeast-directed trough or “fore-deep.” As transgression proceeded, the high ridge feature running through Aqra was overwhelmed, and very thick neritic limestones, including reef components, were deposited over it, and around its margins. The Flysch clastics advanced westward over globigerinal limestones and marls to the vicinity of Kirkuk. Lenticular reef-like masses of organic, detrital limestone developed locally within the clastic trough, and tongued eastward into the Flysch from the massive limestone barriers in the Aqra-Rowanduz area. Indigenous oil is recognized within the Flysch sediments, in several areas in Kurdistan.
The Flysch-type clastics include detritus of green rocks, and radiolarian cherts and limestones of Middle Cretaceous-Jurassic age. At the close of the transgression large sheets of radiolarites slid or were thrust into the northeastern borderlands of Iraq.
The normal development toward compression orogeny in the northeast was cut short at the end of Upper Cretaceous times. Though slight folding adjustments of this date have been detected, and other evidence of compression is available, there is no well-defined system of frontal folds, ahead of the advancing orogen.
The portrayal in Figure 8 of the distributions of the various facies of this complicated interval is much schematized. The thicknesses shown are residual after an important episode of emergence and erosion at the end of Cretaceous time, which was preceded or accompanied by structural adjustments. The isopachs are not a true guide to original depositional thicknesses. The whole region was probably submerged by the end of Maastrichtian time. Absence of sediments of Upper Cretaceous age from the Ga’ara area in the southwest is probably due to early Tertiary erosion.
The globigerinal basinal sediments of the interval have been regarded as potential source rocks; they are richly fossiliferous, and locally bituminous. The neritic detrital limestones of the western margins, and of the Shiranish-Aqra-Rania high, and the Flysch trough, present excellent reservoir potentialities in many areas. Henson (1950a) has described some of the reef developments of Upper Cretaceous age in Northern Iraq, discussing their relationships to oil generation, migration, and accumulation. The neritic porous, Campanian-Maastrichtian limestones of the Qaiyarah area fields contain a large volume of heavy sulfurous oil. Light oil is produced from fractures in indurated, Maastrichtian, marly, globigerinal limestones in the Ain Zalah and Butmah fields, and small volumes of similar oil in similar limestones have been proved beneath the Tertiary producing reservoir at Kirkuk.
Bitumen pebbles in Maastrichtian Flysch conglomerates and reef-type limestones suggest escape to surface of large volumes of oil during the Upper Cretaceous. Although one source for this oil may be seen in the uplifted mass to the east, a nearer and more probable origin is at hand in the high Shiranish-Aqra-Rania ridge feature, which must have attracted migrating oil at this time, and which lacked any adequate seal until late in the Tertiary.
The Cretaceous-Paleocene transition period was one of widespread regression, which bared most if not all of the region, permitting very uneven erosion of different areas, but producing in most localities quite marked evidence of discontinuity in sedimentation. The northeastern area and its hinterland were elevated, and deposition was re-introduced by the downwarping of a broad linear northwest-southeast trough, crossing the region, to the southwest of the position of the Flysch trough of Upper Cretaceous time, and of the reef-crowned Shiranish-Aqra-Rania high of the Maastrichtian. Flysch-type clastics accumulated in great thickness in this subsiding trough, and Paleocene bounding reefs developed locally along its southwestern margins in some areas (Figure 9).
In the northwest and central areas, emergent features inherited from the terminating uplifts of the Cretaceous Period, and probably further differentiated by faulting, provided a partial barrier, running from the south of Jebel Sinjar to the vicinity of Bai Hassan. Lagoonal limestone sediments, lenticular reef-type limestones and fore-reef shoals, and globigerinal marls, are intercalated with Flysch-type clastics and shales to the northeast of this barrier. Reef limestones are well developed over the Jebel Sinjar, and in the foothills running northwest-southeast between Koi Sanjak and the Persian frontier south of Halabja. Inter-relationships of different facies are intricate in the Flysch belt, and cannot be satisfactorily portrayed on a single map.
Southwest of the emergent barrier, and of the Koi Sanjak-Halabja reef belt, Paleocene and lower Eocene deposits are mostly in “basinal” globigerinal facies, and generally thin, with internal breaks which may reflect emergence of flat-lying island highs (or merely non-depositional submerged environments). The limits of the Makhul “island” are unknown on the west; it could extend over most of the “blind” area up to the Syrian frontier, where basinal sediments are shown on the map, or it could be much smaller in area than is indicated. Cherty, phosphatic marls with neritic-littoral limestone developments are found around the western margins of the Ga’ara uplift, in the southwest of the region.
In the Flysch zone the lower Eocene is represented by red beds, which overlap the underlying marine Paleocene and wedge out on to the slope of the northeastern land mass.
Indigenous oil indications are known in the Flysch zone, and the “basinal” globigerinal sediments can be favorably viewed as potential source rocks. But the principal function of rocks of this age interval in known fields is that of caprock marl seal to accumulations in upper Campanian-Maastrichtian fractured globigerinal limestones (Ain Zalah, Butmah, Kirkuk) and neritic limestones (Qaiyarah, Najmah, Jawan, Qasab).
The complications of the Paleocene-lower Eocene are lacking from the paleogeography of the middle and upper Eocene (Figure 10). Introduction of Flysch clastics abated, after deposition of the lower Eocene red beds in the Kurdistan area. A broad basin, receiving globigerinal sediments and trending approximately northwest-southeast, occupied the central parts of the region. One margin of this basin is to be placed around the broad Ga’ara uplift in the southwest. Patchy neritic limestones, with Nummulite faunas, occur along this gently shelving coast, where thicknesses deposited were small. In the northeast the basin was limited against a steeper marginal slope. Nummulitic shoal limestone of considerable thickness developed outward from this slope, and protected a widespread shallow lagoonal area, in which chemically deposited limestones and dolomites were laid down. The lagoonal sediments extend northeastward into strand-line deposits, and interdigitate locally with red sands and silts close to the shore line.
Within the area of globigerinal marl deposition, internal troughs are inferred south of the Jebel Sinjar and in the Euphrates Valley through Anah. These prominent east-west features are regarded as persistent, in some form or other, from Upper Cretaceous time onward. A third east-west trough of thick basinal sedimentation, north of Sinjar, is complicated by the appearance of thick, shoal limestones at its eastern end. The area of thin sedimentation in the central area, west of Kirkuk, may be smaller than is shown, no control section being available between Sadid-Hibbarah and the Syrian frontier. Similarly the northeast-southwest trough connection lying south of Kirkuk and north of Baghdad is conjectural and may not exist as shown.
The lagoonal limestones of the northeastern zone are tight impermeable limestones, without great value as potential reservoir rocks, except perhaps where strongly fractured. The basinal globigerinal milestones are locally excellent source-bed candidates, as they are richly organic, with planktonic foraminiferal faunas throughout, and as they were deposited under’ anaerobic conditions in some areas.
They are locally bituminous at outcrop in Persia (Lees, 1934), though not commonly conspicuously so where seen at surface in Iraq. The thick nummulitic shoal limestones of the linear zone which trends northwest-southeast through Kirkuk are exceptionally porous and permeable in parts. They are excellent potential reservoir rocks, and their potentialities are realized in the Kirkuk field, where they provide part of the accommodation for the 2,000 ft (609.6 m) of oil column of this large accumulation. The underlying globigerinal limestones are also productive at Kirkuk.
In the fields of the Qaiyarah area, the thin middle-upper Eocene globigerinal marls provide part of the seal which separates the accumulations in the Upper Cretaceous and Miocene reservoirs.
Only a very schematic and formalized account of the sediments of this time interval is attempted in Figure 11. The depositional history of these rocks has been studied in detail by R.C. van Bellen (1956), whose observations are drawn upon.
In general, the paleogeography and depositional history of the Oligocene follows quite closely the pattern set in middle-upper Eocene time. The northeastern margin of the depositional area lay much to the southwest of the position of the corresponding boundary of the middle-upper Eocene stages. Reef limestones of Oligocene age, with back-reef limestones to the northeast, were deposited along a linear trend running across the entire region, approximately from northwest to southeast. The reef zone corresponds approximately with the basinal side of the belt of shoal limestones which were deposited in the middle-upper Eocene. The reef and back-reef zones of the Oligocene time are much narrower than the shoal limestone and lagoonal zones of the middle-upper Eocene. Upper Oligocene sediments were deposited after regression, and have much smaller areal distribution than those of the lower and middle Oligocene.
The reef-controlled sediments of the northeastern margin of the basin are repeated on the southwestern margins, where gentler shelving shores result in a much wider areal distribution of the facies belts than is found in the Kirkuk region. The broad, rather featureless basinal area between the reef-girded margins is occupied by globigerinal sediments. Anhydrites also occur in the central parts of the basin in the younger sediments of the interval.
The troughs south and north of Jebel Sinjar, noted from Upper Cretaceous through Eocene stages, are still evidenced in Oligocene to Miocene time by large thickness of basinal sediments. The persistence of the Anah trough, suggested by the 250 ft (76.2 m) contour in Figure 11, may be illusory, as the position of this contour in the area north of Anah and west of Makhul is uncontrolled.
Oligocene sediments were subjected to erosion in some areas during the Aquitanian cycle, and later to extensive exposure following localized structural differentiation, during the time of deposition of the Euphrates limestone and basal Lower Fars sediments. The isopachs indicate found thicknesses, and offer an imperfect picture of depositional conditions. The disparities are greatest around the northeastern margins of the basin, where original deposits of back-reef Oligocene rocks were stripped off, before encroachment of the Lower Fars sea brought erosion to a close.
Most of the porous limestones housing the Kirkuk (Baba dome) and Bai Hassan fields are Oligocene reef or fore-reef limestones. The globigerinal basinal sediments have been considered the likely source for the Kirkuk oil, and they must certainly be included among the possible source beds of the region. In spite of low permeabilities, those parts of such rocks which lie above oil/water level in the Kirkuk field contribute greatly to the total oil-filled pore space in this reservoir. Drainage of oil from these tight sediments, within the present reservoir, is aided by the existence of an extensive fracture system (Daniel, 1954).
After the deposition of upper Oligocene rocks in the lower lying parts of the Oligocene basin, a further regression, dated approximately at the Oligocene-Aquitanian transition, preceded the commencement of lower Miocene sedimentation. The rocks of the lowest Miocene transgression, which are for the most part lagoonal limestones, comprise the Euphrates limestone formation. [Note: Formal definition published by R.C. van Bellen, 1956]. This formation is thickly developed over most of the area in which globigerinal Oligocene sediments were laid down (Figure 12). The Euphrates limestone overlaps the limits of Oligocene transgression in some areas, notably around the southwestern margins of the basin. The main trough of sedimentation continues into northeastern Syria, where thick-bedded anhydrite and salt have been found in some wells. Anhydrites occur in Iraq also, thickening and proliferating towards the deeper parts of the basin.
Deposition was terminated by widespread withdrawal toward or at the end of the Aquitanian stage. In some localities the basal Lower Fars is transgressive over eroded Euphrates limestone.
The upper accumulations of the oil fields of the Qaiyarah area are contained in porous Euphrates limestone. The formation is very fossiliferous locally, corallinacid algae being especially abundant. But the characters of the sediments indicate deposition under aerated conditions, so that the unit is not considered to have contained important source rocks.
