Konert Paleozoic Stratigraphy

GeoArabia, Vol. 6, No. 3, 2001Gulf PetroLink, Bahrain

Paleozoic Stratigraphy andHydrocarbon Habitat of the Arabian Plate

Geert Konert, Shell International,Abdulkader M. Afifi and Sa’id A. Al-Hajri, Saudi Aramco,

and Henk J. Droste, Petroleum Development Oman

ABSTRACT

The Paleozoic section became prospective during the early 1970s when the enormous gasreserves 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 PaleozoicSystem throughout the Middle East, driven by various economic considerations. ThePaleozoic sequences were essentially deposited in continental to deep marine clasticenvironments at the Gondwana continental margin. Carbonates only became dominantin the Late Permian. The sediments were deposited in arid to glacial settings, reflectingthe drift of the region from equatorial to high southern latitudes and back. Followinglate 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 theCarboniferous. The orogeny manifested itself in a change in basin geometry, inversiontectonics, regional uplift and tectonism along the Zagros fault zone. This deformationcaused widespread erosion of the Devonian-Carboniferous and older sections, and wasprobably caused by collision along the northern margin of Gondwana. The Paleozoictectonic super cycle ended with the onset of break-up tectonics in the Permian, and thedeposition of Khuff carbonates over the newly formed eastern passive margin. A majorPaleozoic petroleum system embraces reservoir seal pairs spanning the Silurian to Permiansequences. Hydrocarbons occur in a variety of traps, and are sourced by the Silurian ‘hotshale’. A second petroleum system occurs in areas charged from upper Precambriansource rocks in the salt basins. Hydrocarbon expulsion estimates, taking into accountsecondary 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 complexhydrocarbon geological evolution of the basin, combined with low acoustic contrastsbetween target rock units, difficult surface conditions, tight reservoirs, and deep subsurfaceenvironments, posed significant challenges to exploration and development. The criticalsuccess factor is the continuous innovative effort of earth scientists and subsurfaceengineers to find integrated technology solutions, that will render the Paleozoic playseconomically viable.

INTRODUCTION

The Middle East holds estimated proven reserves of some 625 billion barrels (bbls) of crude and 1,720trillion cubic feet (TCF) of natural gas, nearly 64% and 34% of the world’s reserves, respectively (IHSEnergy 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-Suleymanfield in Iran on the 26th of May, 1908. Crude and condensate production from these reservoirs wasreported to be nearly 8.0 billion barrels/year in 2000.

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

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Figure 1: Location and major tectonic elements of the Arabian Plate and Iran. The present-dayArabian Plate is bordered in the north by the collision zones of the Zagros and Bitlis sutures, andby subduction along the Makran zone. To the south, southwest and west, the Arabian Plate boundaryis defined by the Owen-Sheba transform fault, the rift systems with sea floor spreading in the Gulfof Aden and the Red Sea, and the Dead Sea transform fault zone. The Makran and Zagrosconvergence zones separate the Arabian Plate from the microplates of interior Iran.

Paleozoic Hydrocarbon Habitat, Arabian Plate

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 therebyproviding incentives for substituting gas for oil.

3) Exploration is required to replace produced reserves in order to guarantee future income andmaintain 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 producingfields is often more cost beneficial than developing remote resources. Enhanced recovery programsmay 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 towardsincreasingly 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. Atpresent, 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 thescale of the Arabian Plate and interior Iran remains speculative.

Throughout this paper all ages are based on the most recent geological timescale of Gradstein andOgg (1996) for the Paleozoic, and Harland et al. (1990) for the Precambrian. Also for ease of referencebetween this paper and the sequence stratigraphic study by Sharland et al. (2001) we have placed insquare brackets equivalent surfaces with dates referred to in their publication. These consist of theirinterpreted Maximum Flooding Surfaces (MFS) identified by Period: for example [Silurian MFS S10dated at 440 Ma]; and Arabian Plate (AP) Tectonostratigraphic Megasequences: for example [TMSAP3].

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 theZagros and Bitlis sutures, subduction along the Makran zone, and transform movement along theDead Sea and Owen-Sheba fault zones. The Makran and Zagros convergence zones separate theArabian Plate from the microplates of interior Iran.

