ABSTRACT

The Paleozoic section became prospective during the early 1970s when the enormous gas reserves in the Permian Khuff reservoirs were delineated in the Gulf and Zagros regions, and oil was discovered in Oman. Since then, frontier exploration has targeted the Paleozoic System throughout the Middle East, driven by various economic considerations. The Paleozoic sequences were essentially deposited in continental to deep marine clastic environments at the Gondwana continental margin. Carbonates only became dominant in the Late Permian. The sediments were deposited in arid to glacial settings, reflecting the drift of the region from equatorial to high southern latitudes and back. Following late Precambrian rifting that formed salt basins in Oman and the Arabian Gulf region, the Cambrian-Devonian sequences were deposited on a peneplained continental platform. The entire region was affected by the Hercynian Orogeny, which climaxed during the Carboniferous. The orogeny manifested itself in a change in basin geometry, inversion tectonics, regional uplift and tectonism along the Zagros fault zone. This deformation caused widespread erosion of the Devonian-Carboniferous and older sections, and was probably caused by collision along the northern margin of Gondwana. The Paleozoic tectonic super cycle ended with the onset of break-up tectonics in the Permian, and the deposition of Khuff carbonates over the newly formed eastern passive margin. A major Paleozoic petroleum system embraces reservoir seal pairs spanning the Silurian to Permian sequences. Hydrocarbons occur in a variety of traps, and are sourced by the Silurian ‘hot shale’. A second petroleum system occurs in areas charged from upper Precambrian source rocks in the salt basins. Hydrocarbon expulsion estimates, taking into account secondary migration losses, suggest that some one trillion barrels of oil equivalent (BOE) may have been trapped from the Silurian ‘hot shale’ alone. However, the long and complex hydrocarbon geological evolution of the basin, combined with low acoustic contrasts between target rock units, difficult surface conditions, tight reservoirs, and deep subsurface environments, posed significant challenges to exploration and development. The critical success factor is the continuous innovative effort of earth scientists and subsurface engineers to find integrated technology solutions, that will render the Paleozoic plays economically viable.

INTRODUCTION

The Middle East holds estimated proven reserves of some 625 billion barrels (bbls) of crude and 1,720 trillion cubic feet (TCF) of natural gas, nearly 64% and 34% of the world’s reserves, respectively (IHS Energy Group, 2001). These reserves were discovered in Mesozoic and Tertiary reservoirs in a NW-trending zone from Oman to Turkey (Figure 1), starting with the discovery of the Masjid-e-Suleyman field in Iran on the 26th of May, 1908. Crude and condensate production from these reservoirs was reported to be nearly 8.0 billion barrels/year in 2000.

The Paleozoic section only became prospective during the early 1970s when the enormous Permian Khuff gas reserves were delineated in the Gulf region and Zagros Mountains, and oil was discovered in Oman. Since then, frontier exploration has targeted the Paleozoic System throughout the Middle East for six reasons.

  • 1) Non-associated gas is required to meet the growing domestic demand for electrical power, desalination and to fuel petrochemical plants.

  • 2) Gas and condensates, unlike crude oil production, are not regulated by OPEC quotas thereby providing incentives for substituting gas for oil.

  • 3) Exploration is required to replace produced reserves in order to guarantee future income and maintain OPEC production quotas.

  • 4) Exploration is needed to replace lower value crude reserves with better quality crudes.

  • 5) Some of the producing fields have reached a high level of maturity, and revitalizing producing fields is often more cost beneficial than developing remote resources. Enhanced recovery programs may therefore stimulate the search for cheap local gas to optimize ultimate recovery.

  • 6) The growing market demand continues to offer an incentive to direct frontier exploration towards increasingly more complex geological settings and, during the last decade in the Middle East, towards deeper Paleozoic targets.

In this paper we describe the Paleozoic frontier exploration opportunities of the Middle East. At present, these sequences are only lightly explored, except in Oman and central Saudi Arabia. Therefore, the discussion of the basin evolution and the hydrocarbon habitat of the Paleozoic sequences at the scale of the Arabian Plate and interior Iran remains speculative.

Throughout this paper all ages are based on the most recent geological timescale of Gradstein and Ogg (1996) for the Paleozoic, and Harland et al. (1990) for the Precambrian. Also for ease of reference between this paper and the sequence stratigraphic study by Sharland et al. (2001) we have placed in square brackets equivalent surfaces with dates referred to in their publication. These consist of their interpreted Maximum Flooding Surfaces (MFS) identified by Period: for example [Silurian MFS S10 dated at 440 Ma]; and Arabian Plate (AP) Tectonostratigraphic Megasequences: for example [TMS AP3].

REGIONAL SETTING

Main Tectonic Elements

The boundaries of the present-day Arabian Plate embrace all types of tectonic regimes (Figure 1). They include rifting and sea-floor spreading in the Red Sea and Gulf of Aden, collision along the Zagros and Bitlis sutures, subduction along the Makran zone, and transform movement along the Dead Sea and Owen-Sheba fault zones. The Makran and Zagros convergence zones separate the Arabian Plate from the microplates of interior Iran.

The Arabian Plate basin is asymmetric (Figure 2). To the west it is bounded by the exposed Precambrian Arabian Shield that was uplifted in the Late Oligocene by the Red Sea and Gulf of Aden rift system. The Precambrian basement is also exposed locally along the Arabian Sea and in interior Iran. The shallow basement along the Arabian Sea reflects episodes of uplift associated with the break-away and drift of the Indian Plate.

The basin deepens gently in an easterly direction with maximum depth reached in a foredeep setting in front of the Zagros collision zone (Figure 2). No obvious foredeep is developed along the northern plate boundary, reflecting the escape tectonics of the Anatolian Plate (Turkey). Northward, the basin is accentuated by the intraplate Palmyra and Sinjar troughs. The Aleppo and Mardin highs form stable blocks between this intraplate deformation zone and the collision zone in Turkey.

The Precambrian Arabian Shield consists of accreted island-arc and microcontinental terranes (Stoeser and Camp, 1985; Brown et al., 1989), overlain by post-cratonic sediments and volcanics. The final Precambrian Amar collision (about 640–620 Ma, Brown et al., 1989) fused together the Arabian Plate along the N-trending Amar suture that bisects the Arabian Peninsula at about E45° (Al-Husseini, 2000).