The precise age of the regression which followed the deposition of the Euphrates limestone remains moot. It may be dated within or at the end of the Aquitanian. During this regression much of Northern Iraq was exposed, and substantial erosion of earlier sediments preceded and accompanied the early stages of the Lower Fars transgression.
The Lower Fars sediments were deposited in an intermittently barred, basinal-lagoonal environment. They comprise rhythmic alternations of limestones, anhydrites, and silty marls and shales, with bedded rock salt appearing in the middle part of the series in the central part of the basin. The Lower Fars transgression extended much further to the northeast than did that of Euphrates limestone time, but the reverse situation arose in the southwest, where the Lower Fars converges to disappearance far to the northeast of the present limits of the Euphrates limestone.
Passing northeastward from the center of the Fars basin, the general characters of the series are modified progressively, first salt, then the bedded anhydrites, and eventually the limestones, falling out of the sequence. The entire series thins in the same direction, and also shows onlap convergence onto an eroded surface which is cut in Oligocene rocks in the southwest (as at Kirkuk) passing into Maastrichtian clastics in the northeast. The sediments of the northeastern zone, up to the “nappe front,” are reddish marls, silts, and sands, with only rare subordinate limestones. But in the areas east, north, and northwest of Rowanduz, a prominent limestone unit, commonly conglomeratic at the base, underlies or lies within these clastics. This limestone, which thickens rapidly northeastward into a massive feature-forming unit along the frontier with Turkey, contains the same fauna, and is of the same age as part of the Lower Fars of the main basin. Thickness relations suggest that it marks the margin of a separate basin of “Lower Fars age” which may extend far into Turkey and Persia. The “nappe front” of the thrust sheets of the late-Tertiary orogeny over-rides the anomalous “undifferentiated Fars” rocks of this northeastern basin, preventing full inquiry into their distribution in Northern Iraq.
In the basinal area, the top of the Lower Fars is taken at the top of the highest bedded anhydrite, the overlying succession of shales, silts, and limestones being recognized as the “Middle Fars.” The Middle Fars rocks, generally thin in Northern Iraq, are transitional between the dominantly chemical-lagoonal sediments of the Lower Fars and the entirely clastic Upper Fars. They are included with the Upper Fars and Bakhtiari rocks for the purposes of isopach portrayal in Figure 14. The entire Lower Fars (and Middle Fars) sequence becomes less clastic and more frankly lagoonal and marine from northeast to southwest.
The isopachs of Figure 13 relate only to the Lower Fars sediments of the basinal area, and to those parts of the “undifferentiated Fars” of the northeastern zone which are considered to be equivalent in age to the basinal Lower Fars. The thickness pattern is much simplified, and the actual basinal configuration is certainly far more complicated than is shown.
The flowage of salt under tectonic stresses in the central parts of the basin has produced marked disharmonies between the generally simple, anticlinal structures which have developed in the pre-Fars rocks, and the commonly complex, surficial structures produced in the Fars and overlying sediments. The nature and origin of such salt-facilitated disharmonies have been discussed lately by O’Brien (1950) and Lees (1952), and earlier by many other writers. Salt flowage and imbrication within the Lower Fars render difficult any evaluation of pre-flow and pre-imbrication thicknesses, especially where the only evidences available are those drawn from scattered wells drilled in the crestal areas of anticlines. Some of the thicknesses used in drawing Figure 13 are estimated pre-folding thicknesses, compensated for presumable thinning or thickening in the salt section during folding, according to the positions of control wells on the anticlines. Individual thicknesses may be incorrect by several hundred feet in the central parts of the basin, but the general configuration of contours must be approximately as shown. The map may be in error in the area immediately to the east and southeast of Jebel Sinjar, where available measured thicknesses are inconsistent.
Some of the thin limestones and shales of the Lower Fars of the central parts of the basin show evidence, of deposition under anaerobic conditions; they could be included among the potential source beds of the region. Some of the limestones are appreciably porous and permeable, and some contain oil where they overlie deeper accumulations, as at Kirkuk. A few contain small quantities of oils unlike those encountered in nearby reservoirs; such oils may be indigenous to the Lower Fars. In general the source capabilities and reservoir potentialities are so small as to be negligible. The principal role of the Lower Fars in the oil accumulation process has lain in its ability to provide a plastic cap-rock seal of salt and anhydrite to retain oil in underlying reservoir formations. In this role it is of high significance. The Kirkuk, Bai Hassan, Jambur, and Qaiyarah area fields in Northern Iraq, and all the developed fields of Persia owe the preservation of their oil to Lower Fars cover.
It is the seal for about 15 per cent of the world’s proven unproduced reserves.
Upper Fars and Bakhtiari
After the brief episode of marine sedimentation which followed the deposition of the evaporitic Lower Fars, clastics entered the basin in large volumes from the rising orogen in the northeast. These deltaic-pedimentary clastics which commence with red silts and marls (Upper Fars facies) pass upward gradationally into conglomerates and coarse sandstones (Bakhtiari facies). The major lithological divisions are not differentiable in terms of age. Subdivisions hinge on grade size of the clastics, and there is general increase in grade from southeast to northwest. The rare marine components of the central part of the basin dwindle and vanish in the same direction.
Isopachs of Figure 14 illustrate combined thicknesses of Middle and Upper Fars and Bakhtiari sediments, as measured into the principal synclines. But this is far from being a straight forward pre-folding thickness portrayal. It is known that the late-Tertiary folding, which reached its climax in late Pliocene time, was commencing already in the lower Pliocene, and that it developed intermittently throughout the duration of deposition of the Bakhtiari rocks. In the later stages of Bakhtiari deposition, the crests of the large mountain folds close to the advancing “nappe front” were undergoing erosion, while coarse sediments were being laid down thinly over the rising crests of the southwestern folds, and more thickly in the deepening synclines. A true isopach map should reveal closures of attenuation over every rising structure. Unfortunately, the information which would be required for such a map is not accessible. Figure 14 shows the areal thicknesses, after arbitrary elimination of crestal attenuation. As the entire Miocene-Pliocene sequence was probably limited by near-planar base levels of deposition, the contours also give rough measure of the subsidence of the folded zone which accompanied the late-Tertiary folding. The bold contour spacing and the smoothness of the areal gradients are false. Local gradients, resulting from contemporaneous uprising of the folds, cannot be assessed. They probably sufficed to control the oil-migration regime and to prevent long-distance regional movements, which the regional gradients would have encouraged but for the presence of growing structural traps.
THE OIL FIELDS
Lacking the results of a systematic chemical evaluation of possible source sediments, it has been accepted that any rock unit of appreciable thickness which was deposited in a more or less euxinic environment may be suspected of having generated and liberated oil. These potential source beds range from stinking, black, bitumen-saturated shales and limestones to rocks which are now finely recrystallized, off-white, lithographic limestones. Potential source beds are widespread and occur at many horizons in the sedimentary sequence of Northern Iraq.
The obvious candidates for the roles of primary reservoir formations are the thick coarse sandstones of Cretaceous age, which appear in the southwestern part of the region (Figures 5 and 6), and the reef limestones, oolitic limestones, and neritic limestones of the shallow-water shelves; which occur intermittently through the dominantly calcareous sequence. The calcareous potential reservoir rocks are much more widely spread than are the sandstones, and they occur in the sediments of most ages considered, up to the middle Miocene Lower Fars.
Within the folded zone, at least, the intensity of fracturing encountered in some formations renders them capable of housing and yielding oil in large quantities, even though the rocks may be intrinsically impermeable (Daniel, 1954). Such fracturing must be considered, generally, to be of late-Tertiary origin, and hence not available to house primary oil accumulations of early date.
The cap rock requirements of impermeability, coupled with some degree of plasticity (a high degree of plasticity in the strongly folded zone), appear to be met in several formations, scattered through the stratigraphic sequence, which have wide areal distribution.
Amid the profusion of possibilities for accumulation which are perceptible in the known associations of presumable source, capable reservoir, and competent cap rocks, there are a few combinations that have resulted in oil accumulation, and many which have been barren. The profusion of source and reservoir possibilities has naturally tempted a multiplicity of speculations as to the source and place of origin of the known oils, the modes of their accumulation and the factors controlling their distribution. These problems require consideration now against the background of the developmental history of the region which has been summarized and simplified in the foregoing isopach and facies maps.
Ain Zalah Area
Ain Zalah field
The Ain Zalah field, situated northwest of Mosul, is in a simple anticline, 12 miles long and 3 miles wide, which exposes Lower Fars limestones and anhydrites. Production is drawn from two “pays.” The upper or “First Pay” yields oil from fractures in unpermeated globigerinal limestones of Upper Cretaceous age. The “Second Pay” reservoir is of porous and fractured Middle Cretaceous limestones and dolomites, abetted by fractures in overlying lower Senonian-lower Campanian cherts, shales, and oligosteginal limestones. The two “pays” are separated by about 2,000 ft (609.6 m) of barren, marly, globigerinal limestones similar to those in which the “First Pay” is developed. The geology of this field has been discussed by Daniel (1954)
The generalized stratigraphy is indicated on the cross section (Figure 15). The seal retaining the “First Pay” oil is provided by basinal marls of Paleocene-lower Eocene age. This seal has not been entirely competent everywhere, for oil of type similar to that in the underlying fractured reservoir occurs sporadically in thin, lenticular, porous limestones, in the lower part of the Paleocene marl unit, in restricted areas over the crest of the structure.
The erosional unconformity which separates the Tertiary seal from the Upper Cretaceous reservoir has had no apparent function in aiding oil accumulation. No significant secondary porosity was developed in the Cretaceous limestones during exposure, and the fracture system, in which the oil is now found, was not developed until after the marly cover had been deposited.
Before the discovery of the “Second Pay” it was considered possible that the oil of the “First Pay” originated in the overlying basinal Paleocene-Eocene marls, and that it had been expressed downward into available fractures, across the Paleocene/Cretaceous unconformity, during compaction of these Tertiary marls. This account was not very satisfying, as most of the open fractures which now house the oil were probably not formed until Pliocene folding commenced, by which time compaction rates must have been very small. The Paleocene-lower Eocene marls contain widespread porous limestone beds and thin silty beds which would have been more effective vehicles for reception of expressed oil than were the tight fractures of the Cretaceous limestone; whereas some of these contain oil, most do not, and the rare sporadic saturation can be accounted for by upward leakage from the “First Pay.”
It has also been argued that the Upper Cretaceous limestones were the source for their own oil, which was retained within the rock mass until fractures opened to allow the present segregation. This possibility seems to be denied by the absence of any significant amount of residual oil in the pore spaces of the globigerinal limestones. No mechanism is apparent by which undersaturated oil, housed in very fine pores in an almost impermeable limestone, could be expelled into fractures developing late in the induration history of the rock.
Yet other origins have been suggested, involving the presence of primary accumulations of oil in Upper Cretaceous reef limestones down flank from the present field, or of similar reef accumulations in Paleocene sediments outside the explored limits of the structure. From the time of discovery most geologists who had experience of, the field leaned to the view that the “First Pay” oil entered its present reservoir by migration from below.