The Arabian Plate basin is asymmetric (Figure 2). To the west it is bounded by the exposed PrecambrianArabian 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. Theshallow basement along the Arabian Sea reflects episodes of uplift associated with the break-awayand drift of the Indian Plate.

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

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The Precambrian Arabian Shield consists of accreted island-arc and microcontinental terranes (Stoeserand Camp, 1985; Brown et al., 1989), overlain by post-cratonic sediments and volcanics. The finalPrecambrian Amar collision (about 640–620 Ma, Brown et al., 1989) fused together the Arabian Platealong 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 inheritedmechanically weak trends. These are defined by: (1) N-trending highs as exemplified by the En Nala

Figure 2: This tentative basement depth map of the Arabian Plate basin is partly compiled frompublished data (Best et al., 1993; Buday and Jassim, 1987; Loosveld et al, 1996). The basin isasymmetrically pronounced. In the west it is shallow and bounded by the exposed PrecambrianArabian Shield. The basin deepens gently in an easterly direction and reaches its maximum depthof several kilometers in the foredeep of the Zagros collision zone.

(Ghawar) anticline and the Qatar Arch; (2) NW-trending systems like the Azraq (Wadi Sirhan andJauf) 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 theevolution 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 tosouthern latitudes, and rotated anti-clockwise. By the latest Ordovician (about 445 Ma) the platereached its maximum low-latitude position (about 55° south) and a major polar glacial pulse coveredwestern Arabia (McClure, 1978; Vaslet, 1990).

During the Silurian to Late Carboniferous the plate underwent a major clockwise rotation of about100° without significant latitudinal translations. As a result of the rotation, Oman was located on thesouthern edge of the Plate. By the Late Carboniferous (about 305 Ma), a second glacial phase affectedOman, Yemen and southwest Arabia. During the Permian the Plate moved rapidly to the north.

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Figure 3: Paleolatitude positions of the Arabian Plate during the Paleozoic. During the Paleozoicthe Arabian Plate reached its most southerly position of about 55° S in the Late Ordovician toPermian-Carboniferous. This southerly Plate position coincides with two periods of glaciationaffecting the western and southern margin of the Arabian Plate. Note the over 90° rotation of thePlate orientation between Ordovician and Carboniferous.

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STRATIGRAPHY AND BASIN EVOLUTION

The Paleozoic stratigraphy of the Arabian Peninsula stems mostly from outcrop studies along themargins of the Arabian Shield and in south Oman, and from wells drilled generally over structuralhighs. In interior Iran the available data is essentially derived from surface outcrops that were describedlithostratigraphically before the 1980s. Large areas, especially in the deeper parts of the basin, containonly sparse or no well data. Also most of the basin is covered by older vintages of seismic data that donot adequately image the Paleozoic section. All of these considerations limit our understanding of thePaleozoic sequences.

Figure 4 illustrates the generalized late Precambrian and Paleozoic chronostratigraphic framework ofthe Middle East. This paper does not cover the tectonic and stratigraphic evolution of the plate duringthe late Precambrian and Early Cambrian Najd Rift phase [AP1 from 610 to 520 Ma]. During thisperiod 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 Cambrianage were deposited over most of the plate (Figure 4). These were sourced from interior Gondwana tothe 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 clasticsthat were deposited in a system of alluvial fans grading into braid plains and braid deltas. South ofJordan and north of the Arabian Shield, in the Tabuk outcrops of Saudi Arabia, the Lower CambrianSiq Sandstone unconformably overlies the irregular surface (pre-Siq unconformity) of the Precambrianbasement. It consists of a basal alluvial conglomeratic sandstone overlain by a mixed sand-flat-eoliansandstone complex (D. Janjou, P. Razin, M. Halawani, and W. Roberts, written comm., 2000). Lateraltime equivalents in the south may include the Nimr Group in Oman, which consists of alluvial fandeposits, in addition to playa lake deposits.

