ABSTRACT

A sequence stratigraphic and tectonic model for the mid-Turonian-early Campanian carbonate sequence in North Iraq was constructed based on subsurface lithologic, thin-section and well-log analyses of ten boreholes. The studied sequence is represented by the Gulneri, Kometan and Mushorah formations in the eastern sector (seven wells) and their correlative Khasib, Tanuma, Sa’adi and lower part of the Hartha formations in the western sector of the study area (three wells). Three second-order sequences (from oldest to youngest A, B and C) were identified. Sequence A consists of two third-order sequences (A1 and A2) and is represented by the mid-Turonian Gulneri Formation. Sequence B consists of three third-order sequences: B1 corresponds to the late Turonian-early Coniacian Khasib Formation and correlative lower part of Kometan Formation. Sequences B2 and B3 are exclusive to the late Coniacian-early Santonian Tanuma Formation and correlative middle part of the Kometan Formation. Sequence C is comprised of three third-order sequences of which C1 and C2 encompass the late Santonian Sa’adi Formation and correlative upper part of the Kometan Formation, while C3 corresponds to the early Campanian Mushorah Formation and correlative lower part of the Hartha Formation. Each of the third-order sequences is comprised of transgressive and highstand systems tracts, while the lowstand systems tract is restricted to sequences B2, B3, C1 and C2. These systems tracts are, in turn, comprised of vertically stacked parasequences of shallowing upward packages.

The tectono-depositional model that was deduced from the sequence analysis is characterised by two flat-topped ramps that formed as a consequence of extensional tectonism. Three NW-trending normal faults are interpreted to separate the ramps whose inclined apex was continuously being eroded. The eroded detritus from the apex consists of shallow-marine carbonates that were deposited in basinal sites. Within this structural framework, the ten boreholes are placed within their appropriate depositional sites within the ramp system.

Facies analysis demonstrated an array of facies associations, which generally accumulate in basins that are fed by an influx of eroded shallow-water derivatives. The depositional architecture developed in an open-marine, low-energy, middle- to outer-ramp setting in the case of Gulneri and Kometan formations. A middle–inner ramp setting is attributed to the Khasib, Tanuma and the lower part of the Hartha formations. A transition to a slope setting is suggested during the deposition of the Mushorah Formation; whereas slope-apron facies are common to all formations in the well Quwair-2. Diagenetic modification overprinted the carbonates and its effect on reservoir properties is considered. Recognition of third-order cycles demonstrates the utility of using genetic units and sequence stratigraphy to discern the depositional architecture of these formations. Local tectonic influences, which control sedimentation patterns, eustasy and sedimentation rates are additional determinants for the final stratigraphic framework.

INTRODUCTION

In North Iraq, besides the main producing Oligocene-Miocene reservoir zones, the mid-Turonian-early Campanian succession also represents an important exploration and producing interval. North Iraq is a vast region with many giant oil fields including Kirkuk (Baba, Avanah and Khurmala domes), Khabaz, Bai Hassan, Qara Chauq, Qassab, Qaiyarah, Makhmur and Quwair (Figure 1). This succession encompasses a complete petroleum system (source, reservoir and seal) and is represented by the Gulneri, Kometan and Mushorah formations in the eastern sector, and their correlative Khasib, Tanuma, Sa’adi and lower part of the Hartha formations in the western sector of the study area (Figure 2, after Al-Naqib, 1967). These seven formations manifest significant differences in lithofacies, thickness and distribution, which appears to reflect the interaction of eustasy with the development of separate depocenters associated with tectonism along the margin of the Neo-Tethys Ocean.

Sequence stratigraphic analysis offers a useful technique for understanding depositional systems and the role of eustasy in complex geologic settings. This technique provides a chronostratigraphic and spatial framework for correlating depositional sequences (van Wagoner et al., 1988; Galloway, 1989), including carbonate depositional models (Sarg, 1988). Hitherto, the concepts of sequence stratigraphy have not been considered in detail for the mid-Turonian-early Campanian succession of Iraq. Nor has the relationship between sequence stratigraphy and hydrocarbon petroleum systems been considered for this and other successions in Iraq.

In this paper, a combination of petrographic observations, facies and well-log analyses are used to identify the mid-Turonian-early Campanian second-, third- and higher-order depositional sequences in North Iraq. The third-order sequences are interpreted in terms of low stand system tracts (LST), transgressive system tract (TST), high stand system tract (HST), as well as maximum flooding surfaces (MFS). The sequence boundaries are interpreted as type SB1 (subaerial exposure and erosion), SB2 (minor regression), and SB3 (drowning surface). The interpretation of the stacking patterns within a systems-tract framework is used to understand the relationship between depositional and diagenetic features. The paper also incorporates the influence of tectonism, which profoundly affected the architecture, facies and distribution of the depositional sequences.

PREVIOUS STUDIES

The mid-Turonian-early Campanian succession has been studied in different regions of Iraq (Table 1). Most of theses studies, particularly for North Iraq, generally addressed lithological, palaeontological and environmental interpretations, and the correlation of rock units within Iraq and to adjacent countries (Figure 2; van Bellen et al., 1959, and references therein; Al-Naqib, 1960, 1967; Ditmar et al., 1971, 1972; Buday, 1980). Likewise, the Department of Geology of the North Oil Company of Iraq presented many articles on the sedimentologic, palaeontologic and reservoir characteristics of the oil fields. These studies placed particular emphasis on age determination of the formations. Table 2 summarises the ages of formations based on foraminiferal data from previous studies as well as those gained in the present study (Plate 1). Other works have dealt with the structural and tectonic evolution of the study area, which can be characterised as a “tectonic corridor” (Bolton, 1960; Numan, 1983, 1997, 2000; Lovelock, 1984; Buday and Jassim, 1987; Ameen, 1992; Beydoun, 1991). The type sections of the mid-Turonian-early Campanian formations in the study area are summarised in Table 3 (according to author, locality name, coordinates, thickness, lithology and age).

In brief, the lithology and stratigraphic framework of the seven studied formations are as follows (Figure 2 and 3, Tables 2 and 3). The Gulneri Formation is composed of shaly limestone and calcareous black shale with pyrite-glauconite showing lamination and fissility. The Kometan Formation is divided into three parts. The upper part is light grey, massive, vuggy with a chalky appearance with the development of stylolites and calcite veins. The middle part is reported to be generally shaly with intercalation of limestone containing pyrite indicating anoxic conditions. The lower part is similar to the upper part with intercalation of shaly limestone and calcareous shales.

The Khasib Formation (lower part of the Kometan Formation) consists of limestone with creamy colour, slightly hard and chalky with subordinate calcareous shales. The Tanuma Formation (shales in the middle part of the Kometan Formation) has low porosity, and consists of grey limestone with calcite-filled vugs and minor intercalations of shaly limestone and calcareous shales. The Sa’adi Formation (upper part of the Kometan Formation) embraces white-grey limestone with green marl showing stylolites-calcite vein development and sporadic chert.

The lower part of Hartha Formation is lithologically similar to the Sa’adi Formation, but with higher porosity and oil shows. The Mushorah Formation is characterised by light brown-grey limestone, slightly porous with oil shows; marly limestone and chert are encountered in this formation in all the investigated wells. It is correlated to the lower part of the Hartha Formation; the age of the latter formation is not constrained and is here considered early Campanian (Table 5) rather than late Campanian (Figure 2, Al-Naqib, 1967).