The main structural elements in the Arabian Platform indicate the existence of a number of inherited mechanically weak trends. These are defined by: (1) N-trending highs as exemplified by the En Nala (Ghawar) anticline and the Qatar Arch; (2) NW-trending systems like the Azraq (Wadi Sirhan and Jauf) and Ma’rib grabens of Mesozoic age; and (3) NE-trending systems like the south Syria Platform, Khleissia and Mosul trends. These trends are expressed in the basement structural map (Figure 2), and suggest that rejuvenation of mechanical basement discontinuities played an important role in the evolution of the basin.

Earlier work suggests that during most of the Paleozoic, the microplates of interior Iran, Anatolia (Turkey), together with the Arabian Plate formed part of the continental margin of Gondwana (Beydoun, 1993); however, geological information indicates that interior Iran started to follow its own tectono-magmatic evolution separate from Gondwana at least since the Early Silurian (Konert et al., in press).

Paleo-Plate Positions

During late Precambrian (about 600 Ma) the Arabian Plate was located close to the Equator (Figure 3), and had an E-W orientation with Iran in the north. During the early Paleozoic, the plate moved to southern latitudes, and rotated anti-clockwise. By the latest Ordovician (about 445 Ma) the plate reached its maximum low-latitude position (about 55° south) and a major polar glacial pulse covered western Arabia (McClure, 1978; Vaslet, 1990).

During the Silurian to Late Carboniferous the plate underwent a major clockwise rotation of about 100° without significant latitudinal translations. As a result of the rotation, Oman was located on the southern edge of the Plate. By the Late Carboniferous (about 305 Ma), a second glacial phase affected Oman, Yemen and southwest Arabia. During the Permian the Plate moved rapidly to the north.

STRATIGRAPHY AND BASIN EVOLUTION

The Paleozoic stratigraphy of the Arabian Peninsula stems mostly from outcrop studies along the margins of the Arabian Shield and in south Oman, and from wells drilled generally over structural highs. In interior Iran the available data is essentially derived from surface outcrops that were described lithostratigraphically before the 1980s. Large areas, especially in the deeper parts of the basin, contain only sparse or no well data. Also most of the basin is covered by older vintages of seismic data that do not adequately image the Paleozoic section. All of these considerations limit our understanding of the Paleozoic sequences.

Figure 4 illustrates the generalized late Precambrian and Paleozoic chronostratigraphic framework of the Middle East. This paper does not cover the tectonic and stratigraphic evolution of the plate during the late Precambrian and Early Cambrian Najd Rift phase [AP1 from 610 to 520 Ma]. During this period rift basins formed in the Arabian Gulf, the western region of the Arabian Peninsula, and Oman, where thick salt deposits are found (Hormuz Formation of Iran, and Ara Group of Oman). Al-Husseini (2000) provides a recent review of this early stage of Plate evolution.

Early Cambrian

Following the extensional Najd Rift phase, massive post-rift, continental clastics of late Early Cambrian age were deposited over most of the plate (Figure 4). These were sourced from interior Gondwana to the south and west, and are bounded below by a regional unconformity that represents a stable platform [base of AP2 at 520 Ma].

In Jordan, the post-rift Lower Cambrian Salib Formation (Powell, 1989) consists of continental clastics that were deposited in a system of alluvial fans grading into braid plains and braid deltas. South of Jordan and north of the Arabian Shield, in the Tabuk outcrops of Saudi Arabia, the Lower Cambrian Siq Sandstone unconformably overlies the irregular surface (pre-Siq unconformity) of the Precambrian basement. It consists of a basal alluvial conglomeratic sandstone overlain by a mixed sand-flat-eolian sandstone complex (D. Janjou, P. Razin, M. Halawani, and W. Roberts, written comm., 2000). Lateral time equivalents in the south may include the Nimr Group in Oman, which consists of alluvial fan deposits, in addition to playa lake deposits.

In southwest Iran, the post-rift Zaigun and Lalun formations appear, on the basis of sedimentary structures such as cross-bedding, to be non-marine. These lack age-diagnostic fossils (R. Jones, written comm., 2000); however, the formal Lalun Formation of northeast Iran, has been dated as Early Cambrian.

In northern plate areas, in Syria and southeast Turkey, the Zabuk and Sadan formations correspond to the basal post-rift continental clastics.

Late Early and Middle Cambrian

By the late Early Cambrian, siliciclastic tidal flats were established in marginal settings, which grade basinwards into low-energy carbonate and mixed clastic and carbonate tidal flats followed by subtidal carbonates (Figure 5).

During the Middle Cambrian, a vast shallow carbonate platform covered most of northern Arabia and interior Iran: Burj Formation in Jordan (Amireh et al., 1994), Saudi Arabia and Syria, Koruk Formation in southeast Turkey, Mila Formation of northeast Iran; and ‘Cambrian Dolomite’ of southwest Iran (R. Jones, written comm., 2000). Sharland et al. (2001) equate their MFS Cm20 (510 Ma) with the Burj in Jordan.

Along the basin margin, however, during the remainder of the Middle Cambrian, deposition returned to proximal alluvial fan deposits, interrupted by subordinate marginal marine conditions. In the outcrops of southwest Jordan, for example, above the Burj carbonates, the Umm Ishrin and Disi formations consist of a coarsening-upward succession of alluvial-fluvial deposits interrupted by subordinate marginal marine clastics, which correlate with the Cambrian Risha Member of the Saq Formation of Saudi Arabia (Vaslet, 1990).

In the Tabuk outcrops of northwest Saudi Arabia the Burj is absent. In the subsurface of northwest Saudi Arabia, however, and in Khursaniyah-81 well in eastern Saudi Arabia, the Lower Cambrian Siq is overlain by the Burj Dolomite (Figure 4c). An MFS is identified within its upper part and is characterized by an Early to Middle Cambrian acritarch assemblage (Al-Hajri and Owens, 2000) [MFS Cm20 dated at 510 Ma].

In the salt basin province of south Oman, the Angudan unconformity separates the Cambrian-Silurian Mahatta Humaid Group from the underlying syn-rift Precambrian-Lower Cambrian Huqf Supergroup and Nimir Group (Droste, 1997). This unconformity may mark the onset of subsidence driven by thermal relaxation. The oldest Amin Formation of the Mahatta Humaid Group consists of variably sorted, arkosic sandstones, conglomerates, and subordinate shales that were deposited in a system of alluvial fans grading into aeolian-influenced braid plains and braid deltas. The coincidence of these sediments with the underlying Ara Salt basins indicates that accommodation space was generated by halokinesis (Loosveld et al., 1996; Droste, 1997).

The Miqrat Formation in central Oman, and the Mahwis Formation in south Oman were also deposited in a continental setting during the Middle Cambrian (Droste, 1997). All relict topography appears to have been leveled by Middle Cambrian time and relatively uniform depositional conditions persisted over large areas, which include central Saudi Arabia for the first time.