In southwest Iran, the post-rift Zaigun and Lalun formations appear, on the basis of sedimentarystructures such as cross-bedding, to be non-marine. These lack age-diagnostic fossils (R. Jones, writtencomm., 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 tothe basal post-rift continental clastics.

Late Early and Middle Cambrian

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

During the Middle Cambrian, a vast shallow carbonate platform covered most of northern Arabia andinterior Iran: Burj Formation in Jordan (Amireh et al., 1994), Saudi Arabia and Syria, Koruk Formationin 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 inJordan.

Along the basin margin, however, during the remainder of the Middle Cambrian, deposition returnedto proximal alluvial fan deposits, interrupted by subordinate marginal marine conditions. In theoutcrops of southwest Jordan, for example, above the Burj carbonates, the Umm Ishrin and Disi

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formations consist of a coarsening-upward succession of alluvial-fluvial deposits interrupted bysubordinate marginal marine clastics, which correlate with the Cambrian Risha Member of the SaqFormation of Saudi Arabia (Vaslet, 1990).

In the Tabuk outcrops of northwest Saudi Arabia the Burj is absent. In the subsurface of northwestSaudi Arabia, however, and in Khursaniyah-81 well in eastern Saudi Arabia, the Lower Cambrian Siqis overlain by the Burj Dolomite (Figure 4c). An MFS is identified within its upper part and ischaracterized by an Early to Middle Cambrian acritarch assemblage (Al-Hajri and Owens, 2000) [MFSCm20 dated at 510 Ma].

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Figure 5: During the Middle Cambrian, a vast shallow, carbonate platform covered most of northernArabia and interior Iran. Proximal alluvial fan deposits, interrupted by subordinate marginal marineconditions, prevail along the basin margin.

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

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Figure 6: In northern Arabia, increased clastic influx in the Late Cambrian terminated carbonatedeposition, and a prograding clastic apron was deposited conformably over the Middle Cambriancarbonates. These clastics grade eastward into distal shale dominated marine environments.

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The Miqrat Formation in central Oman, and the Mahwis Formation in south Oman were also depositedin a continental setting during the Middle Cambrian (Droste, 1997). All relict topography appears tohave been leveled by Middle Cambrian time and relatively uniform depositional conditions persistedover 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 inSyria; and Sosink Formation in southeast Turkey), and a prograding clastic apron was depositedconformably over the Middle Cambrian carbonates (Figure 6). These clastics grade eastward intodistal 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. Duringthe Early Ordovician (Tremadoc and Arenig stages), the platform was again inundated, and deepermarine environments become established basinward in the north (Swab Formation in Syria; BedinanFormation in southeast Turkey), and interior Iran (Zardkuh Formation). Mixed clastic and carbonatesettings 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 clasticenvironments (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 toEarly Ordovician palynomorphs (Al-Hajri and Owens, 2000).

In Oman, in the Ghaba and Fahud salt basins a marine-influenced environment of deposition becameestablished in the Late Cambrian. In these basins, Droste (1997) interpreted three depositional sequencesin 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 CambrianAl 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 fromthe Lower Ordovician Ghudun Formation (Droste, 1997). A thin sandy unit locally overlies thisunconformity, but generally a rapid transgression resulted in deposition of middle to outer neriticshales. The primary maximum flooding surface of this sequence is of Llanvirn age [MFS O30 dated at465 Ma], and can be traced from the Saih Nihayda in Oman, to the Hanadir Member of the QasimFormation 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 inthe basin for the first time.

During the remainder of the Middle Ordovician, the Arabian Plate was covered by a major marineprograding clastic sequence. These sediments were deposited in inner-neritic to estuarine or deltaicenvironments. Point sources can be recognized in Oman and northern Saudi Arabia (Figure 7). Basininwards deposition of middle to outer-neritic shales continued during the Middle Ordovician (Swaband 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 recognizedin the Kahfah, Ra’an and Quwarah members of the Qasim Formation in Saudi Arabia, and the HasirahFormation in Oman. Basinwards, the cycles are difficult to recognize as the section consists of anundifferentiated package of essentially middle to outer-neritic graptolitic shales (Zardkuh Formationin Iran, and Bedinan Formation in southeast Turkey).