DATA BASE AND METHODS

Ten boreholes were selected to cover the depositional basin (Figures 1 and 3). The sampled wells were selected on the basis of their geographic location, availability of cores and thin sections. Moreover, cursory checking was made with subsidiary adjacent wells so as to ascertain the lateral equivalence or distribution of affiliated facies. The palaeoenvironmental interpretations and depositional model were based on palaeontological and sedimentological criteria provided by microfacies analysis and examination of more than 1,850 thin sections prepared from cores or cutting samples, and stained with alizarine red for dolomite-calcite differentiation.

Various types of well logs were extensively used in the correlations; these included GR (total gamma-ray), SGR (spectral gamma-ray), SP (spontaneous potential), FDC (formation density compensated), CNL (compensated neutron log), BHC (borehole compensated sonic), THD (high resolution dipmetre) and CPI (computer processed interpretation). In combination with thin-section analysis, these logs provided the means for identifying parasequences, sequence boundaries, stacking patterns, maximum flooding surfaces and changes in facies.

MICROFACIES

On the basis of petrographic characteristics and following the classification of Dunham (1962), seven major microfacies and sixteen submicrofacies, in addition to Calcareous Shale Facies (Sh), were recognised. Their distribution is illustrated for the ten studied wells in Tables 4-1 to 4-10 and Figure 3, and they are arranged as follows:

Lime mudstone microfacies (M)

  • M1: Chalky lime mudstone submicrofacies (Plates 2.1 and 2.2)

  • M2: Peloidal lime mudstone submicrofacies

  • M3: Shaly lime mudstone submicrofacies (Plates 2.3 and 2.4)

Lime wackestone microfacies (W)

  • W1: Bioclastic lime wackestone submicrofacies (Plate 3.1)

  • W2: Planktonic Foraminiferal and bioclast-rich lime wackestone submicrofacies (Plates 3.2 and 3.3)

  • W3: Heterohelicids lime wackestone submicrofacies (Plate 3.4)

  • W4: Rotalids lime wackestone submicrofacies (Plate 3.5)

Lime wackestone/packstone microfacies (W/P)

  • WP1: Whiteinella-rich lime wackestone/packstone submicrofacies (Plate 3.6)

  • WP2: Calcisphaerulids-rich lime wackestone/packstone submicrofacies (Plate 3.7)

  • WP3: Inoceramus and planktonic foraminifera-rich lime wackestone/packstone submicrofacies (Plate 3.8)

  • WP4: Shaly lime wackestone/packstone submicrofacies

Lime packstone microfacies (P)

  • P1: Echinodermal lime packstone submicrofacies (Plates 4.1 and 4.2)

  • P2: Shaly peloidal lime packstone submicrofacies

  • P3: Planktonic foraminiferal lime packstone submicrofacies (Plates 4.3 and 4.4)

Lime grainstone microfacies (G)

  • G1: Bioclastic lime grainstone submicrofacies (Plate 4.5)

  • G2: Red algal-peloidal lime grainstone submicrofacies, consisting mainly of Corallinaceae fragments (Plates 4.6 and 4.7)

Lime rudstone microfacies (R) (Plate 4.8)

Lime floatstone submicrofacies (F)

All the examined microfacies, in general, have been affected to various degrees by leaching (Plates 4.7 and 5.1), dissolution (Plate 5.1), cementation (Plates 3.3, 4.3, 5.1 to 5.4), dolomitisation (Plate 6.1), silicification (Plates 6.2 to 6.4), neomorphism (Plates 2.2, 4.7, 7.1 and 8.1), bioturbation, compaction, glauconite and pyrite formation (Plates 7.2 and 7.3). The distribution of these diagenetic phenomena along the various lithologic sections is indicated for the ten studied wells in Tables 4-1 to 4-10.

Petrographic characteristics show that dissolution in the studied rocks is preferentially enhanced at or near sequence boundaries with the development of vuggy-moldic porosity (Plate 7.4). The bioturbated succession exhibit mottling, and whorled alignment features are affiliated to Thalassinoides, Chondrites and Zoophycus. The association of fine-grained dolomite with stylolites (solution compaction; Plate 3.3) and shaly lime mudstone precluded the influence of either hypersaline or mixing-zone dolomitisation and is in favour of a burial origin (Chillinger et al., 1979; Warren, 2000).

The selective replacement of bioturbated carbonate intervals by silica point to the intervening successive diagenetic preference of such sites to silica emplacement (Minoura et al., 1996; Orti et al., 1997), most probably from biogenic origin. Authigenic pyrite and glauconite filling foraminiferal tests record their production under reducing environment with periodic cessation of deposition following relative sea-level rises (Hein et al., 1974; Bathurst, 1976). Plates 8.1 and 8.2 show examples of microfacies that are interpreted to represent a hardground and condensed section; whereas Plates 8.3 and 8.4 show similar examples of planktonic peaks that characterise maximum flooding surfaces (MFS).

DEPOSITIONAL SEQUENCES

The present study identified three second-order sequences (A, B and C) within the mid-Turonian-early Campanian succession. These sequences span a period of more than 11 million years (My) based on the geological time scale of Haq et al. (1988), and are comprised of eight third-order sequences (Figures 3 and 4). The ages of the depositional sequences and their formational positions are shown in Table 5. The identification of third-order depositional sequences and systems tracts depends mainly on the distinction of the deepening, fining-upwards trends (TST) and shallowing, coarsening-upwards trends (HST), in addition to the signatures of gamma-ray and spontaneous-potential logs.

Second-order Sequence A

This sequence is represented by the mid-Turonian Gulneri Formation and consists of two third-order sequences; from base-up A1 and A2 (Figures 3 and 4, Table 5). The lower part of the Gulneri Formation contains Helvetoglobotruncana helvetica, Whiteinella praehelvetica together with Whiteinella sp. and Hedbergella sp., which implies an early to mid-Turonian age (Table 5). The identification of fauna in the present study was made using thin sections. The deposition of sequence A followed a mid-Turonian drowning event and the resulting drowning unconformity is considered as a sequence boundary of SB3 type with periodic bottom-water anoxia in the outer ramp. The A1 and A2 sequences were identified in wells Qara Chauq-1, Khabaz-1, Quwair-2, Kirkuk-175; whereas the A2 sequences is missing from Kirkuk-116 and Kirkuk-117 (Figure 3). The Gulneri facies were not recorded in the western sector (Qassab-12, Qaiyarah-55 and Makhmur-1), either because of non-deposition or erosion (Figure 3).

Third-order Sequence A1

This sequence generally occupies the lower part of the Gulneri Formation (Figures 3 and 4); however, in Kirkuk-116 and Kirkuk-117 it represents the whole succession. Its thickness varies between 2 and 11 m. The A1 sequence is subdivided into two fourth-order parasequences in Bai Hassan-81 and Quwair-2. The progradational part of the first parasequence occurred during the TST, while the retrogradational part of this parasequence and the second parasequence is in the HST. Other wells show the development of only one parasequence.