Late Cambrian and Early Ordovician

In northern Arabia, increased clastic influx in the Late Cambrian terminated carbonate deposition (Saq Formation in Saudi Arabia; Disi and Umm Ishrin formations in Jordan; Khanasar Formation in Syria; and Sosink Formation in southeast Turkey), and a prograding clastic apron was deposited conformably over the Middle Cambrian carbonates (Figure 6). These clastics grade eastward into distal shale dominated marine environments in the Zagros (Ilebeyk Formation). In interior Iran, however, carbonate deposition persisted into the Late Cambrian (Mila Formation).

The Cambrian-Ordovician boundary in the Arabian Plate is poorly defined in the rock record. During the Early Ordovician (Tremadoc and Arenig stages), the platform was again inundated, and deeper marine environments become established basinward in the north (Swab Formation in Syria; Bedinan Formation in southeast Turkey), and interior Iran (Zardkuh Formation). Mixed clastic and carbonate settings are found on the central Iran microplates (Rickards et al., 1994). Along the basin margin, braid-plain to braid-delta environments were followed by coastal-plain to inner-neritic clastic environments (Umm Sahm Formation in Jordan and Saq Formation in Saudi Arabia).

The boundary between the Disi and Umm Sahm in Jordan is transitional, and difficult to pinpoint. The essentially marine Umm Sahm correlates to the Sajir Member of the Saq Formation (Vaslet, 1990). Over most of Saudi Arabia the upper part of the Saq Formation, is characterized by Late Cambrian to Early Ordovician palynomorphs (Al-Hajri and Owens, 2000).

In Oman, in the Ghaba and Fahud salt basins a marine-influenced environment of deposition became established in the Late Cambrian. In these basins, Droste (1997) interpreted three depositional sequences in the Upper Cambrian and Lower Ordovician Andam Formation of the Mahatta Humaid Group, along with corresponding maximum flooding surfaces. These MFS are within the Upper Cambrian Al Bashair Member [MFS Cm30 dated at 502 Ma]; Lower Ordovician Tremadoc Mabrouk Member [MFS O10 dated at 494 Ma], and the upper Tremadoc Barakat Member [MFS O20 dated at 487 Ma]. These were followed by a stack of prograding braid-delta sequences in an overall transgressive setting. The remainder of the Arenig stage was accompanied by a regression during which a prograding braid-delta system was deposited, consisting of massive quartz sand/siltstones, and subordinate shales (Ghudun Formation; Droste, 1997).

Middle Ordovician

In Oman, the Middle Ordovician Saih Nihayda Formation is separated by a major unconformity from the Lower Ordovician Ghudun Formation (Droste, 1997). A thin sandy unit locally overlies this unconformity, but generally a rapid transgression resulted in deposition of middle to outer neritic shales. The primary maximum flooding surface of this sequence is of Llanvirn age [MFS O30 dated at 465 Ma], and can be traced from the Saih Nihayda in Oman, to the Hanadir Member of the Qasim Formation in Saudi Arabia, to the Hiswa Formation in Jordan (Figures 4 and 7). In Jordan, Powell (1991) renamed the lower part of the Hiswah as the Sahl as Suwwan Formation (Middle Ordovician, Llanvirn). Locally, the shales may be rich in organic matter, indicating restricted water circulation in the basin for the first time.

During the remainder of the Middle Ordovician, the Arabian Plate was covered by a major marine prograding clastic sequence. These sediments were deposited in inner-neritic to estuarine or deltaic environments. Point sources can be recognized in Oman and northern Saudi Arabia (Figure 7). Basin inwards deposition of middle to outer-neritic shales continued during the Middle Ordovician (Swab and Affendi Formation in Syria, Bedinan Formation in Turkey, Zardkuh Formation in Iran).

Late Ordovician

In the Late Ordovician a transgressive-regressive cycle [includes MFS O40 dated at 453 Ma] is recognized in the Kahfah, Ra’an and Quwarah members of the Qasim Formation in Saudi Arabia, and the Hasirah Formation in Oman. Basinwards, the cycles are difficult to recognize as the section consists of an undifferentiated package of essentially middle to outer-neritic graptolitic shales (Zardkuh Formation in Iran, and Bedinan Formation in southeast Turkey).

Time equivalent deposits are absent in most of interior Iran, and only local remnants have been preserved (Reitz and Davoudzadeh, 1995), possibly due to erosion. They are also absent in the Mardin area of southeast Turkey, where the entire Ordovician section has progressively been removed by pre-Silurian erosion (Figure 4). Whether this is due to tectonic processes, or shelf-edge erosion associated with the fall in sea level during the close of the Ordovician, remains to be resolved.

Late Ordovician Glaciation

The base of the uppermost Ashgill deposits is an important unconformity, which formed during the Late Ordovician glaciation of Gondwana [base AP3 dated at 445 Ma]. The polar icecap covered sub-Saharan Africa, and advanced into western Arabia in two major pulses depositing the Zarqa and Sarah formations. Each consists of tillite and pro-glacial clastics, mostly sandstones within incised valleys adjacent to the Arabian Shield and in southern Jordan (Figure 8; McClure, 1978; Vaslet, 1990).

Deep valley systems were incised to depths exceeding 600 m by glacial and fluvial processes, and have been traced into the subsurface of northern Saudi Arabia with seismic data (McGillivray and Husseini, 1992; Aoudeh and Al-Hajri, 1995). The associated major fall in relative sea level is witnessed away from the glaciated areas by a sudden influx of significant amounts of fluvial to deltaic sands on top of deeper marine sediments in parts of the basin (uppermost parts of the Dubeidib Formation in Jordan, and Affendi Formation in southern Syria).

Silurian

With the retreat of the glaciers, a major phase of global warming developed during the Llandovery. Sea level rapidly started to rise and flooded [S10 dated at 440 Ma] the Arabian Platform (Figure 8). Shallow to open marine environments were established in marginal areas, whilst deeper marine environments covered the inundated platform, and extended southward along a subsiding intrashelf basin located in central Saudi Arabia (Husseini, 1991; Mahmoud et al., 1992; Jones and Stump, 1999).

Anoxic water bottom conditions in the sediment-starved basin resulted in the preservation of organic rich shales–the prolific Silurian ‘hot shale’. These are the Qusaiba shale in Saudi Arabia, Mudawwara Formation in Jordan, Sahmah Formation in Oman, Abba Formation in Syria, Dadas Formation in southeast Turkey, and Ghakum Formation in Iran. The Qusaiba is the principal source rock for Paleozoic hydrocarbons in Saudi Arabia (Abu-Ali et al., 1991; Mahmoud et al., 1992).