With the proving of the “Second Pay” oil, the problem of origin of the “First Pay” accumulation was solved. The two oils are of the same gravity, and are chemically similar, such differences as exist being small and explicable. Moreover, the two “pays” are intimately connected, presumably by a fracture network, through the intervening 2,000 ft (609.6 m) of seemingly barren marls and limestones. Production of either “pay” affects the other directly. It is now regarded as certain that the “First Pay” accumulation originated by upward migration of oil which escaped from the “Second Pay.” Such migration has occurred extensively in the past, and migration of fluids is occurring rapidly at present from the “Second Pay” to the “First” as oil is produced from the latter. There is no likelihood that Paleocene-lower Eocene marls, or the Upper Cretaceous limestones themselves, have contributed importantly and directly to the “First Pay” accumulation, though either may have subscribed a very minor proportion of the oil there found (Daniel, 1954).
The origin of the accumulation in the “Second Pay” is more controversial. The principal reservoir formation is provided by the Middle Cretaceous limestones and dolomites. These are terminated by an erosional unconformity representing Turonian and early Senonian emergence. Though the early Ain Zalah wells did not reveal secondarily developed porosity below this break (Daniel, 1954, p. 785), later wells on this and other structures have shown marked dolomitization, leaching, and porosity enrichment below the contact, and extensive, if patchy, distribution of highly permeable rocks is presumable below the unconformity. The overlying sediments are the restricted-fauna, “basinal” shales, cherts, and limestones of the lower Senonian-lower Campanian depositional cycle, which underlie basinal Upper Cretaceous marly limestones; the whole Upper Cretaceous sequence may be regarded as a possible source for the “Second Pay” oil. The Middle Cretaceous and Lower Cretaceous rocks are not euxinic at Ain Zalah; it is very unlikely that they have themselves produced any of the oil now found in the porous sections. The Lower Cretaceous marls and limestones rest on eroded Middle Jurassic limestones and dolomites which are classed among the potential source rocks. There is no evidence of porosity enrichment at this contact and the unconformity cannot have acted as a channel for migrating fluids. Arguable possibilities for the source of the “Second Pay” oil are therefore:
That it originated in overlying source beds and was expressed downwards into the porous Middle Cretaceous limestones and dolomites, across the unconformity, during compaction of the source beds.
That it entered the trap laterally, through the Middle Cretaceous reservoir carrier formation, from some source area outside the present field limits.
That it originated in underlying source sediments and accumulated in the Middle Cretaceous reservoir following vertical migration.
The volume of overlying sediments which were deposited under more or less euxinic conditions is very large, and more than adequate to supply the small accumulation found in the “Second Pay.” But it is very improbable that downward expression of oil can have occurred from the main mass of the Upper Cretaceous rocks, because the least porous beds in the section are found within the lower Senonian-lower Campanian. Within the lower Senonian-lower Campanian, expression would more probably have been lateral, toward the basin margin in the northeast (Figure 7), than across the bedding planes into the Middle Cretaceous reservoir. But in the northeast, toward the convergence of Upper Cretaceous onto Middle Cretaceous, oil may have entered the latter from any part of the lower Senonian sequence, to reach the Ain Zalah trap after more or less extensive lateral migration in the main reservoir-carrier formation. Although downward entry of oil in situ is thought to be improbable, origin of the “Second Pay” oil in Upper Cretaceous and/or lower Senonian source beds within the general area must be regarded as a reasonable possibility.
The volume of euxinic Middle Jurassic and older sediments below Ain Zalah is also very great, and some of these rocks appear to have been effective source rocks. Disseminated residual bitumen is common in Middle Jurassic sediments of Ain Zalah. However, the Middle Jurassic rocks were subjected to prolonged exposure, during Neocomian times, following an earlier history of fairly deep burial. Any indigenous oil present in the upper part of the sequence should have been dissipated during this emergence, and any gas content must have been lost due to release of hydrostatic pressure. No oil should have remained in the Middle Jurassic sediments after subsequent deep burial, and if any under-saturated oil did survive, in housing of low permeability, no adequate mechanism is apparent for its expulsion into a later fracture system. The lower parts of the Middle Jurassic section appear to be sufficiently impermeable and plastic to have acted as fairly competent seals to prohibit upward passage of oil from deeper horizons. The lowest rocks encountered in a deep well on Ain Zalah were Liassic anhydrites. Live oil indications were observed in these, but the oil appears to have been of different type from that now found in the Ain Zalah “pays.” Deeper oils found elsewhere are gas rich; they would be saturated with gas at the depth of the “Second Pay,” yet the actual oil at this depth is very undersaturated. The Lower Cretaceous section includes marl units which should have offered at least a partial seal to oil migrating upward in the structure, but no oil accumulation appears below these potential trapping beds. From these considerations, none of which is conclusive, it is argued that the oil probably did not originate directly from underlying Jurassic or earlier source beds, as has been suggested by Daniel (1954)
The remaining alternative is that the “Second Pay” was fed laterally, via the Middle Cretaceous limestone carrier. The ultimate origin of the oil is not explained by adoption of this alternative, which smacks of the familiar device of thrusting awkward problems into areas where they cannot be investigated. The original source may still have lain within the overlying basinal sediments, or deep down in the Mesozoic section. But the paleogeographic constructions suggest a more probable source. Figure 5 indicates the presence of a localized Lower Cretaceous trough, trending northwest-southeast, which lies a little to the northeast of Ain Zalah. The sediments of this trough, where seen at its northeastern margin, were radiolaria-rich, euxinic shales and limestones. Neritic limestones spread over the trough in the Middle Cretaceous. Depositional gradients would have favored migration of oil generated in this trough, southwestward into the Middle Cretaceous high on which Ain Zalah was placed (Figure 6). Later sedimentary loading in Upper Cretaceous time would have compressed the source sediments, expelling oil into the overlying neritic limestones, while Upper Cretaceous trough developments, west and southwest of Ain Zalah, would have imposed gradients prohibiting further migration in these directions (Figure 8). Paleocene-lower Eocene events include further loading of the source basin by very thick deposits, and further steepening of gradients favoring migration, within the Middle Cretaceous carrier, into the Ain Zalah area (Figure 9). The Paleocene-lower Eocene tilting would have served to concentrate accumulation at the western extremities of the high feature of Middle Cretaceous time (productive wells on Ain Zalah are restricted to the western end of the anticline).
Local structural uplift, of which there is inconclusive evidence, may have served to concentrate oil within an incipient Ain Zalah “high” during Upper Cretaceous time. Gradient changes imposed on Middle Cretaceous horizons by events during middle Eocene to lower Miocene time were relatively small. They abetted or were inadequate to modify significantly either the step gradients favoring upward migration of oil into the vicinity of Ain Zalah from the northeast, or those preventing migration out of the Ain Zalah area toward the southwest. With the onset of late-Tertiary folding, oil in the Middle Cretaceous carrier limestones must have been firmly locked within the confines of the rising anticlinal traps.
The rather complex accumulation history suggested above for the “Second Pay” oil is speculative and equivocable, but more satisfactory in many details than the alternatives of accumulation in situ from overlying or underlying source beds. The origin of the “First Pay” accumulation by upward migration from the “Second Pay” can be regarded as established.
The Butmah field occupies one of two in-line domes of the Butmah anticline, adjacent to Ain Zalah, but lying to the southeast. Oil is produced from fractured Upper Cretaceous limestones in a setting comparable with that of the “First Pay” of Ain Zalah. In Butmah the equivalent of the “Second Pay” of the Ain Zalah field is oil stained but water logged. There is an active water drive in the “First Pay” of Butmah. The oil is similar to that from the Ain Zalah reservoirs. It is believed that primary accumulation occurred in the Middle Cretaceous limestone reservoir at Butmah, as at Ain Zalah, and that the whole of this initial accumulation has escaped upward into the higher fractured-limestone reservoir. The Upper Cretaceous marly limestones are somewhat thinner in Butmah than in Ain Zalah.
The mode of origin deduced for the oil accumulations of the Ain Zalah and Butmah fields is illustrated in Figure 16.
The Kirkuk field is a sinuous anticline, some 60 miles long, divided by two prominent saddles into three major structural culminations. The structure is overthrust from the northeast in the exposed beds, due to sliding on the Lower Fars salt, but the fold at medium depth, below the salt zone of the Fars, is simple and almost symmetrical (Figure 17). The geology of the field has been described recently by Daniel (1954) Production was 23.7 million tons in 1954, and producible reserves have been estimated at 1,000 million tons (McCollum, 1947)
The producing reservoir, named the “Main Limestone,” is made up of back-reef, reef, fore-reef-shoal and globigerinal limestones of middle Eocene to lower Miocene age. [Note: A full account of the stratigraphy of the “Main Limestone” has been published by R.C. van Bellen, 1956]. The “Main Limestone” is terminated by an erosional unconformity which cuts deep into the succession toward the northwestern end of the field. The overlying sediments of the Lower Fars converge by onlap in the same direction (Figure 18). Thin limestones within the lower parts of the Lower Fars are porous and contain oil-they may be considered part of the reservoir but are of negligible volume in comparison with the “Main Limestone.” The field is capped by the salt and anhydrite beds of the Lower Fars “salt zone.” The seal- is imperfect, as there are extensive gas and small oil seepages in the surficially thrust area, southwest of the crest of the southeastern “dome.”
The axis of the anticline cuts obliquely across the shore line and facies trends of the different middle Eocene-Oligocene depositional cycles, each of which is characterized by passage, in a northeasterly direction, from globigerinal “basinal” sediments into nummulitic shoal limestones. The Oligocene cycles show further progression, toward the northeast, through reef limestones into back reef-lagoonal limestones (Figure 18). A thin tongue of lower Miocene limestones enters the succession, below the basal unconformity of the Lower Fars, at the southeastern end of the field. The most porous and permeable constituents of the “Main Limestone” are the fore-reef-shoal sediments, and parts of the reef limestones. The back-reef limestones are porcelaneous and impermeable. The globigerinal sediments have variable porosity but generally low permeability. The reservoir is extensively fractured, and fluid and pressure connection is remarkably free through most of the field.
The cross sections shown in Figures 17 and 18, and the facies and isopach constructions for middle-upper Eocene and lower-upper Oligocene intervals (Figures 10 and 11) illustrate that the porous, neritic limestones of the Kirkuk fold are ideally placed to have received any oils which were expelled from the contemporaneous basinal sediments. These potential source sediments lie to the southwest and south of the present field, and also underlie and form a part of the southeastern dome. The straightforward and simple theory that the oil originated in nearby basinal sediments contemporaneous with the reservoir rocks has held sway for many years. It is apparent from the cross sections that possibilities for large-scale stratigraphic trap accumulations might have been realized, even without the added favor of folding, if the basinal source rocks did in fact yield any oil to the shallow-water limestones at any time after the deposition of the Lower Fars salt seal.
In questioning this obvious mechanism of origin, Henson (1950b) has pointed out the importance of the prolonged exposure of the northwestern end of the field, which preceded deposition of the Lower Fars cover. This emergence would have afforded ample opportunity for dissipation of any indigenous oil, or of any oil which was released from the basinal source beds before the deposition of the Lower Fars. Some sealed cavities in the “Main Limestone” contain oil which differs from that now found in the open reservoir, suggesting that an earlier accumulation did occur, that this was dissipated, and that the reservoir was again sufficiently water-permeated to permit cementation, before the accumulation of the oil which is now found (Henson, 1950a, p. 236). This and other evidence led Henson [Note: Unplublished Company reports, and Henson, 1950b, in discussion.] to suggest that the Kirkuk “Main Limestone” reservoir has been filled by upward migration from a stratigraphic trap of large volume, which would be found to underlie part of the present field.