TST A1 is characterised by shaly lime mudstone microfacies (M3) and calcareous shale lithofacies enriched with organic matter and abundant planktonic foraminifers and calcisphaerulids. This TST is bounded below by an SB3, which demarcates the Gulneri Formation from the underlying Dokan Formation; and above by MFS A1, which is recognised as a planktonic peak.

HST A1 microfacies include planktonic foraminiferal wackestone-packstone and bioclasts of shallow-water fauna (algae, echinoids, etc.), accumulated in a hemipelagic-pelagic environment with a shallowing upward trend and gravitary deposition. The lower part contains extraclasts and composite rock fragments possibly indicating erosion and redeposition. The upper boundary is interpreted as a SB2-type boundary as evidenced by the absence of A2 from Kirkuk-116 and Kirkuk-117, and diagenetic dissolution in Quwair-2.

Third-order Sequence A2

This sequence is 4.5 to 11 m thick, and represents the upper part of the Gulneri Formation in Bai Hassan-81, Khabaz-1, Qara Chauq-1, Quwair-2 and Kirkuk-175 (Figures 3 and 4). In Kirkuk-116, Kirkuk-117 and the western sector, it is not present indicating non-deposition or subaerial exposure (Figure 3).

TST A2 ranges in thickness between 1 and 5 m and consists of shaly wackestone-packstone and calcareous shales with extraclasts. Peloidal packstone is found only in Quwair-2 within this tract. It is bounded below by an SB2-type boundary, and above by MFS A2 as demonstrated by well logs and a high-diversity planktonic peak or calcisphere-rich packstone with Whiteinella sp. and Hedbergella sp.

HST A2 shows a stacking pattern of shallowing upwards units with increasing carbonate content. It starts with MFS A2 and is bounded above by an SB1-type boundary. It is made of three fourth-order parasequences in Bai Hassan-81 and Quwair-2, four in Khabaz-1 and two in Kirkuk-175.

Second-order Sequence B

Sequence B is equivalent to the lower and middle parts of Kometan, Khasib and Tanuma formations, and spans the late Turonian and early Santonian time interval (Figure 4 and Table 5). A major sea-level rise during this period flooded the entire region. The sequence begins with a drowning unconformity (SB3-type) that coincides with the type SB1 sequence boundary. Sequence B also ends with an SB1-type boundary.

Third-order Sequence B1

The basal part of sequence B1 represents the lower part of the Kometan Formation in all boreholes that encountered this formation (Figure 3). It is represented by the Khasib Formation in Qaiyarah-55 and Makhmur-1 but is not present in Qassab-12 indicating positive relief at this location during deposition (Figure 3).

TST B1 has a thickness of 4 and 36 m in Kirkuk-117 and Kirkuk-175, respectively. The facies association consists of deepening upward cycles, which infer retrogradational or back-stepped stratal geometries. TST B1 includes, in Qaiyarah-55, Makhmur-1, Bai Hassan-81, Khabaz-1 and Kirkuk-117 two fourth-order parasequences and the progradational part of a third parasequence, and has a reduced thickness in Kirkuk-116, Kirkuk-175, Qara Chauq-1 and Quwair-2.

A combination SB1/SB3 (essentially an SB1) boundary delineates TST B1 from below. The upper boundary appeared as an MFS reflecting higher accommodation and an equivalent condensed section in Kirkuk-175, as manifested by abundant planktonic foraminifera and associated glauconite, clay minerals and organic material.

HST B1 has lower and upper components. The lower component has an aggradational stacking pattern (constant bathymetry inferring aggradation), while the upper one has a shallowing-up facies trend that may imply a progradational stacking pattern. It has a variable thickness that ranges from 2 m in Kirkuk-175 within the Kometan Formation, to 67 m in Qaiyarah-55 within the Khasib Formation. HST B1 is comprised of six fourth-order parasequence divided between the lower and upper components of this tract. These parasequences are not likely to be consistently recognised across the studied area.

Third-order Sequence B2

This sequence embraces the lower part of the Tanuma Formation and the lower portion of the middle part of Kometan Formation (Figure 4), i.e. the Kometan shale. The upper and lower boundaries are of SB2-type and show the development of LST, TST and HST deposits.

LST B2 is only developed in Khabaz-1 and Bai Hassan-81 as a consequence of a relative sea-level fall and subaerial exposure of the inner and middle parts of the ramp allowing siliciclastic detritus (mainly clays and silts) to be transported further down-ramp as shaly lime mudstone and shale as in Khabaz-1 and Bai Hassan-81. The upper limit appeared as a transgressive surface (TS).

TST B2 facies were laid down following a sea-level rise as pelagic-hemipelagic facies forming the middle part of the Kometan Formation. In general where the Tanuma Formation is recorded, TST B2 retained a peritidal character with an inner-ramp facies that is shallower than subtidal. TST B2 encompasses numerous fourth-order, high-frequency deepening-up stacking patterns within the various localities. The upper boundary corresponds to MFS B2.

HST B2 is characterised by multiple shallowing upward cycles with static to shallowing-up stacking patterns implying possible progradation. This is indicated by the increase of carbonate facies and their final dominance with the inclusion of hemipelagic and periplatform facies. The latter facies are bioclastic lime wackestone, shaly lime wacke-stone, lime floatstone and shaly lime mudstone planktonic foraminifera and bioclastic-rich lime wackestone. This tract is comprised of a variable number of fourth-order parasequences in the investigated wells, and its upper boundary surface is of type 2.

Third-order Sequence B3

The upper part of the Tanuma Formation and the upper middle part of the Kometan Formation are included in sequence B3, which shows a great similarity to B2. It also exhibits the development of LST, TST and HST deposits. The lower and upper boundaries are of SB2 and SB1 type, respectively.

LST B3 deposits are manifested as calcareous shale with noticeable content of quartz grains in wells Qara Chauq-1 and Kirkuk-116.

TST B3 deposits are represented by a variable number of deepening upward, fining upward parasequences (one in Kirkuk-116 to seven in Makhmur-1) with deepening-up stacking patterns. TST B2 is bounded above by MFS B2.

HST B3 deposits are represented by two shallowing-up parasequences in the wells Qassab-12, Qaiyarah-55, Quwair-2, Khabaz-1 and Kirkuk-116, and one in the Makhmur-1, Kirkuk-117 and Kirkuk-175. In Bai Hassan-81 and Qara Chauq-1, HST B3 deposits are not well differentiated. The HST B3 deposits show an increase in hemipelagic and shallow-water derivatives. The lower boundary of this tract is an MFS recognised by a planktonic peak within the Kometan Formation as in Kirkuk-175 and within the Tanuma Formation as in Qaiyarah-55, and rhodolite-rich peloidal packstone in Quwair-2. Most of the well logs (GR and SP) show an abrupt cut at the top of this tract (this subject will be discussed later).

Second-order Sequence C

This sequence comprises the upper part of the studied formations; namely the Sa’adi, upper part of the Kometan, Mushorah and the lower part of Hartha formations (Figure 4). Its age extends from late Santonian to early Campanian (Table 5). Three third-order sequences (C1, C2 and C3) can be differentiated with progressively younger ages.