A second, younger source rock of possibly Wenlock age occurs in the northern parts of the basin (Aqrawi, 1998). The initial transgression was followed by a thick (>1,000 m) coarsening-upward sequence of shales and sandstone of Llandovery to Pridoli age (e.g Qalibah in Saudi Arabia), which prograded basinward (Mahmoud et al., 1992). However, middle to outer neritic environments persisted in the north (Abba Formation in Syria, and Dadas Formation in southeast Turkey) and east (Gahkum Formation in Iran) during the remainder of the Silurian.

Late Silurian and Devonian

The latest Silurian and Devonian periods are poorly represented in the rock record. This is primarily due to Hercynian tectonism, uplifting and the resulting erosion (Figure 9).

In Saudi Arabia, above the regional pre-Tawil unconformity (Wender et al., 1998) the continental clastics of the uppermost Silurian-Lower Devonian Tawil Formation are followed by the marine, Pragian to Emsian Jauf Formation (Al-Hajri et al., 1999), [MFS D10 dated at 402 Ma, and MFS D20 dated at 393 Ma]. Marine incursions also reached Oman where the Misfar Formation was deposited. The latter includes anoxic mudstones deposited in lower coastal plain environments. The absence of pre-Emsian deposits in Syria, Turkey and Iraq suggests a structural high position with respect to the depocenter in Saudi Arabia.

During the Middle and Late Devonian, continental clastics were deposited in Saudi Arabia (Jubah Formation), which in the north are replaced by mixed marine siliciclastics and carbonates (Hazro and Kayayolu formations in southeast Turkey). During this period, continental environments became established in central Arabia, Syria and Iraq, whilst marginal marine environments persisted in Turkey and Oman. The age determination of specifically the deposits in Syria and Iraq is highly uncertain; these sediments may represent the latest Devonian (Aqrawi, 1998).

The return of marine environments, especially during the latest Devonian in the northern region (or their preservation), suggest differential downwarp of the northern margin of Gondwana. Similar relationships can be observed in interior Iran. Uppermost Devonian strata rest directly on Cambrian or Lower Ordovician sequences in the Alborz Mountains in Iran, whilst a more continuous Paleozoic section including older Devonian, is preserved in the basin south of the Mountains (Figure 8, Wensink, 1991). These relationships suggest that the northern margin of Gondwana became tectonically unstable, and herald the onset of the Hercynian Orogeny (see later).

Carboniferous

The Carboniferous is largely missing due to widespread uplift and erosion during the Hercynian Orogeny. However, in Syria, Lower Caboniferous sequences (Doubayat Formation) were deposited and preserved in a NE-trending proto Palmyra trough (Figure 10). The base of the Doubayat is a regional unconformity, becoming angular adjacent to Hercynian uplifts.

The basal part of the Doubayat section in Syria comprises Tournasian to lowermost Visean shallow marine shale, with subordinate sandstone and siltstone, and bioclastic carbonates. Incomplete biozones are indicative of intraformational depositional hiatuses. These are followed by fully marine carbonates of Visean age, reflecting the maximum extent of the transgression. The overlying sequences are part of a regressive complex made up of near shore to deltaic clastics. These sediments range in age up to the Stephanian. Thinning and pinching out of the carbonates, and variations in sand/shale ratios of especially the Middle to Upper Carboniferous sequences suggest that deposition occurred in a shallow, land locked, SW-trending depression. This implies a major change in basin geometry, which may be attributed to the Hercynian Orogeny (see below).

Isolated occurrences of Carboniferous siliciclastics have been penetrated in Saudi Arabia (Al-Hajri and Owens, 2000). They consist of poorly dated, syn-Hercynian continental sandstones of the Berwath and Unayzah-C member, which were deposited in low regions.

Upper Carboniferous deposits outside the proto-Palmyra depression are known from southern Arabia. Here glaciogenic and periglacial deposits of the Al Khlata in Oman (Braakman et al, 1982) and Juwayl Formation (Helal, 1966) have been preserved (Figure 4). The deposits are related to uplifted areas located southeast of Oman (Al-Belushi et al, 1996). Deposition in glacial environments in Oman continued during the Early Permian.

Early Permian

The first extensive deposits following the Hercynian Orogeny are the Upper Carboniferous-Lower Permian clastics that rest with angular unconformity (Hercynian unconformity) on older Paleozoic rocks and basement. They were partly deposited coeval with rift tectonics along the eastern and northern margins of the Arabian Plate. These sediments appear to be missing in Yemen and over the Central Arabian Arch (Figure 11).

Generally the section is made up of braided plain, channel fill, and eolian sandstones and siltstones (Unayzah A and B members in Saudi Arabia) that were deposited in semi-arid conditions (Senalp and Al-Duaiji, 1995). They are replaced basinward by braid plain deposits overlain by shallow-marine near-shore sediments to essentially shallow marine sands in the Zagros mountains (Faraghan Formation; Szabo and Kheradpir, 1978). The thickness of these clastics is variable due to onlap on the Hercynian structures.

The Lower Permian section in Oman embraces shallow marine carbonates (Haushi Limestone of the Lower Gharif Member) of Sakmarian age (Figure 11). The initial transgression is witnessed in the deeper part of the basin by a transgressive lag and marine mudstones (Maximum Flooding Shale, Guit et al., 1995) [MFS P10 dated at 272 Ma], which grade laterally into alluvial and fluvial deposits. They are followed by fine clastics, which may include lacustrine and playa deposits (Middle Gharif member), suggesting diminishing basin topography. The later Artinskian documents a sudden increase in sand content brought in by rivers, probably in response to uplift in the source areas associated with incipient rifting that preceded the formation of the Neo-Tethys margin (Le Métour et al., 1995).

Late Permian and Early Triassic

In the Late Permian, increased accommodation space related to stretching of the crust accompanied the formation of the Neo-Tethys Ocean along the Oman-Zagros suture. The break-up unconformity (pre-Khuff unconformity) marks the birth of this new ocean.

The base of the resulting megasequence [base of AP6 dated at 255 Ma] consists of continental to marine sandstones and shales (basal Khuff clastics, Senalp and Al-Duaiji, 1995) supplied from the west. Northward the continental deposits include coal deposits indicating wetter tropical environments (Kas Formation in southeast Turkey). These were followed by the deposition of extensive carbonates and anhydrites (Khuff Formation in Saudi Arabia and Oman; Dalan and Kangan formations in Iran; Al-Jallal, 1995) over the entire Arabian shelf in shallow marine to tidal flat environments.