Weeks (1950) have contested Henson’s arguments for deep origin of the oils, although agreeing that erosion and emergence prior to Fars deposition would have permitted loss of much indigenous and early-arriving migrant oil. According to Weeks, major oil migration from basin to reservoir may have awaited deposition of some thousands of feet of overburden upon the source sediments, so that major incursion of oil from the basin may have occurred after the deposition of the Lower Fars seal. Thus the Eocene and Oligocene basinal sediments could have been the source for all the Kirkuk oil. In addition, oil could have been furnished from source beds of which the shallow-water equivalents are lost in the pre-Fars unconformity, whereas the Lower Fars and Euphrates limestone could also have contributed appreciably to the large “Main Limestone” accumulation.
It has been suggested by Daniel (1954, p. 805) that large volumes of Oligocene and Eocene oils may have been held in porosity-wedge traps, in the vicinity of Kirkuk, during the pre-Fars emergence, and that these may have joined with oils newly liberated from the basinal source rocks, after the Tertiary folding created a fracture network which destroyed local seals. Similar destruction of seals at depth could have permitted oils from older reservoirs to add to the general accumulation.
In marked opposition to Weeks’ contention that oil does not migrate across the bedding “as a rule,” is the long-held and oft-reiterated view of Lees, that the oil of the Persian fields and of Kirkuk may have migrated upward through many thousands of feet of strata, until arrested beneath the almost perfect seal of the Lower Fars salt (Lees and Richardson, 1940, Lees, 1934, 1953a, etc.). According to this view the voluminous accumulations below the saliferous Fars may be mixtures of oils derived from many underlying sources, each of which may have contributed only a small part of the volumes now found. There is no necessity to postulate exceptionally rich source rocks in order to account for the exceptionally large fields of the Iraq-Iran foothills zone.
The opinions summarized above cover a very wide field, and the divergences between them are so great that no general conclusions can be drawn without consideration of further evidence. Some contentions can be controverted in detail, or assessed in importance, from consideration of the relationships of the Tertiary sediments, their volumes, and their positions in the structure. Some of these arguments have been pursued by Daniel (1954). The major issue lies between those who seek to account for the oil by origin in nearby source rocks contemporaneous with or somewhat younger than the reservoir formations, and those who claim that oil entry has been by vertical migration from lower source sediments or from lower precedent accumulations. Clearly the Kirkuk field is voluminous enough to include oils of both lateral and vertical origins-a first necessity in the analysis of the Kirkuk accumulation is to determine the relative importance of lateral and of vertical migration.
A recent deep test well on the southeastern dome has provided evidence pertinent to this problem. The stratigraphy proved by this well is illustrated schematically in Figure 17. The principal interest of the test, in connection with the present theme, lay in the discovery of small oil accumulations:
In fractures in the upper part of a globigerinal, marly limestone sequence of Upper Cretaceous age.
In porous and fractured, dolomitized, neritic limestones of Albian age.
The Upper Cretaceous fractured reservoir is sealed by impervious Paleocene-lower-Eocene marls which form the core of the “Main Limestone” reservoir. The seal for the Middle Cretaceous reservoir is provided by the lower part of the Upper Cretaceous marly limestones, and by a thin oligosteginal limestone of Turonian age. The accumulation in the Middle Cretaceous appears at the top of a very thick succession of Middle-Lower Cretaceous dolomites and neritic limestones, which yielded abundant evidence of one-time oil impregnation, many hundreds of feet below the base of the present accumulation.
These circumstances are closely comparable with those found in the Ain Zalah field. The parallel extends to the types of oil found, and even to the nature of the minor differences which do occur between the oils in the upper and lower accumulations of both fields.
By analogy with Ain Zalah it may be surmised that the oil in the Upper Cretaceous fractured reservoir at Kirkuk has originated by upward escape from the underlying, porous, Middle Cretaceous reservoir. It may be concluded, from the oil indications through the latter, that the Middle Cretaceous and Lower Cretaceous section has housed, at some time, an oil accumulation many times larger in volume than that which it contains at present. The suggestion is manifest that the vanished oil has ascended through fractures into the overlying “Main Limestone” reservoir, via an intermediate housing in the fractures of the Upper Cretaceous limestones.
This suggestion receives adequate confirmation from analytical data on the three Kirkuk oils.
Figure 19 illustrates for the three oils the specific gravities of distillation fractions of narrow-boiling-point ranges. The volumetric distillation curves themselves are closely comparable for the three oils. The striking agreement in specific gravities at all temperature ranges is sufficient to support the conclusion that the three oils are of common or closely comparable origin.
It is accepted that oils produced in similar environments at widely different times may be closely similar. It is argued that similarity of oils should increase with increasing similarity in the faunal and especially in the microfauna) constituents of the source sediments. Unless and until chemical inquiry reveals significant differences between the three Kirkuk oils, it is claimed that these originated from a common source, or else from source sediments which should evidence deposition in similar environments. The alternative view that the resemblances are purely fortuitous is unassailable except in terms of probability-the closeness of similarity illustrated by the specific gravities of narrow cuts must speak for itself in this connection.
If a common source for the three oils be admitted the problem resolves into that of locating this source. Several formations, or a single one, may be held responsible for the oil. But in any event the source cannot be of Tertiary age, as no opportunity has existed for feeding of Tertiary oil into the Middle Cretaceous reservoir. If there is a common source for the three oils, the “Main Limestone” accumulation must have originated by upward migration, either from the source directly, or through an intermediate accumulation in the Cretaceous.
The possible sources for the oil in the Middle Cretaceous reservoir are:
The basinal globigerinal limestones of the overlying Upper Cretaceous and/or the Turonian oligosteginal limestones.
The underlying rock sequence.
Distant sources including 1. and 2. above, entry of oil having been effected through the Middle Cretaceous carrier-reservoir.
The overlying basinal limestones of Upper Cretaceous and Turonian age are devoid of such conspicuous residual impregnation with oil as might be expected if they had functioned as very fruitful source rocks. It is concluded that they were not the directly responsible source sediments. Distant representatives of these units may have supplied an initial accumulation within the Middle Cretaceous, outside the limits of the structure.
The underlying section, proved in the deep test, includes massive neritic limestones and dolomites, underlain by neritic marls with porous limestone tongues, which follow a calcareous mudstone sequence some 2,000 ft (609.6 m) thick. The neritic sequence cannot be considered a source-bed sequence; all the sediments were deposited under aerated conditions which must have permitted ready oxidation of any indigenous organic contents. Marl seals within the sequence do not appear to have retained any accumulations of oil, suggesting that no oil has passed upward through them.
The thick mudstone sediments suggest a euxinic depositional environment in which some oil may have been generated. But this sequence is almost unfossiliferous, and it is difficult to imagine voluminous oil generation in its rocks. Probably it retains most if not all of its original slender stock of indigenous oil, owing to its low permeability. The thick plastic mudstones -provide now an impervious seal, to prevent or hinder upward migration of any oil which may be available in deeper sediments.
The competency of the mudstone unit as a cap rock is illustrated by the discovery below it, in the deep test, of a radiolaria-rich unit of shales, limestones, and cherts. This unit, which was deposited under euxinic conditions, was bitumen-soaked, yet productive of small quantities of a light, gas-rich oil, free from asphaltenes, which is completely unlike any of the oils in the overlying reservoirs. These productive radiolarian rocks are manifestly source rocks which still contain their original charge of oil, and which are themselves now sufficiently fractured to allow slow release of their oils when the overlying mudstones are penetrated. The lowest rocks encountered in the test were shales and interbedded anhydrites of upper Kimmeridgian age, which are also noteworthy for their sealing capabilities rather than for their source potentialities. The richly productive radiolarian sediments, which are of Tithonian age, are thus enclosed between two impressively competent sealing units. The equivalents of the radiolarian rocks, where seen in the Kurdistan mountain zone, are exceedingly bituminous.
The deep-test evidence indicates that there is little possibility of any appreciable volume of pre-Cretaceous oil having entered the structure by vertical migration, in the vicinity of the well, because of the barrier to such migration offered by the mudstone sequence. The mudstone section thins, and may disappear, toward the northwest end of the field, so that the prohibition is not an absolute one for the structure as a whole. Perhaps significantly, the northwestern dome shows anomalous conditions in the presence of a deep gas cap and of gas-saturated oil in the “Main Limestone.” All the oils of the southeastern and central domes are undersaturated, though under producing conditions a small gas cap has developed in the southeastern one.
The contention of Lees that vertical migration may have extended from deep down in the sedimentary section appears to be incorrect for the main Kirkuk accumulations. The undersaturation of the three oils of the southeastern dome is further evidence that no extensive migration from great depth has occurred. Oil of deep origin would presumably have yielded gas to higher formation in which it accumulated, thus enriching any initially undersaturated oil body which might have gathered there before the arrival of the deeper-originating oils.
Though possible contributions of large volumes of oil from the directly overlying and underlying sections cannot be excluded, a more satisfactory account is that the original accumulation occurred in the Middle Cretaceous reservoir-carrier limestones, beyond the limits of the structure. The obvious candidates for the role of source-beds supplying these limestones are the thick, contemporaneous, basinal sediments, lying to the east of the field.
The Middle Cretaceous reservoir limestones of Kirkuk, and the underlying Lower Cretaceous neritic rocks, pass eastward at small distance into basinal radiolarian sediments, which are regarded on field evidence as rich potential source rocks (Figures 5 and 6). The Middle Cretaceous porous limestones of Kirkuk pass southwestward into a dense, marly and evaporitic rock sequence, which must have prohibited migration in that direction. Late Cenomanian and early Turonian events raised a broad structural high feature coursing through Kirkuk on a northwest-southeast trend. At this time, also, emergence resulted in development of localized secondary porosity in the upper parts of the potential reservoir. In mid-Turonian time the region was resubmerged, and a thin plastic marl unit was laid down over the eroded surface of the Middle Cretaceous neritic limestones (Figure 7). The earliest sediments of the Upper Cretaceous depositional cycle were also plastic marly limestones, which contributed to the original seal overlying the reservoir limestone.
In the Kirkuk area, marly limestone sedimentation continued through Upper Cretaceous time, but in the “basinal” area, to the east, very thick Flysch-type clastics were deposited, imposing heavy loads on the underlying source sediments (Figure 8). The depositional load advanced from northeast to southwest across the basin, thus favoring continuously the expulsion of fluids into the reservoir-capable neritic limestone zone, and particularly into the broad, northwest-southeast “high” of Cenomanian time on which the Kirkuk structure lies (Figure 6). In conjunction with this advancing load, a narrow localized trough sank or was impressed to accommodate the clastics. This trough also migrated from northeast to southwest, producing progressive, transient, steep gradients, which further favored the southwestward migration to which fluids within the basinal sediments were predisposed by the compressional factors. Oils which were expressed from the Middle and Lower Cretaceous source rocks in the permeable, neritic, carrier limestones, suffered lateral migration within these carriers, in response to changing regional gradients. Upward escape was forbidden by the overlying globigerinal limestone-and-marl succession, and escape to the southwest was ruled out by passage into impermeable anhydritic sediments.