Third-order Sequence C1

A sea-level rise covered the whole study area resulting in the deposition of the upper part of the Kometan and the lower part of the Sa’adi formations. Sequence C1 is delineated by an SBl-type lower boundary and an SB2-type upper one. The lower boundary represents a regional unconformity between the Tanuma and Sa’adi formations evidenced by several palaeontological studies. The unconformity also occurs between the middle and upper parts of Kometan Formation, and hence it is interpreted as an SB1. The upper boundary represents continuous deposition of the upper part of the Kometan Formation in the deep sites of the basin, and shallowing and restricted exposure within the Sa’adi Formation. It is therefore interpreted as an SB2.

LST C1 deposits followed a prolonged withdrawal of the sea, which exposed most of the platform to subaerial exposure, as evidenced by a high rate of dissolution (moldic and vuggy porosity), which was occasionally filled by calcite cement. The LST C1 deposits were emplaced at the margin of the deep basin as a shallowing upward accumulation with possible progradational geometries as seen in Qara Chauq-1, Khabaz-1 and Kirkuk-116 (Figure 3). The occurrence of packstones, grainstones and rudstones with bioclasts, intraclasts and extraclasts, together with shale and quartz grains, possibly indicates a short swift transgressive event above a ravinement surface. The thickness of this interval is 8 m in Qara Chauq-1 and 3 m in both Khabaz-1 and Kirkuk-116.

TST C1 represents the return of pelagic sedimentation as a result of relative sea-level rise that continued the upward-deepening, fourth-order stacking pattern as in Kirkuk-175, Quwair-2, Khabaz-1, Bai Hassan-81, Qassab-12 and Qaiyarah-55. The first (deepening-up stacking pattern) represents the earlier phase of this tract, while the second (retrogradational stacking pattern) represents the late phase. TST C2 is bounded upwards by MFS C2 that was diagnosed by a planktonic peak (Figure 3).

HST C1 was deposited during the still-stand and the subsequent sea-level fall. It is represented by a stacking pattern of shallowing upward parasequences that accumulated in the middle-inner ramp as in Qassab-12 and Qaiyarah-55.

Third-order Sequence C2

This sequence developed during latest Santonian-earliest Campanian and is represented by the upper part of the Kometan and Sa’adi formations (Figure 4 and Table 5). The upper and lower boundaries are of SB2 type.

LST C2 is a “ramp-margin wedge systems tract” (i.e. ramp-margin lowstand, Emery and Meyers, 1996) formed during a sea-level lowstand. It includes gravity-flow deposits of shallow-water-derived bioclastic packstone-floatstone or mudstone-wackestone in the distal part. This tract is only distinguished in Qara Chauq-1 (5 m thick) and Quwair-2 (4 m thick) (Figure 3).

TST C2 is composed of variably stacked parasequences with an aggradational pattern, showing a deepening upward trend with decreasing grain size as in Qassab-12, Qaiyarah-55, Qara Chauq-1 and Khabaz-1. Its lower boundary appears either as an SB2 or transgressive surface, while planktonic-rich wackestones-packstones in the upper part represent MFS C2 in the wells Qara Chauq-1, Bai Hassan-81 and Kirkuk-116. A shaly mudstone microfacies in the wells Qassab-12, Qaiyarah-55, Makhmur-1 and Quwair-2 separates this TST from the overlying HST and corresponds to MFS C2.

HST C2 is characterised by numerous parasequences with a progradational stacking pattern in which shallow-water derivatives show a slight increase at the expense of the hemipelagic-pelagic counter-part. This is capped by an SB2-type surface, which separates the Sa’adi and Hartha formations (and Sa’adi and Mushorah), excluding Kirkuk-175 where this upper boundary is within the upper part of the Kometan Formation or as a shallowing upward unit.

Third-order Sequence C3

This sequence is developed in the upper part of all the studied formations (Figure 4). It is associated with the lower Hartha Formation facies in Qassab-12, Qaiyarah-55, Makhmur-1, or the Mushorah facies as in Qara Chauq-1, Bai Hassan-81, Khabaz-1, Quwair-2, Kirkuk-117, whereas in Kirkuk-175 it occupies the top of the Kometan Formation (Figure 3). An early Campanian age is assigned to sequence C3 (Table 5). The depositional style is that of a ramp geometry, which developed locally to a rimmed-shelf at the end of the early Campanian.

TST C3 consists of deepening-up parasequences in Quwair-2, Bai Hassan-81, Qara Chauq-1, Khabaz-1 and Qaiyarah-55, appearing as a deepening upward facies with a high shale fraction. The facies exhibit interbedding of pelagic and periplatform deposits containing bioclasts and extraclasts. This is capped by MFS C3 where planktonic foraminifera, calcispheres and organic-clay materials are concentrated.

HST C3 is comprised of fourth-order parasequences with a shallowing-up to static stacking pattern. It shows the greatest thickness in Bai Hassan-81 and Qara Chauq-1, which probably can be attributed to highstand shedding from a nearby “carbonate factory”. The carbonate deposits dominate the parasequences with wackestone-floatstone that is rich in planktonic-benthonic fauna and bioclasts of crinoids. Dolomite-enriched facies as in Qassab-12, Qaiyarah-55 and Khabaz-1, and periodic anoxia with increasing organic materials, appear in Quwair-2 and Khabaz-1. The upper contact is represented by an SB3-type boundary (drowning surface) as clearly distinguished in Bai Hassan-81 and Quwair-2, and accompanied by a one-metre-thick bed of rudstone-peloidal grainstone with glauconite enrichment.

STRUCTURAL SETTING AND EVOLUTION OF THE RAMP SYSTEMS

Besides eustatic sea-level changes, the present study also considers the important influences and complex interplay of structural movements and climate on the depositional system. In this context, we consider how subsidence/eustacy and sediment supply affect accommodation, drowning and anoxia. These factors can be combined to relate the bounding surfaces to their system tract, stacking patterns, geometry of the stratal units and component lithofacies within a chronostratigraphic framework. Similarly, all these factors combine to impart diagnostic lithologic, faunal and diagenetic attributes to depositional facies.

Extensional tectonism that is dominated by normal faulting results in subsidence and the development of the sedimentary basin and accommodation space. In contrast, compressional tectonism results in uplift disconformities that accompany reverse and thrust faulting or inversion (Burchette and Wright, 1992). Normal-listric faulting with a general NW-strike is a common feature of the Arabian Platform as a consequence of the Permian-Triassic opening of the Neo-Tethys Ocean along the Zagros Suture (Le Pichon and Sibuet, 1981; Numan, 1997, 2000). During the Late Cretaceous (early Turonian), the Neo-Tethys closure promoted syntectonic uplift of the northeastern rim of the Arabian Platform, but excluded the outer sector of the foreland basin (Numan, 2000; Figure 5, after Sharland et al., 2001).

The inferred distribution of the NW-trending (Zagros) normal faults in North Iraq (F1, F2 and F3) is shown in Figure 1. The activation of these faults resulted in the initiation of tilted half-grabens and perched/suspended basins (Santantonio, 1994). Furthermore, strike-slip faults (S1, S2 and S3) divided North Iraq into transverse blocks with a general NE-trend (Figures 1 and 6). The reactivation of the strike-slip faults during the Late Cretaceous was in response to the obduction of the Neo-Tethyan crust prior to the collision of the Arabian Plate with the Iranian and Turkish plates in Eurasia (Hancock and Atiya, 1979; McBride et al., 1980; Buday and Jasim, 1987; Numan, 2000; Figure 5 after Sharland et al., 2001). The tilting of the blocks through differential subsidence produced an inclined surface, which constituted the foundation of two depositional ramps as tilted half-grabens (Figure 7).