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Figure 7: In the Middle Ordovician a rapid transgression resulted in deposition of middle to outerneritic shales over most of the Arabian Plate. This primary maximum flooding surface is of Llanvirnage and corresponds to MFS O30 dated at 465 Ma (Sharland et al., 2001). Locally the shales are richin organic matter, indicating restricted water circulation. Stippled areas indicate outbuilding deltasduring the subsequent regressive stage recognized in Oman and northern Saudi Arabia.

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(On top of Latest Ordovician glacial valleys)Early Silurian: Environments of Deposition

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Figure 8: With the retreat of the glaciers at the end of the Ordovician global sea level rose andresulted in widespread flooding of the Arabian platform [MFS S10 dated at 440 Ma]. In the EarlySilurian, shallow to open-marine environments were established in marginal areas, whilst deepermarine environments covered the inundated platform and extended southward along a subsidingintrashelf depression located in central Saudi Arabia. Anoxic water bottom conditions in thesediment-starved basin resulted in the preservation of organic-rich shales, the principal sourcerock for Paleozoic hydrocarbons in Saudi Arabia.

Time equivalent deposits are absent in most of interior Iran, and only local remnants have beenpreserved (Reitz and Davoudzadeh, 1995), possibly due to erosion. They are also absent in the Mardinarea 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 associatedwith 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 theLate 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 andSarah formations. Each consists of tillite and pro-glacial clastics, mostly sandstones within incisedvalleys 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, andhave been traced into the subsurface of northern Saudi Arabia with seismic data (McGillivray andHusseini, 1992; Aoudeh and Al-Hajri, 1995). The associated major fall in relative sea level is witnessedaway from the glaciated areas by a sudden influx of significant amounts of fluvial to deltaic sands ontop of deeper marine sediments in parts of the basin (uppermost parts of the Dubeidib Formation inJordan, 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 marineenvironments covered the inundated platform, and extended southward along a subsiding intrashelfbasin 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 organicrich shales–the prolific Silurian ‘hot shale’. These are the Qusaiba shale in Saudi Arabia, MudawwaraFormation in Jordan, Sahmah Formation in Oman, Abba Formation in Syria, Dadas Formation insoutheast Turkey, and Ghakum Formation in Iran. The Qusaiba is the principal source rock for Paleozoichydrocarbons 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-upwardsequence of shales and sandstone of Llandovery to Pridoli age (e.g Qalibah in Saudi Arabia), whichprograded basinward (Mahmoud et al., 1992). However, middle to outer neritic environments persistedin the north (Abba Formation in Syria, and Dadas Formation in southeast Turkey) and east (GahkumFormation 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 primarilydue 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 clasticsof the uppermost Silurian-Lower Devonian Tawil Formation are followed by the marine, Pragian toEmsian Jauf Formation (Al-Hajri et al., 1999), [MFS D10 dated at 402 Ma, and MFS D20 dated at 393Ma]. Marine incursions also reached Oman where the Misfar Formation was deposited. The latterincludes anoxic mudstones deposited in lower coastal plain environments. The absence of pre-Emsiandeposits in Syria, Turkey and Iraq suggests a structural high position with respect to the depocenter inSaudi Arabia.

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

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Devonian (Emsian): Environments of Deposition

MedSea

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Arabian Gulf

Gulf of Aden

Caspian Sea

ArabianSea

Red Sea

(Devonian in Metamorphic Complexes)

Shallow MarineClastics / Carbonates

Deltaic

Marine

Shallow MarineMainly Shales

Shallow MarineClastics / Carbonates

Continental Clastics (age determination uncertain)

Figure 9: A large delta front developed in Saudi Arabia, which in the north is replaced by mixedmarine siliciclastics and carbonates. Continental environments became established in central Arabia,Syria and Iraq, whilst marginal marine environments developed in Turkey and Oman.

The return of marine environments, especially during the latest Devonian in the northern region (ortheir preservation), suggest differential downwarp of the northern margin of Gondwana. Similarrelationships can be observed in interior Iran. Uppermost Devonian strata rest directly on Cambrianor Lower Ordovician sequences in the Alborz Mountains in Iran, whilst a more continuous Paleozoicsection 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).