By the end of the Cretaceous the entire basinal area of Middle and Lower Cretaceous time had been buried beneath at least 2,000 ft (609.6 m) of rapidly deposited sediments. The main parts of the basin, lying directly east of Kirkuk, had been subjected to overburden loads corresponding to depositional thicknesses two or three times as great. By this time most of the oil may have been expressed into the neritic carrier formations, to migrate, where prompted and where possible, into structural or up-dip, porosity-wedge traps. Much of this postulated oil would have concentrated on the Middle Cretaceous “high” feature running below Kirkuk (Figure 6).
Cretaceous-Paleocene emergence and subsequent events would have sufficed to promote minor, gravity-prompted adjustments or shifts, within the reservoir-carrier. Paleocene-lower Eocene depositional history repeats that of the Upper Cretaceous, and may have resulted in expression of additional oil from the source beds (Figure 9). Gradients favored further persuasion of the oil southwestward. Middle Eocene to lower Miocene sedimentation (Figures 10-12) probably accompanied only minor gradient changes within the Middle Cretaceous.
Lower Fars to late Pliocene conditions (Figures 13 and 14) introduced a sharp northeasterly gradient, favoring migration away from the porosity-reduction facies boundary of Middle Cretaceous times, and also a marked northwesterly gradient upward from the center of the Bakhtiari “basin” toward Kirkuk. These latter day regional-gradient changes conspired to favor migration, within the Middle Cretaceous neritic limestones, into or through the general vicinity of Kirkuk. As the broad anticlinal structures were already forming at the commencement of Bakhtiari deposition, conditions were ripe for the concentration of very large volumes of Middle-Lower Cretaceous oils, in the Middle Cretaceous reservoir, within the rising Kirkuk structure.
The foregoing account is an outline of the most credible mode of accumulation of the original oil in the Middle Cretaceous reservoir of Kirkuk. It involves reference back to the original depositional basin, and investigation of the events to which this basin has been subjected. The account requires expulsion of oil from source sediments as they underwent compaction, due to sedimentary loading, and it demands more or less free lateral migration over a few tens of miles within neritic limestone carriers. It is not an unavoidable account, but it is the most convincing of the several alternatives which have been conceived and investigated.
If the account is accepted, and other alternatives are rejected, significant arguments can be brought to bear on the origin of the “Main Limestone” oil. The postulated source beds for the Middle Cretaceous accumulation are radiolaria-rich, ammonitiferous marls and shales, deposited in a markedly euxinic environment. This combination of faunal and environmental characteristics was not repeated, after the end of the Middle Cretaceous times, in the sediments of any part of the region. The close similarity in composition of the “Main Limestone” and earlier oils, already stressed, has been taken to indicate close similarity in the faunal and environmental factors controlling the nature of the source sediments. Subject to acceptance of this premise it may be concluded that the “Main Limestone” oil originated in radiolaria-rich, ammonitiferous sediments, deposited in a euxinic environment, and most probably in the same source sediments as fed the Middle Cretaceous accumulation. Further, it may be concluded that the Oligocene and Eocene basinal sediments, with their very different, rich, foraminiferal faunas, have contributed little or nothing to the existing “Main Limestone” accumulation.
The preferred account for the origin of the Kirkuk oils may be summarized as:
Primary accumulation in Middle Cretaceous neritic limestone carriers, in porosity traps, or incipient structural traps, at about the end of Cretaceous time.
Secondary readjustment into the rising Kirkuk structure during early phases of Mio-Pliocene folding.
Rupture of Turonian and Upper Cretaceous cap rocks, and escape of some oil into the fracture reservoir in Upper Cretaceous limestone.
Rupture of Paleocene-lower Eocene seal and vertical escape of most of the Middle Cretaceous accumulations into the Tertiary reservoir, leaving residual accumulations in the original reservoir, and in the fractures of the Upper Cretaceous limestone.
This account is illustrated schematically in Figure 20.
Other alternatives are apparent which do not absolutely deny the absence of oils of Oligocene-Eocene origin from the “Main Limestone” accumulation. Thus the Middle Cretaceous accumulation could originate from overlying Upper Cretaceous globigerinal limestones, and could have filled the Upper Cretaceous fractured reservoir, whereas somewhat similar oils, produced by somewhat similar globigerinal source beds of Eocene-Oligocene age could have supplied the “Main Limestone.” The similarities between the several oils seem to the writer much greater than can be satisfactorily accounted for by this alternative, but the need for further chemical investigation is recognized.
If Eocene-Oligocene sources have supplied all or most of the “Main Limestone” accumulation, as advocated by Weeks (1950), then there has been no entry of appreciable volumes of oils of pre-Cretaceous origin into any accumulation. The Jurassic source-bed conditions and source organisms were completely different from those of Upper Cretaceous and later ages, and should have produced significantly different oils.
But if Jurassic earlier sources have contributed significantly to any accumulation they have contributed more or less equally to all, and the inference is strong that the three oils were originally one.
Bai Hassan field
The newly proved Bai Hassan field lies parallel with the Kirkuk structure, some six miles to the southwest. This field is probably underlain by neritic limestone developments of Middle and Lower Cretaceous age (Figures 5 and 6).
The reservoir unit now under development is equivalent to the “Main Limestone” of Kirkuk. The oil is saturated, and there is an extensive gas cap. The “Main Limestone” crest is much deeper than in the Kirkuk domes, but there are no surface seepages.
Apart from the presence of exceptional gas content the “Main Limestone” oils of the two fields are closely comparable. Significantly, the resemblance between the Bai Hassan “Main Limestone” oil and the oil from the Kirkuk Middle Cretaceous reservoir is even closer than that between the two “Main Limestone” oils, except at the two extremes of distillation.
As in the case of Kirkuk it is postulated that the “Main Limestone” accumulation has migrated vertically upward from an earlier pool, which was housed in Middle Cretaceous neritic limestones, the probable origin being seen in basinal sediments of Middle-Lower Cretaceous age lying a few tens of miles to the east. The anomalously high gas content may be a result of later addition of gas, from a deeper source, from Eocene-Oligocene basinal marls, or perhaps within the “Main Limestone” from another anticline. The mode of origin of the accumulation is portrayed in Figure 20.
The Jambur field, also under current investigation, lies almost in line with Bai Hassan, some 45 miles to the southeast. It is parallel with the Kirkuk structure but separated from the southeastern dome of Kirkuk by a deep syncline. The productive reservoir is equivalent to the “Main Limestone” of Kirkuk. Again there is a deep gas dome, and the oil is gas-saturated. Jambur lies farther into the Oligocene-Eocene basin than Kirkuk, and the pre-Fars break is very much smaller than on the Kirkuk structure. Jambur is thus better situated to have retained initial Oligocene-Eocene oils than is Kirkuk. The oil is very different from that of Kirkuk, being markedly less dense, and free from asphaltenes. The specific gravities of narrow boiling-point cuts, taken in fine-fractionation distillation, differ considerably from those of the corresponding fractions of the Kirkuk oils. At first sight it might appear that Jambur really does contain Oligocene-Eocene oil, and that the “Main Limestone” oil of Kirkuk, being chemically different, has a different origin.
However, the Jambur “Main Limestone” is permeated by copious bitumen residues, even high up in the gas cap of the present accumulation. Vet the produced oil is free from asphaltenes. It appears that there was originally an accumulation of heavier oil in the structure, and that the light oil with gas is a very late arrival. The ubiquitous bitumen was probably deposited from the original oil by solvent precipitation during invasion by the later, lighter oil. Again the suggestions may be made that the bitumen represents residues of an early-originating accumulation of Eocene-Oligocene oil which was inspissated during the pre-Fars emergence, and that the present oil arrived later, following genesis in the same basinal source beds, after deposition of Fars cover. This account does not bear close examination. The bitumen is too widespread, too deeply distributed, and too voluminous to be an inspissation residue, and similar “residues” are lacking in Kirkuk, where they might be expected to occur. It is improbable that the same source beds should yield first an asphaltene-rich oil, capable of leaving large volumes of heavy residuum in the formation, and, at a later time, a light oil, without asphaltenes, which would be incapable of dissolving any of the ubiquitous residuum of the first accumulation.
The distribution of bitumen in depth indicates that it was deposited from an oil column of considerable height, which could have come into place only after considerable development of late-Tertiary folding. The initial accumulation may be dated not earlier than Lower Bakhtiari “time.” At this stage in development, the Eocene-Oligocene potential source beds had been buried under more than 4,000 ft (1,219.5 m) of Miocene and perhaps early Pliocene sediments. Although the initial accumulation could have been of oils of Eocene-Oligocene source, it is also possible that it could have resulted from upward migration from an underlying Middle Cretaceous reservoir, as suggested for Kirkuk. Cenomanian neritic limestones probably underlie the northwestern end of the structure (Figure 6). The later-arriving, light, gas-rich oil could be of Eocene-Oligocene source, but this again appears to be unlikely, because compaction due to sedimentary loading must have been very slight at the late stage in history at which the incursion must be placed. It is suggested, tentatively, that the light-oil invasion took place from below through fractures created by late-Tertiary folding, and originally from some such previously imprisoned source rocks as the radiolaria-rich Tithonian sediments which were encountered in the Kirkuk deep test.
Jambur is a new field, as yet imperfectly explored. The opinions advanced above are in high degree speculative, and their retraction may be required by new findings at any time. But Jambur does not fit comfortably into a simple history of Tertiary source feeding Tertiary reservoir. The peculiarities of this accumulation are more readily explicable by vertical than by lateral migration.
The oil fields of the Qaiyarah area occur in the anticlinal traps of the Qaiyarah, Najmah, Jawan, and Qasab structures. There are two productive reservoirs, the upper one comprising the porous, neritic-lagoonal Euphrates limestone, of lower Miocene age, and the lower one being neritic limestones of Upper Cretaceous (upper Campanian-lower Maastrichtian age). The upper reservoir is more consistently porous and permeable than the lower, which shows marked variation in quality from dome to dome.
Qaiyarah, Najmah, and Jawan are successive individual culminations on a single sinuous fold axis, which trends approximately northwest from the western bank of the Tigris at Qaiyarah (Figure 21). Qasab is a parallel anticline, with two domes, which is offset to the northeast, and separated by a broad, shallow, synclinal saddle, from Jawan.
The oils of the two superimposed accumulations are similar. Gravities range from 11.5° to 18° A.P.I. in the Upper Cretaceous reservoir, and from 11° to 19° A.P.I. in the Euphrates limestone reservoir. All oils are exceedingly sulfurous, the Euphrates limestone accumulation more so than the oil in the Cretaceous reservoir. The oils are density stratified, the heaviest lying in the deepest parts of the structure. The oil/water contact in both reservoirs is tilted, being highest in the northwest, in Qasab. Edgewater stringers appear in the Euphrates limestone accumulation, high above the bottom-water level, along the southwestern flanks of the Najmah and Jawan domes.
There are small gas domes in the Euphrates limestone pools in all structures, but the only gas dome found in the Cretaceous pools is in Qasab. The Cretaceous oils in the Qaiyarah and Najmah fields are undersaturated. The oil accumulations are continuous through the four domes in the Cretaceous reservoir, but the synclinal saddle between the Qasab and Jawan domes is lower than the oil/water level in the Euphrates limestone.