The resulting Turonian-Campanian depositional basin is interpreted in terms of two ramps delineated by three normal faults (F1, F2 and F3; Figures 1 and 7): the NE ramp (between F1 and F2) and the SW ramp (between F2 and F3). Subsidence in the hanging walls and uplift (and erosion) promoted a major change in basinal configuration; especially with the development of sub-basins containing contrasting thickness of the successions and their component lithofacies. The distribution and thickness of the Turonian-Campanian succession indicate that it blanketed an irregular topography over the hanging wall and footwall of the half-grabens.

The high-standing footwall would have been subjected to episodic uplift and erosion resulting in flattening and transition to a flat-topped ramp (Santantonio, 1994). During sea-level fall, the higher shoulder evolved into a wide shelf accumulating shallow-water facies. These deposits were later transported down-slope and redeposited as storm accumulations and gravity-flow deposits, and possibly slumps, or as gravity rock-falls adjacent to the deeper part of the ramp (Figure 8). In contrast, the flat-topped ramp, during relative sea-level rise, was inundated by deep-water, thus producing condensed pelagic deposits. The latter deposits may have been reworked and transported towards the outer deep ramp leaving a thin or eroded top.

During the final Miocene-Pliocene phase of the tectonic inversion, the outer-ramp deep-zone would have “popped-up” as an elevated anticline compared to the inner ramp zone, which would have been a low-amplitude swell or flat area. The area north of the Kirkuk structure is devoid of any swell-depression features, and as such it is allocated to the flattened part of the northeast ramp (Figure 7). Likewise, the region to the south-southeast of the Qara Chauq and Khabaz structures is located at the flat-topped SW Ramp, whereas the Makhmor and Qaiyarah areas were in perched basins (Figure 7). On the other hand, the Qara Chauq structure occurs in the deep outer part of the NE Ramp where the succession has a greater thickness. Multiple phase inversion along normal faults is reflected by the present-day higher elevation of the Qara Chauq and Makhul structures.

DROWNING AND ANOXIA

Differential and rapid subsidence of the carbonate platform can induce drowning (Schlager, 1981) as well as its demise. This is commonly reflected as a sharp contact of deep planktonic facies overlying shallow facies accompanied by increasing clayey and organic matters, or as a condensed section due to sediment starvation. A drowning event may straddle the entire basin in response to regional subsidence. In the studied succession a drowning event that was due to regional subsidence is manifested at the start of second-order sequence A. In contrast the drowning events associated with the start of sequence B and the end of sequence C are assigned to local differential subsidence.

Anoxia is associated with drowning. It is related to an episode of increased productivity and an expanded oxygen minimum zone that together promote the accumulation of organic-rich sediments. This episode takes place on local or ocean-wide scales. It can occur in either deep or shallow basins with restricted vertical circulation, or those experiencing drowning (Jenkyns, 1980; Legget et al., 1981; Kuhnt et al., 1986).

Anoxic-dysaerobic cases of local magnitude are recorded in the present study and diagnosed by enrichment in organic matter or facies with high abundance of Heterohelix sp., Inoceramus sp. and calcisphaerulids. These together endure oxygen deficiency during the TST in the outer-ramp zone and appear in the A1 and A2 sequences, and the lower part of sequences B1 and C1. However, transgressive marine pulses result in the development of maximum flooding surfaces (MFS) that produce planktonic foraminifera-rich facies.

TECTONO-STRATIGRAPHIC SYNTHESIS

In the mid-Turonian, the flexure of the northeastern edge of the platform triggered differential subsidence, which induced tilting of the block bounded by the F1 and F2 normal faults (Figures 5 to 7). This tilting initiated the northeastern ramp upon which the Gulneri succession was established following widespread drowning. This extensive ramp stretched from the Greater Zab River in the northwest (Figure 1), and southeast where it connected with the open sea. To the northeast it was linked to a Tethyan basin, and the area of the Qara Chauq field represented its southwestern limit.

Relaxation, throughout the late Turonian-early Coniacian, accomodated the deposition of sequence B1 (Khasib and correlative lower part of Kometan formations), with the suspended basins constrained in a zone near the Makhmur and Qaiyarah fields. In contrast, sequence B1 is absent in the southwest ramp (Qassab field), which indicates either non-deposition or erosion. Outside the study area, however, the distal deeper part (Makhul area) may be preserved in part.

Reactivation of strike-slip movement along older faults (Buday and Jassim, 1987; Numan 1997, 2000) (S1 to S3, Figures 1 and 6) caused local basin uplift, which together with the simultaneous fall of sea level ended the deposition of sequence B1. In this regard, Chatton and Hart (1961) interpreted these tectonic events as vertical epeirogenic movements (intra-Coniacian) that caused a regional unconformity or shallowing of the basin. The thickness variation and missing parasequences in the B2 and B3 sequences may reflect local subsidence or uplift, where either syn-rift deposits became exposed or non-deposition occurred.

The deposition of the B3 sequence ended in the early Santonian following a eustatic sea-level fall. The subsequent period of tectonic relaxation preceeded the deposition of the upper part of the Kometan and Sa’adi formations (which are included in the C1 and C2 sequences). This relatively quiescent episode was contemporaneous with differential subsidence, which produced dysaerobic-anoxic conditions in some parts of the basin.

The boundary between the C2 and C3 sequences separates the Santonian from the Campanian strata, and records the start of deposition of the Mushorah and the lower part of the Hartha formations. At that time a global compressional episode (Santonian event; Guiraud and Bosworth, 1997; Bosworth et al., 1999) was active and may have modified the configuration of the basin. This event created local topographic irregularities wherein uplifted inner-ramp region received Hartha facies. The generated slope provided a conduit through which the shallow-water derivatives accumulated as Mushorah facies. This means that the deposition of the Hartha Formation had started in the early Campanian and acted as a “carbonate factory” from which Mushorah deposits were derived. Meanwhile the upper part of the Kometan succession in the outer ramp remained without any facies change.

The superposition of these events in the basin is reflected by the change in the depositional topography from ramp to shelf. As the inner ramp became uplifted, it mimicked a shelf environment while the middle ramp acquired a slope configuration capped by Mushorah facies. During the late Campanian, the Shiranish sediments accumulated in grabens during extension, whereas the upper part of the Hartha Formation was deposited on horsts.

DISCUSSION

A1 and A2 Sequences: Gulneri Formation

The A1 and A2 sequences are attributed to the mid-Turonian Gulneri Formation. Several publications have described the sedimentary environment of the Gulneri strata as basinal (e.g. van Bellen et al., 1959; Buday, 1980). Hammudi (1995), based on the relative abundance of planktonic-benthic ratio of foraminifera, interpreted the depositional setting of the Gulneri Formation as upper-middle slope.