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Basement

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Devonian

Basement

Carboniferous

in Proto-Palmyra tro

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Hercynian Subcrop

MedSea

Gulf of Oman

Arabian Gulf

Gulf of Aden

Caspian Sea

ArabianSea

Red Sea

Abu Dhabi

Damascus

Cambro-Ordovician

Cambro-

Ordovician

SAUDI ARABIA

Basement

Figure 10: Isolated occurrences of Lower Carboniferous sequences are preserved under theHercynian unconformity (e.g. in the NE-trending proto-Palmyra trough in Syria and in isolatedwells in Saudi Arabia). In other areas of the Plate, the Carboniferous is largely missing due towidespread uplift and erosion during the Hercynian Orogeny, and older Paleozoic rocks subcropunder the unconformity.

Konert et al.

Carboniferous

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

The basal part of the Doubayat section in Syria comprises Tournasian to lowermost Visean shallowmarine shale, with subordinate sandstone and siltstone, and bioclastic carbonates. Incomplete biozones

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Early Permian: Environments of Deposition

MedSea

Gulf of Oman

Arabian Gulf

Gulf of Aden

Caspian Sea

ArabianSea

Red Sea

Abu Dhabi

ContinentalClastics

Deltaic toShallow Marine

Erosion

Depositional

Directio

Depositional

Direction

Depositional

Directio

Figure 11: In Saudi Arabia the Lower Permian deposits are continental. Towards the southeast,marine influence is evident with shallow-marine carbonates being deposited in Oman. Arrows inSaudi Arabia indicate the location and depositional direction of the main channel complexes in theoverlying basal Khuff clastics.

are indicative of intraformational depositional hiatuses. These are followed by fully marine carbonatesof Visean age, reflecting the maximum extent of the transgression. The overlying sequences are partof a regressive complex made up of near shore to deltaic clastics. These sediments range in age up tothe Stephanian. Thinning and pinching out of the carbonates, and variations in sand/shale ratios ofespecially 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 beattributed to the Hercynian Orogeny (see below).

Isolated occurrences of Carboniferous siliciclastics have been penetrated in Saudi Arabia (Al-Hajriand Owens, 2000). They consist of poorly dated, syn-Hercynian continental sandstones of the Berwathand 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 JuwaylFormation (Helal, 1966) have been preserved (Figure 4). The deposits are related to uplifted areaslocated southeast of Oman (Al-Belushi et al, 1996). Deposition in glacial environments in Omancontinued during the Early Permian.

Early Permian

The first extensive deposits following the Hercynian Orogeny are the Upper Carboniferous-LowerPermian clastics that rest with angular unconformity (Hercynian unconformity) on older Paleozoicrocks and basement. They were partly deposited coeval with rift tectonics along the eastern andnorthern margins of the Arabian Plate. These sediments appear to be missing in Yemen and over theCentral 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 andAl-Duaiji, 1995). They are replaced basinward by braid plain deposits overlain by shallow-marinenear-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 Hercynianstructures.

The Lower Permian section in Oman embraces shallow marine carbonates (Haushi Limestone of theLower Gharif Member) of Sakmarian age (Figure 11). The initial transgression is witnessed in thedeeper 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 Gharifmember), suggesting diminishing basin topography. The later Artinskian documents a sudden increasein sand content brought in by rivers, probably in response to uplift in the source areas associated withincipient 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 accompaniedthe 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 marinesandstones 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 carbonatesand 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.

Konert et al.

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. Duringregressions, restricted evaporitic environments became established on the western part of the platformprotected by shoals from open seas in the east (Figure 12). In the High Zagros and Oman Mountainsdeep-marine environments were established.

Reef Prone ShelfEdge Carbonates

Continental Clastics

Evaporites

ShallowMarine Clastics

Dolomite

ShallowMarine

Carbonates

Mixed ShallowMarine Clastics and Evaporites

OoliticShoals

SAUDI ARABIA

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Carbonates

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Late Permian: Environments of Deposition

MedSea

Gulf of Oman

Arabian Gulf

Gulf of Aden

Caspian Sea

ArabianSea

Red Sea

Figure 12: The Upper Permian (Kungurian to Tartarian) is characterized by the deposition ofcarbonates and anhydrites over the entire Arabian shelf in shallow-marine to tidal- flat environments(modified from Al-Jallal, 1995). Deeper marine carbonate deposits occur at the eastern tip of theplate in Oman and Iran.