The cover for the Euphrates limestone accumulation is provided by anhydrites, limestones, and marls of the Lower Fars, which lacks salt in this area. The seal between the two accumulations is provided by thin Oligocene, Eocene, and Upper Cretaceous globigerinal marls. The Cretaceous neritic limestone reservoir is underlain by oligosteginal limestones of Turonian age (Figure 7) which transgress over eroded Albian evaporitic limestones and shales. A thin Aptian-Barremian succession of marls and limestones intervenes between the base of the Albian and the top of the eroded Upper Jurassic in the Najmah dome. No wells have passed through the Albian on the other three domes.
The origin of the oils of the Qaiyarah area fields is controversial. Indigenous origin for the Euphrates limestone oils has been argued, as has a history of generation in the underlying globigerinal sediments. Indigenous origin is improbable, as the rocks were deposited in well aerated waters. The underlying globigerinal sediments are generally rather unusually rich in benthonic faunal components, suggesting that they also were not deposited under euxinic conditions. The Cretaceous limestone oils have also been reckoned indigenous, or derived from overlying basinal sediments or from underlying basinal Turonian or lagoonal Albian formations. None of the suggested source rocks is significantly bituminous. If the similarity of the oils in the two reservoirs is due to origin from a common adjacent source, the parent sediments must be the intervening globigerinal marls of Eocene-Oligocene and Upper Cretaceous age, which are not impressive for their source potentialities. They are also quite inadequate in volume to have supplied from the resources of a small area the major accumulations which are now found. Early-arising gradients, which might have aided drainage of large areas of source beds, were lacking from this area, as was any localization of heavy depositional loading. The most satisfactory account for the origin of the Euphrates limestone oils is that they have originated from precedent accumulations in the underlying Upper Cretaceous limestone reservoir, or from a yet deeper source which fed both reservoirs. The tilted oil/water contact and the intermediate waters in the Qaiyarah-area fields suggest that entry occurred in the southeastern plunge, and independently in the southeastern plunge of Qasab, and that the entering oils have never adjusted themselves entirely to the hydrostatic conditions, perhaps because of their high viscosity and ready emulsification.
For the Cretaceous limestone accumulations, origin in contemporaneous basinal source beds has been suggested. But isopach and facies constructions suggest that, in this event, entry into the system should have been from the northeast, north, northwest, or west, whereas gravity distribution, gas and oil distribution, and the tilted oil/water contacts infer entry from the southeast or east. The possibility of origin from deeper sources, directly underlying the whole area, appears to be ruled out by a deep test well on the Najmah fold, which penetrated a thick Albian-Liassic section without encountering any significant porous reservoir rocks or obvious source beds.
A clue to the origin of the Qaiyarah heavy oils is afforded by bitumen deposits and impregnations in the basal Lower Fars sediments of the Hit-Awasil area, on the Euphrates, and of the Fatha Gorge, on the Tigris, south of Qaiyarah. These impregnations and deposits resulted from extensive seepage in these areas, in the period immediately following deposition of the Euphrates limestone. The seepage oils were very heavy and very sulfurous, approximating to soft bitumen. Vast quantities of sedimentary bitumen and anomalous sediments were spread over large areas as leakage proceeded into the shallow Lower Fars sea. [Note: The Miocene activity of the seepages of the Awasil area, and the nature of their rejuvenation in the present erosion cycle, were first remarked by W.T. Foran, in unpublished reports of the British Oil Development Comapany]. Eventually the seepages were choked by these deposits themselves, and by the later sediments of the Lower Fars. In the Awasil area, Pleistocene erosion has cut through the thin seal, and the seepages are again operative, pouring forth several thousands of tons of bitumen and heavy inspissated oils each year, together with large volumes of sulfurous waters and low-pressure sulfur-laden gases.
The Awasil area and Fatha area seepages are seemingly identical in origin, and carry identical bitumen. Those of the Awasil area are linearly arranged, and correspond in position to a line or zone of pre-Maastrichtian faulting. The modern seepages at Fatha are linked by similarity of oils with those of Qaiyarah area, and with the reservoir oils of the Qaiyarah area fields.
In the Awasil area, a deep well has shown partial impregnations of Maastrichtian limestones, and of Middle and Lower Cretaceous sandstones, by heavy sulfurous oils and bitumen, which are identical with those of the seepages. Structural closure is probably lacking in these reservoirs. The sporadic distribution of impregnation in continuously porous rocks suggests that they are in relation to a fault zone, through which oil has ascended, irregularly, with occasional more or less accidental lateral incursions into porous rocks. The well penetrated a thick Kimmeridgian anhydrite section, and yielded, from below this, some dense oil, with very high sulfur content, which was superficially indistinguishable from the seepage oils of Awasil or Qaiyarah.
In the Awasil area the heavy sulfurous oils are clearly of pre-Kimmeridgian origin, despite their wide distribution in sediments of several younger ages. Their appearance in overlying reservoir formations, and their copious seepage to surface in Lower Fars times, and at the present time, have been facilitated by faulting of early date, perhaps several times repeated, which has broken the Kimmeridgian anhydrite seal, along a line coursing more or less north-south from Hit to Abu Jir. On the evidence of regional correlation, the actual primary reservoir for the heavy oils is almost certainly the upper part of the underlying Jurassic, corresponding approximately in stratigraphic position with the fabulously productive “Arab Zone” of the Arabian fields.
In the Qaiyarah area, as in Awasil, the tectonic setting is one of block movements with north-south bounding faults of late Jurassic or earliest Cretaceous age. The Kimmeridgian anhydrite cover is absent from the area west of the Tigris, due to Neocomian uplift and erosion. The underlying Upper Jurassic, including the presumed porous reservoir unit, has been eroded off the Qaiyarah-Qalian-Atshan tilted block but is believed to remain, intact below anhydrite cover, in the down-fauted block lying east of the Tigris (Figures 3 and 4).
It is argued that the heavy oils of the Qaiyarah fields originated in basinal sediments which occupy a north-south trough passing below Kirkuk (Figure 3). Oil was expressed westward from this trough, during terminal Jurassic and early Cretaceous time, under the impulse of heavy loading (Figure 4) and encouraged by steepening westward gradients. This oil accumulated in contemporaneous neritic-oolitic limestones, beneath Kimmeridgian anhydrite cover, against the face of an uplifted block, along the Tigris River line. Much oil escaped to surface, and remaining oil may have been partially inspissated, in post-Kimmeridgian to Hauterivian time. Some of the retained oil, later buried beneath Cretaceous-Pliocene sediments, migrated upward, through and across the fault zone, into the reservoir-capable limestones of the Qaiyarah-Qasab area. Entry into these reservoirs was from the eastern ends of the Tertiary fold structures. Vertical movement of oil may have commenced during early Lower Fars times, or earlier, and may have continued intermittently through most of the late-Tertiary period. Much Upper Jurassic oil escaped directly to surface, up the fault zone itself, and much continues to escape, through present-day seepages lying along the Tigris River line, at Qaiyarah, Nimrud, Hammam Ali, Khanuqah and elsewhere.
At present the existence of porous Upper Jurassic reservoir rocks, beneath the Kimmeridgian anhydrite cover, is only conjectured. Exploratory drilling now in progress will shortly confirm or deny this conjecture, strengthening or weakening the proffered account of the genesis of the oils of the Qaiyarah area accumulations. The deduced history of the oil of these accumulations is illustrated, somewhat diagrammatically, in Figure 22.
PAST AND PRESENT DISSIPATION OF OIL
In addition to the known oil fields, North Iraq offers prolific evidence of past and now dispersed accumulations on a grand scale. There are also many active and some relict seepages which should be considered in a full inquiry into the factors controlling distribution of accumulated oil within the region. Full discussion of the seepage occurrences is outside the scope of the present superficial survey.
Impressive bitumen impregnations occur in the Berat Dagh, at Aqra, Bekhme, and elsewhere, sporadically distributed through a bank- reef- and shoal-limestone complex of Maastrichtian and upper Campanian age (Henson, 1950a). Impregnations are found in the crestal region, and also low down on the flanks and in the core of what is now a very large, steep-sided anticline. The reef-complex passes northeastward, by intertonguing, into the thick sediments of the Upper Cretaceous Flysch basin, and southwestward into globigerinal basinal sediments (Figure 8). Some, and perhaps all of the oil which was responsible for the heavy impregnations may have originated in either or both of the flanking basins, as suggested by Henson (1950a). The fore-reef tongues and porous parts of the massive limestones are locally bitumen saturated, but elsewhere free of hydrocarbons. It is supposed that large volumes of oil accumulated in localized porosity traps, prior to late-Tertiary folding. Doubtless there was great wastage by loss to surface during this primary accumulation period, for paleogeographic reconstruction indicate that the Berat Dagh area was never covered by any really competent seal. When late-Tertiary folding occurred, the local trap conditions were destroyed by fracturing, and the oil migrated into the main anticlinal trap, and to almost immediate dissipation at surface (Henson 1950a). Judging by volume and nature of the residual bitumen, the original porosity-trap accumulations were very large, and also the original oil was heavy and asphalt rich.
Berat Dagh impregnation
The Berat Dagh impregnation appear to demonstrate that oil in quantity was generated in, and liberated from, sediments of Upper Cretaceous age. In the cases of the known oil fields the contributions from such sources are believed to have been negligibly small. The productivity of the Upper Cretaceous source sediments, even at Aqra, is not entirely proven, as the reef developments of this age rest directly upon porous and permeable Middle Cretaceous limestones, without any intervention of a seal formation, in the area where impregnation is heaviest. Possibly the copious bitumen is residual from secondary accumulations, within the Upper Cretaceous reef-limestone complex, of oils which entered the uplifted Aqra area through Middle Cretaceous carrier-limestones, and which were generated in basinal sediments of Middle-Lower Cretaceous age. The source area would have lain to the east and north of the Aqra-Bekhme range (Figure 6). Heavy loading and steep gradients, imposed during Upper Cretaceous and Paleocene-lower Eocene time (Figures 8 and 9), would have favored extensive migration into the Aqra area. Presence of residual oils in the Middle and Lower Cretaceous rocks shows that such migration did occur. Bitumen impregnation in the lower parts of the Upper Cretaceous also suggests filling from below, as oil entering laterally, before folding, would have ascended through the porous rock mass. Pebble-armored bitumen balls occur abundantly within the lower parts of the Upper Cretaceous section, demonstrating that oil in considerable quantity was seeping to surface early during Upper Cretaceous deposition, in the area where spectacular impregnation is now found in later beds.