The present model postulates that a widespread mid-Turonian marine transgression drowned the NE Ramp, as consistent with the highest global sea level (Haq et al., 1988). The transgression caused the “carbonate factory” to cease production and thus decreased the sedimentation rate, while accommodation space increased; these factors would explain the reduced thickness of the Gulneri Formation. The microfacies, fossil assemblage and spatial relationship in the Gulneri Formation indicate that it was deposited over the NE Ramp, precluding the Quwair–2 well, which represents the toe of the slope (Figure 7). Evidence for the toe-of-slope is based on the facies association that is dominated by shallow-water detritus, mainly peloidal gainstone/packestone, enriched by red algae rhodolites and some basinal facies rich in planktons.

The facies associations and faunal assemblages range from inner to outer ramp and are associated with periplatform hemipelagic facies. The deepest outer-ramp setting is characterised by the enrichment and assortment of planktonic forams (Heterohelix sp., Globotruncana sp., Dicarinella sp., Marginotruncana sp., Helveteglobotruncana sp., Whiteinella sp., Globigerinelloides and Calcisphaerulids) together with benthic foraminifera (Textularia sp., rotalids, etc.) and bioclasts, which are also found at the sites of Khabaz-1, Qara Chauq-1 and Bai Hassan-81. Flooding surfaces as indicated by the high diversity of planktonic foraminifers (planktonic peak), and their equivalent condensed sections appeared in the A2 sequence in Kirkuk-175. Anoxic-dysaerobic periods left their imprints as evident from the restriction of foraminifers species to Heterohelix sp. (Heterohelicidal wackestone, W3) accompanied by calcispheres and Inoceramus sp. (W/P2 and W/P3 submicrofacies, respectively).

The shallow-water bioclasts (echinoids, Inoceramus sp., rotalids and red algae), abundant in some facies, can be described as storm deposits (tempestites), gravity-flow deposits or slope-apron deposits (Coniglio and Dix, 1992), as in Quwair-2, Kirkuk-116 and Kirkuk-175. These bioclasts were reworked from the flat-top of the ramp. In contrast, sequence A2 at Quwair-2 is characterised by peloidal grainstone/packstone dominated by red algae rhodolites (G3 submicrofacies), while at Kirkuk-175 and Bai Hassan-81 it contained extraclasts and composite rock fragments recording the deposition as base-of-slope apron adjacent to a highly inclined slope. The well Bai Hassan-81 is deflected by the movement along strike-slip faults that follow the Lesser Zab River. The same model is proposed for the Quwair-2 area, which may also represent the base-of-slope apron.

B1 Sequence: Khasib and Lower Kometan Formations

Sequence B1 includes the Khasib Formation or the lower part of the Kometan Formation. The late Turonian to early Coniacian sea-level rise matches the age of this sequence and followed the lowest level of sea attained in the Cretaceous (Haq et al., 1988; Haq and Al-Qahtani, 2005). The sea-level drop preceding the deposition of the B1 sequence led to an increase in siliciclastic input and subdued carbonate production under restricted conditions.

All previous investigation concluded that Khasib sediments accumulated within inner, middle and outer-ramp settings (Buday, 1980; North Oil Company, 1990a, b; Hammudi, 1995). The present study holds this view, but adds that a cold climate prevailed during the deposition of the basal part, which later changed to tropical conditions as pointed out by Beydoun (1991). Sequence B1 is manifested in the NE Ramp (Kirkuk-116 and Kirkuk-117, Quwair-2, Bai Hassan-81, Khabaz-1 and Qara Chauq-1), and shallower SW Ramp (Makhmur-1, Qaiyarah-55 and Qassab-12); it is missing in Qassab-12, which occupied the erosion-prone, flat-top zone of the SW Ramp (Figure 7).

In the present model the B1 sequence marks the second drowning event that extended further west than the A1 drowning event to include Makhmur-1 and Qaiyarah-55 (Hammudi, 1995). It is envisaged that the Makhmur and Qaiyarah structures were located in suspended basins (Figure 5). They document the weaker influence of faulting parallel to F2 at the northeast edge of the SW Ramp facing the deep-zone at Qara Chauq; the remainder of the ramp was a high area. Tectonically driven subsidence and uplift is manifested by an increase in accommodation space and sediment accumulation in the basin on the southwest side of the SW Ramp. The swells or highs on the northeast side of the SW Ramp suffered from erosion so as to cause the omission of sequence B in the Chemchemal area (at the flattened top) in the area to the north and north-east of Kirkuk structure.

The high frequency of bioclastic facies (storm deposits), upwards in the succession, indicate progradation toward the sea due to highstand shedding. The progradation simultaneously restricted the siliciclastic influx in a proximal direction, thus re-establishing a greater thickness in the HST versus TST in Qaiyarah-55 and Makhmur-1 (Figure 3). In Makhmur-1, the facies are determined by low-diversity faunal content of the chalky mudstone (M1 submicrofacies) owing to deposition under relatively restricted conditions similar to those of perched basin.

Several dysaerobic-anoxic intervals occur in the Khasib facies as in Qaiyarah-55 at depth 997–999 m and in Makhmur-1 at 1,182–1,186 m, and are indicated by the impoverishment of foraminifera (dwarfed-variety) and the enrichment in organic matter and calcispheres. This is assumed to be related to a marked oxygen-minimum zone, probably coinciding with flooding by oxygen-depleted water.

The lower part of the Kometan Formation is correlated to the Khasib Formation, and their setting was interpreted as basinal (van Bellen et al., 1959; Youkhana, 1976; Buday, 1980; North Oil Company, 1988, 1990, 1992a, b). Hammudi (1995) assigned the lower part of the Kometan Formation to an upper-middle slope environment. In the present study, a middle-outer ramp setting is postulated (facies W2, W/P1, P3 and M3) and particularly in the NE Ramp. In this context, Qara Chauq field coincides with the deeper outer-ramp zone, while the Kirkuk area is close to the flat-topped sector. In contrast, Khabaz-1 and Bai Hassan-81 are viewed as being the result of local subsidence caused by minor faults. The northern terminus is bounded by a steep slope (located on bifurcated fault) whose (P2, G3 submicrofacies) base-of-slope apron sediments accumulated as in Quwair-2 area. Reworked bioclasts included echinoids and inoceramids, as well as algae and rare rudist or bryozoa. This sequence was encountered in cores in wells Kirkuk-116 and 117 and spot cores in Bai Hassan-81 and Qara Chauq-1.

B2 and B3 Sequences: Tanuma and Middle Kometan Formations

The B2 and B3 sequences encompass the Tanuma Formation and its correlative middle part of the Kometan Formation. Their depositional environment, as deduced from facies association, shows that the two sequences were deposited in a continuous manner as part of the second-order B sequence.

Most of the previous studies of the Tanuma Formation have precluded the present study area. The majority have interpreted central south Iraq to occupy the inner-middle shelf (Buday, 1980; Al-Hamdani, 1986). Hammudi (1995) extended the environment to include restricted lagoon and outershelf settings.