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 removalof 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 ArabianPlate underwent multiple phases of compression during this orogeny. The structural observations areconsistent with a NW-directed principle compressive stress.

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

The Hercynian Orogeny resulted in a major change in basin geometry as revealed by the Hercyniansubcrop (Figure 10). This map shows a NE-trending basement high protruding into the basin in centralArabia, the Central Arabian Arch. Facies patterns and thickness variations in Devonian-Silurian andolder 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 (WadiSirhan and Jauf) graben were also activated, and are associated with large uplifts accompanied bydeep erosion (Figure 13).

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

Additional evidence for Hercynian tectonism stems from structural observations. Figure 13 illustratesthe structural and stratigraphic relationships in the northern Arabian Plate. The section shows thatthe 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 aregional-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 southSyria Platform (compare with Figure 2). Folded Ordovician-Silurian rocks can also be observed belowthe base Triassic and younger unconformities farther south. Finally, the distribution of the Carboniferoussequences suggests that the area was affected by a phase of differential uplift prior to deposition in theTriassic.

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

The NS-trending Hercynian uplifts, such as Ghawar, are bounded by reverse faults, suggesting thatthe uplift was due to a regional compressive stress field. In general, post-Hercynian pre-Permianerosion reduced the relief, but not completely, as indicated by thickness and facies variations in theUnayzah Formation. Many of the Hercynian faults bounding the major N-S uplifts were reactivatedduring the Triassic and especially during the Late Cretaceous, as indicated by the dramatic thickeningof 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-HercynianCarboniferous sequences generally rest on Ordovician or older deposits. Devonian rocks are onlylocally preserved. No folding or reverse faulting is known in Oman, suggesting that Hercynian eventswere essentially vertical in nature.

Konert et al.

10,000

Precambrian Salt

ambrian/P recambrian

Silurian

Precambrian Basement

SAUDI ARABIA QATAR

Dilam Mazalij Ghawar

Tertiary

Dukhan Southern GulfSalt Basin

INDEX MAP

West East

Cretaceous

Jurassic

Ordovician/Cambrian

Triassic

Permian

Devonian

Arabian Shield

Section

Figure 14: Geologic traverse through Saudi Arabia to Qatar (see also Alsharhan and Nairn, 1994).For location of section see index map.

AzraqGraben

Basalt Plateau

JORDAND

LowerCambrian

U.- M. Ordovician

L. OrdovicianM. Cambrian

Jurassic

Triassic

Carb.TertiarySilurian Cretaceous

-1,000

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-6,000

ASouth

SilurianDepositional

Thinning

0Datum: 107 m above mean sea level

South Oman Salt Basin Ghaba Salt Basin

Tertiary

Cretaceous

Ordovician/Cambrian

Precambrian

suorefinobraCDevonian

Ordovician

Permian

Jurassic

South NorthE F

Precambria

Figure 15: Geologic traverse through Oman. For location of section see index map.

Figure 13: Geologic traverse from Jordan through Syria to Turkey. For location ofsection see index map.

0 5 100

NorthSYRIA TURKEY

-2,000

-4,000

-6,000

EuphratesGraben

SinjarTrough

DiyarbakirBasin

Upper-MiddleOrdovician

Devonian

CarboniferousTertiary

Permian

Silurian

Devonian

Triassic

Silurian

Konert et al.

Further evidence for Hercynian movements, though still highly speculative, is derived from theSanandaj-Sirjan ranges of Iran (see Figure 1) and the Oman Mountains. In the former, intensely foldedmetamorphic 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). Inthe Oman Mountains, the Permian rests unconformably on highly deformed and metamorphozedLower Paleozoic rocks attributed to the Hercynian Orogeny (Mann and Hanna, 1990). The deformationcombined with the metamorphism indicates that the future Zagros margin was possibly a zone oftranspressional movements.