Pila Spi area
At Pila Spi, close to the Persian frontier southeast of Sulaimania, oil and bitumen permeate the Maastrichtian Flysch, the overlying Eocene-Paleocene clastics, and the lagoonal middle-upper Eocene limestone, in a setting which is now anticlinal. The accumulation may have predated much of the late-Tertiary folding, but if it did not, the total oil column in this now dissipated oil field must have been many thousands of feet thick. But the nature of the seal for such a large accumulation is obscure. Post-Eocene sedimentation was thin, and the Lower Fars is atypical in this area, in that salt and anhydrite are lacking. The significance of the Pila Spi impregnation is that the oil most probably originated within the Upper Cretaceous Flysch, or perhaps from an unknown Upper Cretaceous reef accumulation now buried beneath the very thick Flysch sediments. The Middle Cretaceous sediments are in basinal facies in this area, and it cannot be argued that the oil has migrated upward from a primary accumulation in a neritic Middle Cretaceous limestone reservoir. Origin from an even deeper primary reservoir or source rock can still be argued, but no candidate is apparent for the role of reservoir rock. It is preferable to admit that the Upper Cretaceous Flysch basin has generated oil in large quantity, and that accumulation occurred in this locality because of special trapping facilities which remain obscure.
Bitumen pebbles occur in some profusion in Paleocene-Lower Eocene conglomerates in the Pila Spi area, indicating that large oil accumulations were undergoing dissipation, somewhere in the region, at the time of deposition of the conglomerates. The source for these water-born bitumens may be seen in such cap-less accumulations as that already described from Berat Dagh. The basal Paleocene pebble bed of southwestern Persia is also characterized by derived bitumen (Kent, et a1., 1951, p. 149).
A third, impressive, bitumen-impregnated structure, now dissected, is Pir-i-Mugrun, west of Sulaimania. The rocks of this large anticline show passage from basinal globigerinal-radiolarian sediments of Lower-Middle Cretaceous age into massive, much dolomitized, rudist bank-reef limestones of the same ages. The lateral passage has been illustrated and discussed by Henson (1950a). The reef limestones and associated massive limestones are very bituminous, as are the fore-reef-shoal tongues which pass out into the basinal sediments. The basinal sediments themselves are classed as fertile source rocks. Pir-i-Mugrun provides a cross section of the facies interdigitation between basinal source rocks, which are believed to have supplied the Kirkuk accumulation, and the reservoir-carrier limestones, in which primary accumulations of the Kirkuk oil is deemed to have occurred, some 80 miles to the southwest.
Pir-i-Mugrun, and other very large mountain anticlines which now expose the neritic Middle Cretaceous limestones, were probably traps for large accumulations of migrant oil, within this reservoir formation, before Pliocene or later erosion destroyed their seals. Dissipation of such accumulations, as well as of accumulations of the Berat Dagh type, may account for the origin of detrital bitumen in the middle and upper Bakhtiari sediments.
The actual volume of wastage from oil accumulations of the Berat Dagh, Pila Spi, and Pir-i-Mugrun types cannot be guessed at, but it must have been very large. The periods of dissipation of such accumulations can be surmised, and the surmises checked to some extent by the distribution of sedimented hydrocarbons in the rock sequence. The picture which emerges for the mountain zone is one of three principal phases of wastage. The first phase corresponds to the Middle-Upper Cretaceous transition, when oils of Middle Cretaceous origin escaped from neritic reservoirs, through windows in the sealing sediments such as are found at Berat Dagh. The second phase corresponds to the terminal Cretaceous regression, when poor-quality seals covering the Upper Cretaceous reef complexes were eroded, and when tectonic fracturing may have disturbed primary porosity traps in the reef sediments. The third phase corresponds to the main anticlinal folding, which destroyed primary porosity traps and encouraged migration into rising structures, but which fractured the low-quality seals capping these structures, and which also laid the crests of the higher structures open to Pliocene erosion.
These three phases of wastage are but little evidenced in the foothills belt, where the known oil fields lie. The Middle-Lower Cretaceous oils had perhaps not entered this area during the time of post-Cenomanian erosion, and they may have been already in place, below an adequate Upper Cretaceous cover, during the Cretaceous-Paleocene emergence. Abundant bitumen in Middle and Upper Cretaceous limestones of some tested structures, where Upper Cretaceous cap rocks are lacking, may represent inspissation of oil during this emergence. The main folding episode certainly promoted vertical migration, and in many cases such migration may have continued to surface, but losses of this origin cannot be assessed. Some tested structures, which have only mediocre cap rocks, show strong bitumen impregnation in Tertiary reservoirs. Such impregnation may indicate slow loss of original oils during the folding or passage of large volumes of oils through the unsealed traps to the surface during the post-folding period.
Two additional episodes of widespread dissipation, affecting the Upper Jurassic oils of the foothills and foreland zone, have already been considered in connection with the origin of the oils of the Qaiyarah area fields. The first of these episodes is illustrated by the emergence and erosion of tilted blocks, embracing the Qaiyarah, Qalian, and Atshan areas, at about the Cretaceous/Jurassic transition period. Large volumes of Upper Jurassic oil were dissipated at this time, evidence in the form of abundant bitumen pebbles being preserved, in shallow-water deposits of the transgressive Lower Cretaceous, in some areas of continuous Jurassic-Cretaceous sedimentation. Early Cretaceous dissipation of Upper Jurassic oils, similar to that argued for the Qaiyarah area, may have occurred elsewhere in the region west of the Tigris.
The second episode of wholesale dispersion of the Upper Jurassic oils, corresponding to the onset of Lower Fars sedimentation, resulted in losses to surface, conservatively estimated at hundreds of millions of tons, in the areas around Awasil and Fatha, where sedimentary bitumens are prominent at the base of the Lower Fars. The known areas of Miocene seepage may be only a small part of the total area in which such seepages occurred, and the residual bitumen may be only a small part of the oil which escaped into the shallow Lower Fars seas.
Active seepages of bitumen, heavy oil, and gas, all rich in sulfur, characterize the Awasil area, and the vicinity of the Tigris. Most of these seepages are considered to be oils of Upper Jurassic origin, though the reservoirs now being drained may be of much younger age. Some seepages of very heavy, sulfurous bitumens, occurring far from the Tigris, may mark reappearances of heavy sedimented oils, which were deposited with the Lower Fars after escape into the Lower Fars seas.
Other important seepages are prominent in the crestal regions of large folds which contain or have contained oil accumulations in the Tertiary limestones, and which are capped by saliferous Lower Fars. These seepages are distributed through the foothills belt between Kirkuk and the Persian frontier, mostly in locations closely related to surficial thrust faults which are accommodated in the Lower Fars “Salt Zone.” Gas seepages are most common, commonly associated with heavy inspissated oils, and locally with very light oils, which are perhaps of condensate type. Secondary mineralization of gypsum into aragonite, and of calcareous marls into “gach-i-turush,” have been described from analogous seepage areas in Persia (Lees and Richardson 1940; Lees, 1953b). Similar mineralization products are found in association with many of the gas seepages of Northern Iraq.
The current rates of gas loss from some structures, and the long history of dissipation which must be allowed, require either that the initial charges of oil must be seriously depleted of their original gas content, or that additional gas must be entering the traps to make good the losses. On the other hand the volume of fluid oil lost to the surface from these seepages must be very small, in comparison with the capacity of the traps, unless the rates of oil escape are much smaller now than in the past. The conclusion is reached that original accumulations of oil in the pre-Fars Tertiary limestones of the salt-covered structures have probably survived the Pleistocene erosion cycle with little loss. But some redistribution of oil from structure to structure, across the spillways, may have followed gas-volume changes consequent upon gas seepage, upon changes in hydrostatic pressure, or upon entry of additional gas or gas-rich oil from depth.
The presence of a gas seepage does not confirm the existence of an underlying oil accumulation, and the rate of escape of gas is not directly related to the volume of oil which is present in the underlying trap. Only one field (Kirkuk) of the five known fields which contain oil of Middle Cretaceous origin is marked by surface gas seepages. Some structures with very large gas seepages may have very small oil columns, or may contain no oil whatever.
Seepages in the strongly folded mountain zone are for the most part associated with residues of accumulations of the Berat Dagh, Pir-i-Mugrun, or Pila Spi types, though a few may originate by vertical migration from surviving, unexposed but probably inspissated accumulations in reservoir limestones of Middle Cretaceous or older ages. Some so-called seepages are in reality merely exceedingly bituminous “source rocks,” stripped of their liquid oils, which exude semi-solid bitumens locally under the high prevailing summer temperatures. The Tithonian radiolarian shales and limestones, and some similar sediments of pre-Kimmeridgian Upper Jurassic age are the most markedly bituminous of such rocks; they have sufficient hydrocarbon content in some areas to render them usable as fuel.
The evidences of relict seepage cannot be considered in detail. Many drilled structures west of the Tigris have revealed thick sections, heavily impregnated by bituminous residues, which indicate one-time saturation by fluid oils which have escaped to surface, or which have been slowly inspissated in depth, presumably by seepage loss, under circumstances which remain to be explained, instance by instance. “Dikes” of hard bitumens, cutting subvertically through Fars sediments, and also recorded from basinal marls of Upper Cretaceous to Oligocene age, present further unsolved problems. The significance of these occurrences of hydrocarbons are not entirely clear. But both the “dikes” and the relict pools may be cited as direct evidence of extensive migration across the bedding.
SYNTHESIS AND DISCUSSION
The known oil fields of Northern Iraq are believed to have drawn at least the bulk of their supplies from only three principal source formations. Each of the three formations which are regarded as productive source rocks comprises basinal sediments, deposited under more or less euxinic conditions, and characterized by radiolarian faunas.
The youngest of the recognized fertile source beds, which are of Middle-Lower Cretaceous age, have been productive of accumulations of medium gravity oils (30°-38° A.P.I.) with about 1-3 per cent sulfur. The oldest recognized source beds, considered to be of pre-Kimmeridgian Upper Jurassic age, are held responsible for heavier oil (10°-20° A.P.I.) with much higher sulfur content (6-10 per cent). The intermediate Tithonian source beds appear to yield very light oils, up to 48° A.P.I., with less than 1 per cent of sulfur, and without any significant asphaltene content.
The occurrence of a light-oil source between two groups of sediments which produce comparatively heavy oils denies any glib generalization relating gravity of oil to depth of burial or to age. In the case of the oils of Middle-Lower Cretaceous origin, the accumulations in the Upper Cretaceous fractured limestone reservoirs, in Ain Zalah and Kirkuk, are lighter than those in the Middle Cretaceous reservoirs, whereas in Kirkuk the “Main Limestone” oil is lighter yet. This upward improvement may relate in part to adsorption of surface-active heavy components of the oil during migration through a tortuous fracture system. In addition the “Main Limestone” oil of Kirkuk may have been diluted by small volumes of light oil of Upper Jurassic (Tithonian) origin, which may have entered the reservoir from the plunging ends of the structure.
Probable major contributions of Upper Cretaceous Flysch-basinal and globigerinal-basinal sediments to contemporaneous reef-type reservoirs are recognized, and accumulations of indigenous oils within the Flysch sediments are acknowledged. But known accumulations fed from such sources lie in the mountain zone and are now largely dispersed. Original oils were probably heavy, asphalt rich, and sulfurous.
Productivity of the Upper Cretaceous source beds may have been restricted to the Maastrichtian-upper Campanian reef belt, and to the Flysch trough itself. No oil field containing oils of such sources has been located in Northern Iraq, but the Raman and Garzan fields of southeastern Turkey are in a setting somewhat comparable with that of the Berat Dagh impregnations.
Oligocene, Eocene, and Paleocene basinal sediments, which are dominantly globigerinal marls, have probably subscribed little or no oil to any present oil accumulation considered. Indigenous Miocene oils may occur in the Lower Fars, but accumulations attributable to such source are negligible in volume.