The Khuff Formation includes at least four depositional sequences. During maximum transgression, carbonates oversteped the clastic realm and rested on basement over the Central Arabian Arch. During regressions, restricted evaporitic environments became established on the western part of the platform protected by shoals from open seas in the east (Figure 12). In the High Zagros and Oman Mountains deep-marine environments were established.

HERCYNIAN OROGENY

The Hercynian Orogeny affected the Arabian Plate from the Late Devonian to the Early Carboniferous. Fission track studies, combined with organo-chemical studies in Turkey to Oman, indicate the removal of several kilometers of sediments over uplifted areas. Changes in basin geometry, regional uplift, basement-cored uplifts, and the evidence of folding and inversion tectonics, suggest that the Arabian Plate underwent multiple phases of compression during this orogeny. The structural observations are consistent with a NW-directed principle compressive stress.

The Carboniferous, synorogenic sequences were deposited in continental to shallow-marine environments, embracing Visean carbonates in Syria. The Carboniferous clastics were mainly derived from the erosion of older clastics in uplifted areas.

The Hercynian Orogeny resulted in a major change in basin geometry as revealed by the Hercynian subcrop (Figure 10). This map shows a NE-trending basement high protruding into the basin in central Arabia, the Central Arabian Arch. Facies patterns and thickness variations in Devonian-Silurian and older sequences suggest that the Arch either formed or was rejuvenated during the Hercynian Orogeny, and persisted into the Mesozoic. This high is overprinted by N-trending basement-cored uplifts (e.g. Ghawar anticline), which juxtapose various rock units. The NW-trending faults in the Azraq (Wadi Sirhan and Jauf) graben were also activated, and are associated with large uplifts accompanied by deep erosion (Figure 13).

The proto-Palmyra and its northeasterly extension occur just south of a zone where uplift and erosion exposed Ordovician strata in the area of the Aleppo and Mardin highs. Northward, younger rock units have been preserved in the Diyarbakir basin, implying the uplift of a regional, ENE-trending foreland bulge, running parallel to the Central Arabian Arch.

Additional evidence for Hercynian tectonism stems from structural observations. Figure 13 illustrates the structural and stratigraphic relationships in the northern Arabian Plate. The section shows that the Cambrian to Silurian rocks form a single structural entity. The Lower Devonian hiatus (Figure 4a) may be due to vertical movements. The Ordovician-Silurian sequences are truncated and folded at a regional-scale prior to the deposition of the Carboniferous resulting a major angular unconformity (below the Berwath Formation in Saudi Arabia). The axial zone appears to coincide with the south Syria Platform (compare with Figure 2). Folded Ordovician-Silurian rocks can also be observed below the base Triassic and younger unconformities farther south. Finally, the distribution of the Carboniferous sequences suggests that the area was affected by a phase of differential uplift prior to deposition in the Triassic.

The cross-section shown in Figure 14 follows the trend of the Central Arabian Arch and extends from the Arabian Shield across several large structures in central and eastern Saudi Arabia to the Qatar Arch. Pre-Permian strata are clearly truncated by erosion below the Hercynian unconformity. This extensive erosion, particularly of the Devonian section, demonstrates that the structures were uplifted by thousands of meters during the Carboniferous (Figure 10).

The NS-trending Hercynian uplifts, such as Ghawar, are bounded by reverse faults, suggesting that the uplift was due to a regional compressive stress field. In general, post-Hercynian pre-Permian erosion reduced the relief, but not completely, as indicated by thickness and facies variations in the Unayzah Formation. Many of the Hercynian faults bounding the major N-S uplifts were reactivated during the Triassic and especially during the Late Cretaceous, as indicated by the dramatic thickening of the Aruma Group on the flanks of these uplifts. Not all the structures shown on Figure 14, however, are Hercynian in origin. For example, the Harmaliyah anticline, located immediately east of Ghawar, preserves the most complete Devonian section in Saudi Arabia and is clearly post-Hercynian in origin.

Figure 15 highlights the geological relationships in the southern Arabian Plate. Here the post-Hercynian Carboniferous sequences generally rest on Ordovician or older deposits. Devonian rocks are only locally preserved. No folding or reverse faulting is known in Oman, suggesting that Hercynian events were essentially vertical in nature.

Further evidence for Hercynian movements, though still highly speculative, is derived from the Sanandaj-Sirjan ranges of Iran (see Figure 1) and the Oman Mountains. In the former, intensely folded metamorphic Devonian complexes have been found (Figure 9, Davoudzadeh and Weber-Diefenbach, 1987; Thiele et al., 1968). These are overlain by non-metamorphic Permian. Although still sparse, radiometric dating indicates an Early Carboniferous age for the metamorphism (Crawford, 1977). In the Oman Mountains, the Permian rests unconformably on highly deformed and metamorphozed Lower Paleozoic rocks attributed to the Hercynian Orogeny (Mann and Hanna, 1990). The deformation combined with the metamorphism indicates that the future Zagros margin was possibly a zone of transpressional movements.

HYDROCARBON HABITAT

Source Rocks

Our working definition of a petroleum system differs from published definitions (Magoon and Dow, 1994). A petroleum system is here defined as the total space occupied by all hydrocarbons derived from one chemically distinguishable source rock interval. This definition places the emphasis on establishing hydrocarbon availability. Understanding the Silurian petroleum system yields one of the keys to unlocking most of the Paleozoic resources.

Organic-rich source rocks exist throughout the basin at the base of the Silurian shales (Figure 8). These are dark-gray to black, containing marine algae, acritarchs and abundant chitinozoans and graptolites. Source rock quality and thickness varies with depositional environment, as demonstrated by ‘source out’ into a shallow-marine bioturbated, sandy, micaceous claystone facies in basin margin settings. Source rock net thickness varies from hundreds of meters in the Rub’ Al-Khali Basin, to some 50 m in the northern basin, to a few meters in marginal settings (Mahmoud et al., 1992; Aoudeh and Al-Hajri, 1995; Jones and Stump, 1999). In Jordan, Syria and Iraq, in areas of greater accomodation space, a younger source-rock level has been observed of probable Wenlock age.

Oil to oil, and oil to source-rock correlations indicate the presence of Silurian derived fluids over a wide geographical area from Turkey to Oman, and from Saudi Arabia to Qatar (Figure 16). They occur as mixtures or end-member crudes that have distinct chemical fingerprints (Grantham et al, 1987; Abu-Ali et al., 1991; Cole et al., 1994).