The depositional environments of the B2 and B3 sequences also conforms to the two flat-topped NE and SW ramps. Facies associations (M3, Sh and W1-W3) that accompanied sequences B2 and B3 in Qassab-12 infer a transition from an inner-ramp setting with tidal flat or peritidal facies inner ramp, to a middle-ramp realm. The transition followed a sea-level rise as implied by facies W2, W3 and W/P2 bearing Heterohelix sp. and enriched with calcispheres resting on the southwest flat-topped high. A similar interpretation is adopted for the coeval inner shelf (Qaiyarah-55 and Makhmur-1), which apparently remained undisturbed during the accumulation of the basal part of sequence B2. Whereas the top of sequence B3 shows an increase in facies G2, W1 and F related to gravity-flow deposition, possibly in a suspended basin. The sporadic occurrence of W3 and M3 facies is considered as an indication of dysaerobic sediments enriched with organic matter.

The middle part of the Kometan Formation has been interpreted as recording a depositional setting similar to that of the Tanuma Formation. In the present model, Kirkuk field defined the middle ramp and Qara Chauq field the outer ramp. Bai Hassan and Khabaz fields lay in between the middle and outer ramps, and Quwair-2 well acquired characteristics of base-of-slope apron. The intercalation of facies W2 (in Khabaz-1 and Bai Hassan-81) adherent to G1 and P3 (in Kirkuk-175 and Kirkuk-116) favour their accumulation as reworked gravity-flow deposits. The occurrence of W3 with Heterohelix sp. in Khabaz-1, Bai Hassan-81 and W/P2-rich in Inoceramus sp. in Qassab-12, Khabaz-1 and Bai Hassan-81 document episodic dysaerobic conditions.

Sequence stratigraphic analysis of the Tanuma and correlative middle part of the Kometan formations indicates that their thickness varies laterally because the corresponding third-order B2 and B3 sequences consist of a variable numbers of higher-order parasequences. This variability (i.e. “parasequences skipping”) is due to several circumstances:

  • Erosion of certain palaeohighs during exposure that results in the omission of parasequences from the third-order HST.

  • Parasequences with similar carbonate lithology that lack shale intercalations become amalgamated, and cannot be recognised on logs.

  • Facies dislocation down-dip, along steep slopes, causes their disappearance from shallow depositional sites and results in incomplete depositional sequences.

  • Differential subsidence and drowning result in condensed sections, with a thickness of only a few centimetres; these cannot be sampled nor detected by logs.

  • Inability to distinguish between fourth and fifth-order fluctuations because the sedimentation rate is variable in different areas; in such cases the gamma-ray log is the only means of differentiation.

Parasequence-skipping results in pronounced thickness variation of the succession. This is related to differential subsidence, varying sedimentation rate and erosion.

C1 and C2 Sequences: Sa’adi and Upper Kometan Formation

The C1 and C2 sequences incorporate the Sa’adi and correlative upper part of the Kometan formations. Drowning was restored at the beginning of the deposition of the second-order C sequence during the late Santonian as a consequence of tectonic subsidence. All previous studies stated that the deposition of the Sa’adi Formation occurred in the middle to outer shelf, which extends to upper slope. In this study the depositional setting is similar to that of the previous sequences and characterised by the two flat-topped NE and SW ramps.

Facies associations of the Sa’adi Formation point to their deposition in middle to inner ramp. This is indicated by the TST facies of sequence C1, which included M1, M3 and Sh in Qassab-12 where W1 with planktonic foraminifers and Calcisphaerulids occur. Furthermore, intercalation of facies F and R indicate signs of storm or gravity-flow deposits. HST C1 deposits at Qassab-12 show a pronounced increase in thickness in comparison with TST C1 deposits. This can be attributed to high stand shedding caused by higher production in the “carbonate factory”. A similar situation is noted in the C2 sequence in Qassab-12 and Qaiyarah-55. In contrast, the reverse relationship is documented in Makhmur-1, which points to episodes of immersion followed by erosion that separate C2 and C3. The C2 sequence is similar to C1 except that deposition occurred in a slightly calmer environment with a local dysaerobic event in the HST with an increase in dolomite content at Qassab-12.

The upper part of Kometan Formation is allocated to basinal environments (North Oil Company, 1988, 1990c, 1992a, b), or an upper-middle slope setting (Hammudi, 1995). The present model indicates the upper part of the Kometan Formation accumulated in a middle-outer ramp zone. This is typified in Qara Chauq-1 and Kirkuk-116 within the LST gravity-flow deposits, particularly in the facies G2, W1, W2, F and W/P3 in Qara Chauq-1, whereas W1 and P2 mostly indicate a peritidal environment.

TST deposits in the C1 and C2 sequences are dominated by basinal facies (middle-outer ramp), which include M1 to M2, in addition to P3 and G1. The HST deposits of C1 and C2 show a resemblance to the TST deposits but with a relative increase in facies W1, W/P3, G2 and G3, particularly in Quwair-2. On the other hand, the silicified facies (SL) in the TST deposits in the C1 and C2 sequences at Bai Hassan-81 and Qara Chauq-1 may reflect nutrient-rich upwelling sea water, and the emplacement of neoformed silica in bioturbated deposits.

The thickness of the HST of the C1 and C2 sequences shows a pronounced increase versus those of the TSTs in Qassab-12 and Kirkuk-175. This may be due to the higher production of carbonates during the HST deposition. Alternatively, Qassab-12 and Kirkuk-175 could have been in a shoreface position during the TST, which experienced reworking and sub-elevation (erosion) by waves and currents, and eventually resulted in a reduction in the thickness. Similarly, the HST C2 deposits exhibit a similar increase in Qassab-12 and Qaiyarah-55. In contrast, the TST C2 deposits show an obvious increase in thickness when compared with the HST deposits in Khabaz-1 and Makhmur-1 and a less pronounced increase in Kirkuk-175. The detailed study of facies and their association suggests that this may be due to the omission of a greater part of the HST deposits during the next transgression.

C3 Sequence: Mushorah and Lower Hartha Formations

The C3 sequence constitutes the uppermost deposits of the succession and embraces the Mushorah and correlative lower part of Hartha formations. In general, the Mushorah Formation is correlated to the top hemipelagic part of the Kometan Formation. The latter, in particular, has received special attention because of its stratigraphic position and reservoir importance. Some investigators view the Kometan and Mushorah as facies in one formation (Ditmar et al., 1971; Buday, 1980; Hammudi, 1995), while others prefer their separation with the Mushorah being considered as an independent lithologic unit (Dunnington, 1953; Hart, 1959, 1960; Chatton and Hart, 1961; North Oil Company, 1988, 1989, 1990c, 1992a, b).

The present model conforms with the formational status of the Mushorah whose depositional site is assigned to a slope environment (as a result of increase in the slope of depositional substructure). Petrographic data proved the existence of proper Mushorah facies in Kirkuk-116, Bai Hassan-81, Khabaz-1 and Qara Chauq-1, while Kirkuk-117 and Quwair-2 showed only an intercalation of Mushorah facies. With respect to Kirkuk-175, it appears that the deposits of the C3 sequence are an extension of the upper Kometan facies and are defined as basinal equivalent to the Mushorah Formation. In the westward direction (Qassab-12, Qaiyarah-55 and Makhmur-1) the C3 deposits comprise a part of Hartha Formation.