The foregoing tentative interpretations arise from consideration of the geological and paleogeographical factors, only lightly held in check by reference to the chemical and physical properties of the oils. Detailed chemical investigation of reservoir, seepage, and residual oils, of bitumens and of source sediments is now necessary, in order to verify or deny the deduced source-to-reservoir relationships. Until the data from such investigation are available, the stated interpretations may be regarded as preferred hypotheses, and alternative, if less probable accounts remain arguable, though they cannot be pursued in the discussion which follows.
The absence of any significant contribution of oil from the globigerinal sediments of the Tertiary basins may be ascribed to innate inability of such sediments to generate oil, to the absence of conditions permitting liberation of oil generated, or to early loss of oils which were generated and liberated before adequate cover was deposited over the reservoir area. The Kirkuk accumulation appears to lack Paleocene-Oligocene oils, in spite of ideal spatial relationships of basinal and reservoir facies, and in spite of favorable post-Oligocene loading, gradient, and tectonic factors. The hostile incident of Oligocene-Lower Miocene emergence seems inadequate to account entirely for the deduced absence or near-absence of Tertiary oils. Hence, for Kirkuk, the failure of the Tertiary globigerinal sediments to yield oil in quantity suggests that they were incapable of generating oil in quantity.
The Upper Cretaceous, lower Senonian, and Turonian basinal globigerinal deposits also seem to have contributed little or no oil to any known oil field (though the Berat Dagh impregnations may have drawn upon Upper Cretaceous globigerinal sediments, as remarked above). Assessment of these rocks as unproductive is less positive than in the case of their Tertiary facies equivalents. One alternative working hypothesis for the origin of the primary accumulations in the Middle Cretaceous reservoirs accepts a source in the overlying, younger, globigerinal sediments, rather than in the Middle Cretaceous and older radiolarian sediments. This alternative hypothesis is rejected, at present, because it does not account with any perfection for the distribution of oil fields and of dry structures in Northern Iraq, whereas the thesis that origin lay in the Middle-Lower Cretaceous basin explains such distribution.
Stigmatization of the Turonian to Oligocene globigerinal-basinal sediments as unproductive in the area considered should not be taken to infer that these sediments are everywhere negligible as source rocks. The evidence is quite inadequate to sustain any such prejudice. The interpretations put forward as to sources of known accumulations are tentative, and even if they should be confirmed for the accumulations concerned, other source sediments could well have supplied other as yet unknown accumulations in other areas, or even in the same area. The parts of Northern Iraq which remain untested are large, and unexplored areas in adjacent territories are larger. Major accumulations, drawing on globigerinal source rocks of any age from lower Senonian to Miocene, could exist within the region. Nevertheless, it may be significant that despite the presence of thick basinal sediments of lower Senonian-to-Oligocene age in Syria, commercial oil has not been found, though adequate cap-rock seals have been proved in some good structures, where fracture production, at least, might have been expected. In this large and at present unproductive region the basinal Upper Jurassic, Tithonian, and Middle-Lower Cretaceous radiolarian source sediments of Northern Iraq are lacking.
Because of differences in areal distribution of sources, carrier-reservoirs, and caprocks associated with the different basins in Northern Iraq, and because of the different times at which primary migration and accumulation occurred, the differently originating oils are now found to occupy different and to some extent mutually exclusive parts of the region.
The distribution of the Kirkuk-type oils relates primarily to the distribution of the reservoir-carrier facies of the Middle Cretaceous, and to the stratal gradients and loads imposed during stages following immediately upon the deposition of the Lower-Middle Cretaceous source sediments. Where favorable reservoir facies of the Middle Cretaceous is absent, and where favorable gradients did not develop to induce primary migration, oil of this origin is lacking.
The Qaiyarah-type oils are distributed in close relation to the shelf-like margin of the Upper Jurassic, and to the position of erosional and tectonic discontinuities in the Kimmeridgian anhydrite cap rocks, and hence in part to the position of block-bounding faults of late Jurassic or very early Cretaceous date, as well as to buried faults of later origin.
The distribution of the very light oils, which are tentatively attributed to origin in the Tithonian basinal deposits, relates directly to the distribution of the source beds, in which the oils are still largely contained. Distribution within the Tertiary limestone reservoirs relates to the presence or absence of a thick plastic “shield” of Berriasian mudstone and to the nature and thickness of the overlying Cretaceous sediments. No primary accumulations of the light Tithonian oils have yet been encountered, and perhaps none exists within the region.
All the discussed present-day accumulations share a similar early history. In each case expression of the oils from the basinal source rocks has been westward or southwestward, into reservoir-carrier formations of the same general age as the source formations. In each case this expression has accompanied heavy depositional loading of the source sediments, and the development of steepening marginward gradients, following shortly after sedimentation of the source beds. In each case primary accumulation has occurred in porous limestones, developed along the shoreward margins of the basin, and primary concentration has been into structural traps, or into stratigraphic traps of some nature.
The geographical distribution of the known oil fields has thus been controlled dominantly by the paleogeographies of the depositional basins in which the source beds were deposited, and by the gradients and loads which have been imposed upon source beds and more or less contemporaneous reservoir formations. The primary accumulations have come into place as a result of more or less important lateral migration. The one partial exception to these generalizations is provided by the very light oils, of probable Tithonian source, which were denied opportunity for westward expulsion from the source sediments by lack of communication with any porous reservoir formation. In general, oils of this origin must have awaited liberation by fracturing during late-Tertiary folding. If any accumulations of such oils exist, they must overlie the source sediments themselves, in circumstances permitting vertical migration into higher reservoir formations.
The pre-Turonian emergence and erosion influenced the distribution of primary accumulations in the Middle Cretaceous reservoir by limiting the area in which porous reservoir rocks survived, and by causing development of localized secondary porosity. The pre-Neocomian emergence and erosion dictated, to a large extent, the whereabouts of stratigraphic and fault-trap accumulations of pre-folding age, within the Upper Jurassic primary reservoir. Other unconformities have had little or no direct control upon the distribution of existing oil fields.
Vertical migration has played a dominant role in the subsequent development of all known oil field accumulations. Every reservoir either owes its oil content to migration from below, or has been greatly or entirely depleted by escape to higher reservoirs, or to the surface. The dissipation of the one-time oil fields at Berat Dagh, Pir-i-Mugrun, and Pila Spi is also attributable to vertical loss.
The Ain Zalah field illustrates partial depletion of the primary accumulation in the Middle Cretaceous reservoir, and consequent accumulation of oil in the fracture housing of the “First Pay” as a result of vertical migration, possibly at the time or late-Tertiary folding (Figure 16).
The Butmah field shows further development than does Ain Zalah. The depletion of the Middle Cretaceous limestone reservoir is entire, and all the original oil content is here found in the upper fractured reservoir, below Paleocene-Lower Eocene marl cover (Figure 16).
In the Kirkuk field (southeastern dome) the process of upward migration has followed the same course as at Ain Zalah, but the initial accumulation was much larger, and the Paleocene-Lower Eocene marl cover was much thinner. Small accumulations remain in the primary Middle Cretaceous reservoir and in the intermediate Upper Cretaceous fractured-limestone reservoir, but the bulk of the oil has ascended into the overlying “Main Limestone.” During the Pleistocene erosional cycle, gas seepage to surface commenced and the “Main Limestone” oil is now in process of losing the greater part of its original endowment of dissolved gases (Figure 20).
The Bai Hassan field probably originated in the same manner as did Kirkuk, but crestal seepage has not yet commenced and the oil column is gas saturated. Additional gas appears to have entered this structure from below, at some time after the main incursion of the oil of Middle Cretaceous origin. It is suspected that the source of the excess gas lay (or lies) in the Tithonian radiolarian-rich source beds, which underlie the Kirkuk field, but which are there denied upward passage by the thickness and plasticity of the intervening Berriasian mudstone shield. Some gas and light oil from this source may be creeping upward into the northwestern dome, and into the southeastern end of the southeastern dome of the Kirkuk structure.
Jambur probably originated, like Bai Hassan, as a “Main Limestone” accumulation of upward-migrated Middle Cretaceous oil. It has been modified by addition of a different and lighter, gas-rich oil, which has entered either vertically from an underlying source (again, suspectedly, the Tithonian radiolarian sediments), or laterally, across a spillway, from a filled structure lying to the southeast or east, which was itself filled from such source.
The heavy noncommercial oils of the two reservoirs of the four Qaiyarah area fields have a common origin in a deeper accumulation of Upper Jurassic source, housed in Upper Jurassic porous limestone reservoirs. It is argued that oil entered vertically into the Qaiyarah trap, and thence into Najmah and Jawan. The Qasab accumulations may have been fed separately from another part of the same fault zone. In addition to the lateral feeding there may have been vertical leakage from the lower to the upper reservoir, within the limits of the individual domes. The Tertiary reservoir is currently undergoing gas and some oil depletion by vertical seepage on the Qaiyarah, Jawan, and Qasab domes (Figure 22).
Although this synthesis lays great stress upon the importance of vertical migration in controlling the stratigraphic distribution of present oil accumulations, it is not suggested that oil has ascended from indefinite depths to fill available anticlines wherever these are capped by a sufficiently competent seal. Instead, it seems that the known accumulations do not include any significant contribution from sources older than Upper Jurassic. If oil was generated in pre-Upper Jurassic basins, it has not ascended into known reservoirs in Cretaceous and Tertiary rocks, and therefore it may be sought, in the first instance, in primary accumulations in reservoir formations of similar age to the source sediments, in areas which may be remote from the known oil fields. Middle Jurassic and older oils may be found in pre-Upper Jurassic reservoirs, beneath some of the structures which are productive from Middle Cretaceous or younger reservoirs.
If the interpretations offered are correct, over 95 per cent by volume of the known oil field reserves of the region have come into place by substantial migration across the bedding. This finding is in sharp contrast with conditions holding in the fields of the Basrah area, and of Arabia, where vertical migration has been unimportant and local, influencing the history of perhaps only a fractional percentage of the total oil in place.
The writer is indebted to the management and to the Chief Geologist of Iraq Petroleum and associated companies for opportunity to prepare and permission to publish this paper. Acknowledgements are gratefully extended to many colleagues in the past and present employ of these companies for their contributions to the stratigraphic synthesis. Particular thanks are due to R.C. van Bellen whose interpretations of Tertiary sections have been freely drawn upon, to R.G.S. Hudson who has been responsible for stratigraphic determinations of most of the Mesozoic macrofaunas, and to R. Wetzel, whose field studies in Kurdistan and elsewhere have provided many of the thickness controls and sample collections on whick the map constructions have been based.
H.V. Dunnington was a Divisional Paleontologist with the Iraq Petroleum Company Limited, Kirkuk, Iraq. He read this paper before the American Association of Petroleum Geologists (AAPG) in New York, on March 30, 1955. The manuscript was received by AAPG on April 5, 1955, and published in 1958 in “Habitat of Oil”(Ed. L.G. Weeks), AAPG, p. 1194-1251. The paper was also reprinted in “Geology and Productivity, Arabian Gulf” (AAPG Foreign Reprint Series No. 2) compiled by A.E.L. Morris. GeoArabia thanks AAPG for permission to reprint this paper (in color) and to distribute it electronically.