Estimating the availability and quality of Silurian-sourced hydrocarbons is often hindered by complex burial histories. These include burial cycles interrupted by major uplifts, especially during the Hercynian Orogeny. This may render the interpretation of Vitrinite Reflectance (VR) measurements difficult in terms of timing, especially considering the uncertainties in thermal history.

In some areas, deep Paleozoic source rocks may have generated their hydrocarbon potential prior to the Carboniferous, but these may have been lost to the surface during the Hercynian orogeny. In other areas, source rocks only reached the oil window prior to Hercynian uplift, leaving only potential for gas generation during the subsequent burial phase. Therefore, it is critical to understand the generation histories through the application of inorganic paleo-thermometer measurements.

The predicted cumulative volumes of oil and gas expelled from the Silurian shale depocenter, contained within the present oil window, range from 430 to 760 bbls of oil, and 1,540 to 2,575 TCF of gas. Cumulative volumes of oil and gas expelled from the Silurian within the present-day gas window range from 3,000 to 3,600 bbls of oil and 21,595 to 39,200 TCF of gas. Assuming that about 90% of the predicted volumes were lost either due to migration losses or model inaccuracies, then between 48 billion to 83 billion barrels of oil and oil equivalents are predicted to be recoverable where the source rock is within the oil window. For the gas window, another 380 to 439 bbls of oil and oil equivalent are predicted with the same assumptions. The Paleozoic exploration frontier may, therefore, offer a Hydrocarbon-Initially-In-Place (HIIP) of 1 trillion BOE reservoired from the Silurian shale alone.

Although the Silurian shale is the principal hydrocarbon source rock, recent geochemical evidence from the Shamah oil field in Oman indicates that the Unayzah condensates are derived from a different source rock, as yet unidentified.

The late Precambrian rocks in Oman constitute another group of established petroleum systems (Terken and Frewin, 2000). These may also be present in the other Precambrian salt basins (Figure 1). Hydrocarbons derived from these source rocks have been found in reservoirs spanning the entire Phanerozoic. The hydrocarbons have been linked to several source-rock intervals deposited in the pre- to syn-rift sequences. They embrace carbonate source rocks, which contain mainly Type I/II organic matter with total organic carbon contents (TOC) of up to 7%. Silicilyte source rocks are found in intrasalt settings. They have variable TOCs ranging up to 10%, and may occur in massive sections up to 1,750 m thick. They are considered world class source rocks that are characterized by anomalously low activation energies.

Finally, a group of hydrocarbons have been defined in Oman, the so-called ‘Q’-oils that have distinct geochemical characteristics (Grantham et al., 1987). The exact source of these hydrocarbons remains to be identified, though they appear to be Precambrian in character.

Reservoirs and Seals

The stratigraphic diagrams in Figure 4 show the relationship between the main Paleozoic source rocks, seals, and reservoirs. These are generalized schemes and local exceptions are to be expected.

The Permian-Carboniferous sandstones (Unayzah, Gharif and Al Khlata) and carbonates (Khuff) contain the main reservoirs of the Silurian petroleum system. They are sealed by intraformational claystone and shale, or by tight carbonates and evaporites. The regional seal in Saudi Arabia and Oman is Triassic shales of the Sudair Formation, which completely separate the Silurian hydrocarbon system from the overlying Mesozoic systems. Lack of seals renders little prospectivity to the Permian sequences in northern Arabia.

The Carboniferous to Devonian sequences may include excellent reservoirs, especially the Devonian of eastern Saudi Arabia. The presence of only local seals, combined with rapid lateral facies variations, render these sequences of limited regional prospectivity. Exceptions include structures where the reservoirs subcrop Permian seals, and are juxtaposed across faults against a sealing facies.

The Silurian section may also include reservoirs in the form of sand deposits within the shale dominated outer neritic environments. Generally these reservoirs are thin and their quality difficult to predict. This play was recently confirmed by discoveries in Saudi Arabia and Iraq.

The Silurian ‘hot shale’ forms the ultimate seal to the pre-Silurian section. The latter embraces excellent reservoirs, which may be down-charged, or side-charged by faults, from the hot shale. An example is the Abu Jifan field in eastern Saudi Arabia, in which sandstones of Ordovician Sarah and Qasim Formation are the main reservoir.

The pre-Silurian section becomes an important target in addition to the Permian in those regions underlain by Precambrian source rocks. The trapping potential in the Cambrian-Ordovician basin margin sections, made up of massive coarse clastics, depends on truncation, which juxtaposes them against Permian-Carboniferous or younger seals. Seal potential increases basinward in parallel with changes in environment of deposition towards more marine settings (lower sand/shale ratios). However, reservoir quality deteriorates especially due to increased burial, and the presence of reservoirs becomes highly dependent on the diagenetic history.

ESTABLISHED PLAYS

The established frontier Paleozoic plays include the Permian Khuff, Permian-Carboniferous Unayzah and Haushi, as well as Devonian and Cambro-Ordovician targets (Figures 16 and 19). The Precambrian-Cambrian Huqf plays of Oman are not discussed here.

Khuff Play

Gas was initially discovered in Permian-Triassic Khuff carbonates in the Awali field of Bahrain in 1949. Subsequent gas discoveries were made in deeper pool tests of the major structures in Abu Dhabi, Iran, Oman and Saudi Arabia. In 1971, the world’s largest gas field, the North Dome Khuff reservoir was discovered in Qatar. The Khuff is the largest non-associated gas reservoir in the world, with approximately 750 TCF of recoverable reserves (Figures 18 and 19).

The quality of the Khuff gas depends upon the amounts of non-hydrocarbon gases, mainly H2S, CO2, and N2. The amount of H2S increases with temperature and depth, reflecting in situ conversion of hydrocarbons to H2S by thermochemical reduction of anhydrite sulfate. The amounts of other gases, such as N2 and CO2 contaminants, appear to increase with depth and source-rock maturity.

The gas accumulations occur in up to four separate reservoirs, each consisting of oolitic grainstones and intertidal dolo-mudstones that are capped by anhydrite seals (Figures 17 and 18). On a regional scale, reservoir development is, in part, related to the relative position on the carbonate shelf, and the development of higher energy facies on shoals that may straddle structural highs and shelf margin reefs (Al-Jallal, 1995).

The quality of the reservoirs varies from excellent to poor, with abrupt lateral and vertical variations in porosity and permeability. These are controlled by dolomitization, leaching, fracturing, and cementation (particularly by anhydrite). Leached zones often form the better portion of the reservoir. Reservoir porosity types range from primary intergranular to secondary oomoldic. Reservoir permeability is equally variable, depending upon leaching of either matrix and cement components, or the extent of fracture development. Production may be both from the matrix and from fractures, but productivity generally improves with the presence of fractures.