Facies association in the Mushorah Formation are represented by W2 microfacies with planktonic foraminifera and bioclasts. The facies associations M1, W/P2 enriched with calcispheres, P1-crinoidal bioclasts and P3 dominated by planktonic foraminifera are considered as base-of-slope apron deposits (Figure 9) juxtaposed with basinal deposits. In Kirkuk-117, the Mushorah facies are subdued due to abundant Kometan facies punctuating the succession. The inverse relationship appears in Quwair-2 where shallow-water facies F, W2, G2 and R constitute the majority of the uppermost parts of the TST and HST parasequences. Several levels, rich in glauconite, are encountered as reworked grains from shallower zones including their prolonged contact with oxygenated sea water. This is manifested by the inclusion of glauconite within the base-of-slope apron facies (in Quwair-2) derived from the erosion of north and northeast palaeohighs (ramp top) encrusted with a hardground. In this context, the basinal facies in Kirkuk-117 point to condensed intervals in pelagic carbonate platforms (Figure 7).

The TST C3 deposits in Quwair-2, Kirkuk-116, Kirkuk-117 and Kirkuk-175 consist of two recognisable parts that are associated with a relative sea-level rise driven by subsidence.

Regarding the Hartha Formation, with its great reservoir potential, its deposition started in Campanian, and together with the Maastrichtian Tayarat Formation, it is considered as a shallow-water equivalent of the deep-water Shiranish Formation (van Bellen et al., 1959; North Oil Company, 1990a, b). The lower boundary of the Hartha Formation remains undated and uncertain due to insufficient biostratigraphic studies.

Tentatively, the present study retains early Campanian age for the depositional start of the Hartha Formation. This age interpretation is consistent with the presence of a “carbonate factory” that exported bioclasts facies of the correlative Mushora Formation (W2/P1). The lower part of Hartha Formation is characterised by its carbonate mud-supported texture, which is at variance with the grain-supported upper part. In addition, the lower part contains intercalations of W1, P2, G2 and F microfacies, which defined storm and debris flow deposits. The mud-supported lower part exhibits a spectrum of facies, such as M1 of the HST in Qaiyarah-55 and Makhmur-1, M3 of the TST in Qassab-12 and in both systems tracts in Qaiyarah-55. In general, facies W1 with bioclasts and benthic forams is the principal facies in all wells and in both systems tract and with which W2 and W4 are locally associated. Moreover, all wackestone and mudstone facies contain variable amounts of floating dolomite, which in Qassab-12 and Qaiyarah-55 exceeds 50%.

CONCLUSIONS

The mid-Turonian-early Campanian succession in northeast Iraq is made of three second-order sequences (A, B and C) separated by sequence boundaries (SB) of type 1 and 3. These sequences are subdivided into eight third-order sequences, which in turn consist of numerous parasequences. Second-order sequence A with its two third-order sequences (A1 and A2 separated by SB2) is represented by the Gulneri Formation. Sequence B is comprised of three third-order sequences (B1, B2 and B3) and corresponds to the Khasib and Tanuma formations, which are correlated to the lower and middle parts of Kometan Formation. Three third-order sequences (C1, C2 and C3) are interpreted in sequence C, which incorporates the Sa’adi Formation and the basal part of the Hartha Formation and their correlative upper part of Kometan and Mushorah formations.

Each third-order sequence is subdivided into a TST and HST; whereas the B2, B3, C1 and C2 sequences also involved an LST and the constrained development of a ramp margin wedge systems tract. The TST is characterised by deepening/fining upward parasequences implying retrogradational or/and aggradational stacking pattern. The HST is marked by shallowing/coarsening upward implying progradational/aggradational stacking patterns. The variable number of parasequences encountered is is due to erosion/reworking from palaeohighs or their incorporation in condensed sections. Drowning manifests itself as SB3-type sequence boundaries at the bottom of sequence A and the top of sequence C, reflecting differential subsidence. The latter event was accompanied by the development of anoxic/dysaerobic episodes as indicated by organic-matter enrichment and abundant Heterohelix sp., Inoceramus sp. and calcispheres.

The depositional setting included two flat-topped ramps established on fault-bounded blocks, which experienced differential subsidence along normal faults. The ramp changed to base-of-slope apron as in Quwair-2. At the end of the Santonian, the inner zone in the NE Ramp probably changed to a slope along which the Mushorah sediments accumulated, and the Hartha deposition started on the shelf. The Gulneri and Kometan formations were deposited in middle-outer ramp settings with shallow-water intercalations as storm or gravity-flow deposits, while in Quwair-2 they appear as base-of-slope apron deposits. The Khasib (B1), Tanuma (B2 and B3) and Sa’adi (C1 and C2) are interpreted as deposits in middle-inner ramp settings extending from hemipelagic to peritidal setting. Sequence C3 includes the Mushorah (slope apron) and Hartha (inner-middle ramp) formations, whose reworking from shallow sites sourced the bioclasts associated with the facies of the Mushorah Formation.

The sequence stratigraphic analysis that is presented in this paper provides a new mid-Turonian-early Campanian framework that can be used for more detailed reservoir characterisation and exploration in North Iraq. The framework will, in the future, be extended to other regions in Iraq and the Arabian Peninsula. The second and third-order sequences and maximum flooding surfaces provide a reliable chronostratigraphic system that can be used for regional correlations. The structural evolution of North Iraq suggests that a complex system of normal and strike-slip faults was active in the Late Cretaceous times and that inversion took place in the Oligocene and Miocene. This structural complexity, taken together with depositional and diagenetic aspects of the succession, may offer opportunities for the development of stratigraphic traps.

ACKNOWLEDGEMENTS

The authors thank the Ministry of Oil of Iraq and the North Oil Company in Kirkuk for their support and permission to publish this paper. They thank Kevin Kveton, Anthony (Tony) Lomando and Chevron for the technical and financial support that made this publication possible. The comments by two anonymous reviewers were very helpful in improving the manuscript and are very much appreciated. Moujahed Al-Husseini and Gulf PetroLink are thanked for editing the paper and designing the graphics.

ABOUT THE AUTHORS

Sabah Noori Saleem Haddad is a Chief Geologist at the Department of Geology at North Oil Company (NOC) in Kirkuk, Iraq. He received his BSc in Geology from Mosul University (1976), and MSc in Geochemistry and Mineralogy of Carbonate Rocks from Mosul University (1980), and PhD in Carbonate Sequence Stratigraphy from Mosul University (2004). He joined NOC in 1983, and worked as a Wellsite Geologist for about three years, then he worked as Sedimentologist in the research section in the same department for about ten years, he became the Head of the research section in 1996. In 1999 he joined Mosul University for his PhD project. From 2002 till now he became the Head of well programming and data bank section. His main subject of interest is the carbonate sedimentology and carbonate sequence stratigraphy.

sabah_n_saleem@yahoo.com

Momtaz Ahmed Amin is a Professor of Sedimentology in the Department of Geology in Mosul University, Iraq. Momtaz received his MSc in Carbonate Sedimentology from the University of Newcastle upon Tyne, England in 1975 and received a PhD in Sedimentary Geochemistry from Sheffield University, England, in 1979. Since then he was a lecturer in sedimentology, clay mineralogy and geochemistry. His research focuses on carbonate-evaporite facies association and their diagenetic alterations; transition of carbonate – siliciclastic sequences; geochemical differentiation and clay mineralogy of marine-non marine sediments. He has supervised many PhD and MSc students in his areas of expertise and participated in various geological projects.