For these reasons, petrophysical evaluation and geologic modeling of the Khuff reservoirs is hampered by uncertainties. On the other hand, these factors suggest that the Khuff has considerable potential for stratigraphic traps, as yet unexplored. 3-D seismic data has proven to be a good approach for delineating zones of Khuff porosity.

There is some uncertainty about the history and paths of hydrocarbon migration into the Khuff, particularly in areas where basal Khuff shales and tight carbonates seal the accumulation in the underlying clastics. It is likely that reactivated older faults, such as those on the west flank of Ghawar (Figure 19), provided pathways for vertical migration into the Khuff from hydrocarbon kitchens in flanking regions (Wender et al., 1998).

Unayzah/Gharif Play

Oil in the Permian Gharif sandstones was first discovered in 1972 in the Ghaba North structure in Oman, and the subsequent campaign demonstrated the economic viability of the play throughout Oman. In Saudi Arabia, the potential of the Permian Unayzah was confirmed in 1979 by a gas discovery in the Qirdi field. The Unayzah play became much more significant in 1989, when super light oil was discovered in Hawtah-1 in central Saudi Arabia. Another 16 Unayzah fields have been discovered in the past decade. The fields are structural closures along Hercynian basement-cored uplifts, that may be transpressional in origin (Simms, 1995). Moreover, the stratigraphic variability of the Unayzah, influenced by paleotopography and the continental environments of deposition, lends a stratigraphic component to entrapment (Evans et al., 1997).

The Unayzah oils range from 48o to 53o API gravity and their gas/oil ratio (GOR) is less than 90 m3/m3. The low GOR is attributed to solution of methane in waters in an active hydrodynamic system driven by influx of meteoric water from outcrops along the western edge of the basin (Figure 14). The Silurian source rocks in central Saudi Arabia are immature, and the Unayzah oils were evidently generated in the deeper parts of the basin and migrated about 200 km westwards towards the basin margin (Abu Ali et al., 1991).

The Unayzah and overlying basal Khuff clastics are composed of alluvial, fluvial, and eolian facies. The Unayzah includes three sandstone reservoirs, designated informally as A, B, and C, which are separated by silt- and mudstone (McGillivray and Husseini, 1992). The sandstone reservoirs are laterally discontinuous, and their quality varies depending on sorting and the amount of diagenetic quartz, kaolinite, illite/smectite cement. Intergranular porosity ranges up to 30% and permeability up to one darcy, particularly in the eolian sandstone facies. The top seal are transgressive shales at the base of the overlying Khuff Formation.

The Unayzah play was extended during the last decade to target gas in the deeper basin (>3,700 m) near facilities in eastern Saudi Arabia. The gas exploration campaign has resulted in the discovery of six additional Unayzah gas/condensate fields near Ghawar field, such as Waqr and Tinat (Figure 19).

The Unayzah deep gas play presents additional challenges, which include poor seismic imaging of the Paleozoic section and abrupt variation in reservoir quality due both to stratigraphy and diagenesis. The problems of deep seismic imaging and reservoir heterogeneity are both being addressed by the acquisition of high-effort, 3-D seismic surveys to reduce reservoir and trap risks.

Devonian Play

Gas in the Devonian Jauf sandstone was initially discovered in 1980 by a deeper pool test in the north Ghawar field. Subsequent tests showed that the Devonian section was mostly eroded from the crest of the structure. The discovery in 1994 of Jauf gas in a combination structural-stratigraphic trap along the flank of the Ghawar structure was a major exploration success, especially in light of the poor seismic imaging of the pre-Khuff section (Wender et al., 1998).

The Jauf reservoir consists of shallow marine sandstones with relatively high porosities (up to 25%). This is unusual given their burial to over 4,300 m. Unlike other pre-Khuff siliciclastics, which have undergone extensive silica cementation, the Jauf reservoir is weakly cemented with authigenic illite that coats grain surfaces, which apparently has inhibited quartz cementation and preserved porosity. The abundant illite also lowers resistivity values due to the excess bound water and the high cation exchange capacity of illite. This can cause pessimistic water saturation estimates and lead to potentially bypassed low-resistivity pay zones (Wender et al., 1998).

The cross-section in Figure 19 shows the structural relationships of the Devonian Jauf. On structures like Ghawar that were subjected to a large amount of Hercynian uplift, the Jauf is eroded from the crest and preserved along the flanks. The play is defined by the lateral truncation of the reservoir against sealing faults, or by its top truncation by the Hercynican Unconformity, with top seal provided by the basal shales of the Khuff. The Jauf may also be preserved over the crest of low relief structures like Waqr, where it is a purely structural play.

Cambrian-Ordovician Plays Saudi Arabia

Several discoveries have been made in Upper Ordovician structural traps, including Dilam and Abu Jifan in central Saudi Arabia, Kahf and Jalamid in northern Saudi Arabia, and Wadi Sirhan and Risha in Jordan (Figure 16). These, mainly gas fields, are sourced and sealed by the overlying Silurian shales. Where the Silurian is missing due to Hercynian erosion, the Permian Khuff forms the seal.

The Ordovician reservoirs are generally of poor quality. In the deeper basins, the sandstones have low porosities and permeabilities due to compaction and extensive cementation by quartz overgrowths during burial. Petrographic studies indicate that any significant porosity is secondary, due primarily to dissolution of early intergranular carbonate cement. The early carbonate cementation was localized in areas where Hercynican erosion placed the Khuff carbonates unconformably above the Ordovician. This cement was probably derived from the Khuff, and occurred at shallow depths before the sandstones underwent significant compaction. The subsequent dissolution of carbonate cement preceded hydrocarbon migration into the reservoirs.

Oman

In interior Oman, the Upper Ordovician (Caradoc) Hasirah was deposited in a tide-dominated, sandy deltaic (or estuarine) environment, fed by braided rivers from an overall southerly source. These pass basinward into undifferentiated marine mudstones and claystones, and interbedded, laterally discontinuous, sandy mass flows, which are deposited in outer shelf environments (Figure 20). The latter sandstones have excellent reservoir qualities, with porosities reaching 32%, due to reworking and rapid deposition. The Hasirah sediments were deposited in an active salt-withdrawal basin in ponded geometries, and occur at an average depth of 3,000 m. They constitute potential stratigraphic traps that are mapped with seismic amplitude and AVO techniques.

Since the first Upper Cambrian Haima gas/condensate discovery in 1989, Oman has booked about 17.6 TCF of reserves. The gas occurs in the Barik Sandstone at depths exceeding 4,000 m (main objective, Figure 21). The reservoirs are found in salt-cored domes that may be compartmentalized by faults. Initial reservoir pressures are about 500 bar and temperatures are 125 to 140°C, providing a considerable challenge in terms of deep-well engineering. The condensate to gas ratio varies from 0 to 950 m3/106m3. Hydrocarbon columns are in the order of 100 m to greater than 200 m.

The Barik reservoirs represent a sandy braid delta interrupted by periodic flooding events. The latter result in the deposition of a non-reservoir heterolithic, shallow-marine faces. Eustatic changes gave rise to eight stacked flow units. Reservoir characteristics vary with overall position within the depositional system; average porosity and permeability is in the order of 8 to 10% and 1 to 2 mD. Local variations in reservoir parameters also depend on diagenetic history, and especially on the presence of an early oil charge that inhibited quartz overgrowth and dolomite cementation. The presence of higher quality thief zones complicates reservoir management through introducing a risk of early water breakthrough. In addition, reservoir performance is highly dependent on fractures. The Barik reservoir is mapped by specialized seismic acquisition and processing techniques, and its production is optimized by reservoir models that account for fractures. Hydraulic fracture stimulation of wells plays a key role in the development of the field.

CONCLUSION

The Paleozoic Arabian Plate offers major opportunities to discover and delineate new energy reserves. The system includes multiple reservoir objectives in continental and marine clastics, and in Permian carbonates. Hydrocarbons were mainly derived from the prolific Silurian ‘hot shale’ that extends over most of the basin. Tectonostratigraphic relationships indicate that the platform southwest of the Zagros Suture was generally stable until the Hercynian Orogeny that started in the latest Devonian and climaxed in the Early Carboniferous. The orogeny is manifested by regional upwarps (Syria, Central Arabia and Oman) and sags (Palmyra and Rub’ Al-Khali), and narrow N-trending basement cored uplifts (e.g. Ghawar field). In the Early Permian, rifting along the eastern margin led to the opening of the Neo-Tethys Ocean.

The prospectivity of the Paleozoic section is largely determined, in addition to the sedimentary facies patterns, by the pre- and post-Hercynian burial and thermal histories, which dramatically impact reservoir quality and availability of hydrocarbons. A non-traditional approach is required to constrain thermal histories due to the complex burial/uplift history. Although porosity was largely destroyed during the deep burial of the section, it was locally preserved due either to the presence of an early diagenetic phase, or to early emplacement of hydrocarbons. Moreover, secondary porosity was selectively created in thin carrier beds by leaching during fluid flow.

Exploration and development success will depend on significant innovations to meet the challenges posed by low acoustic contrasts between the target rock units, difficult surface conditions, tight reservoirs, and deep subsurface environments. The history of hydrocarbon exploration in the Arabian Plate has yielded a wide variety of new and often unexpected hydrocarbon plays spanning the Tertiary to Precambrian section. Exploration success in these plays, driven by creative geologists, was often much to the surprise of the established views.

ACKNOWLEDGMENTS

This paper is based on the work of numerous individuals who cannot all be justly mentioned. Special thanks are due to D. Evans, A. Al-Hauwaj, M. Husseini, M. Mahmoud, A. Neville, H. McClure, J. McGillivray, A. Norton, M. Rademakers, M. Senalp, and L. Wender from Saudi Aramco, and W.O. Bement, H.G. Hoetz, P.J.F. Jeans, A.T. Jones, B.K. Levell, M.P. Ormerod, M.A. Partington, J.G.M. Raven, A.N. Richardson, P. Spaak and W.G. Townson from Petroleum Development Oman (PDO) and Shell. The authors assume full responsibility for their own conclusions. The authors are grateful to Petroleum Development Oman LLC, Saudi Aramco, Shell International Exploration and Production B.V., the Oman Ministry of Oil and Gas, and the Saudi Arabian Ministry of Petroleum and Mineral Resources for permission to publish this paper.

This paper was presented in an earlier form at the American Association of Petroleum Geologists (AAPG) Pratt II Conference, San Diego, California, January 12–15, 2000; and at GEO 2000, Bahrain, March 27–29, 2000. The present revised version was substantially modified and makes reference to the work of GeoArabia Special Publication 2 by Sharland et al. (2001). We thank Moujahed Al-Husseini and Joerg Mattner of GeoArabia, and Peter Sharland from Lasmo for their assistance in preparing the revised version. The design and drafting of the final graphics was by Gulf PetroLink.

Geert Konert is Principal Geologist for Shell International Exploration and Production B.V. in Research and Technical Services, Rijswijk Netherlands. He graduated from the University of Amsterdam in Geology, Structural Geology and Geochemistry in 1981 and joined Shell the same year. Geert has worked on various exploration assignments in Brunei, The Netherlands and Oman, and has been involved in E&P projects in the Middle East. His main area of interest is the tectonic evolution of the Middle East.

Abdulkader M. Al-Afifi is Chief Explorationist, Southern Area Exploration, with Saudi Aramco. He received a BSc degree in Geology from King Fahd University of Petroleum and Minerals, Dhahran, an MSc from the Colorado School of Mines, and a PhD degree from the University of Michigan, Ann Arbor. Abdulkader worked previously with the US Geological Survey Mission in Jeddah prior to joining Saudi Aramco in 1991. He is a member of the American Association of Petroleum Geologists and Society of Petroleum Engineers.

Sa’id Al-Hajri is Chief Geologist of the Regional Mapping and Special Studies Division of the Saudi Aramco. He holds a BSc in Geology from the King Fahd University of Petroleum and Minerals, Dhahran, and an MSc in Geosciences from Penn State University. Sa’id is professionally interested in the Palaeozoic palynology and stratigraphy of northern Gondwana. He is a member of CIMP, AASP, BMS and the DGS, and has published several papers on geological and palynological subjects.

Henk H.J. Droste joined Shell in 1984 after receiving his MSc in Geology from the University of Amsterdam. He worked as a Carbonate Geologist with Shell Research in The Netherlands and as a Sedimentologist in the Regional Studies Team of Shell Expro in London. He was transferred to PDO Oman in 1992 where has been working as a sedimentologist in the Exploration Laboratory, Geologist/Seismic Interpreter in Exploration, Production Geologist of the Yibal Field and as a Team Leader of the Regional Studies and Geological Services Team. In 2001 he was posted to the Carbonate Research Centre located in the Sultan Qaboos University of Oman.