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Silurian and Precambrian: Petroleum System Map

North Dome Field

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Abu Sa’fah

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Silurian Source Rockand Ordovician Reservoirs

Precambrian Source Rock and Cambrian to

Permian Reservoirs

Silurian Source Rock and Carboniferous/Silurian Reservoirs

Silurian Source Rock and Devonian Reservoirs

Abu Dhabi

Figure 16: Geochemical studies prove the presence of Silurian and Precambrian derived fluids inmany fields over a wide geographical area from Turkey to Oman, and from Saudi Arabia to Qatar.These fluids occur as mixtures or end-member crudes that have distinct chemical fingerprints.

HYDROCARBON HABITAT

Source Rocks

Our working definition of a petroleum system differsfrom published definitions (Magoon and Dow, 1994).A petroleum system is here defined as the total spaceoccupied by all hydrocarbons derived from onechemically distinguishable source rock interval. Thisdefinition places the emphasis on establishinghydrocarbon availability. Understanding theSilurian petroleum system yields one of the keys tounlocking most of the Paleozoic resources.

Organic-rich source rocks exist throughout the basinat the base of the Silurian shales (Figure 8). Theseare dark-gray to black, containing marine algae,acritarchs and abundant chitinozoans andgraptolites. Source rock quality and thickness varieswith depositional environment, as demonstrated by‘source out’ into a shallow-marine bioturbated,sandy, micaceous claystone facies in basin marginsettings. Source rock net thickness varies fromhundreds of meters in the Rub’ Al-Khali Basin, tosome 50 m in the northern basin, to a few meters inmarginal settings (Mahmoud et al., 1992; Aoudehand Al-Hajri, 1995; Jones and Stump, 1999). InJordan, Syria and Iraq, in areas of greateraccomodation space, a younger source-rock level hasbeen observed of probable Wenlock age.

Oil to oil, and oil to source-rock correlations indicatethe presence of Silurian derived fluids over a widegeographical area from Turkey to Oman, and fromSaudi Arabia to Qatar (Figure 16). They occur asmixtures or end-member crudes that have distinctchemical 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 complexburial histories. These include burial cyclesinterrupted by major uplifts, especially during theHercynian Orogeny. This may render theinterpretation of Vitrinite Reflectance (VR)measurements difficult in terms of timing, especiallyconsidering the uncertainties in thermal history.

GAMMA RAY RESISTIVITYDEN/NEU POR

FORMATION/

MEMBER(API)

LITHOLOGY

.2 2,000

(OHM-METERS)

DENSITY (GRAMS/CMI)

2.0 3.0

45.0 -15.0NEWPORC (%)

SUDAIR

D-ANHYDRITE

Sandstone

Siltstone

Limestone

Dolomite

Anhydrite

Reservoir

Hercynian Unconformity

Figure 17: Composite log of thePaleozoic section in the Ghawar areain eastern Saudi Arabia. Reservoirsections are indicated in red (Jauf,Unayzah A and B, and Khuff A, B,and C reservoirs).

Konert et al.

7 65 4

approx. 4,100 ft

approx. 5,700 ft

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

The predicted cumulative volumes of oil and gas expelled from the Silurian shale depocenter, containedwithin 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 windowrange from 3,000 to 3,600 bbls of oil and 21,595 to 39,200 TCF of gas. Assuming that about 90% of thepredicted volumes were lost either due to migration losses or model inaccuracies, then between 48billion to 83 billion barrels of oil and oil equivalents are predicted to be recoverable where the sourcerock is within the oil window. For the gas window, another 380 to 439 bbls of oil and oil equivalent arepredicted with the same assumptions. The Paleozoic exploration frontier may, therefore, offer aHydrocarbon-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 evidencefrom the Shamah oil field in Oman indicates that the Unayzah condensates are derived from a differentsource rock, as yet unidentified.

The late Precambrian rocks in Oman constitute another group of established petroleum systems (Terkenand 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 entirePhanerozoic. The hydrocarbons have been linked to several source-rock intervals deposited in thepre- to syn-rift sequences. They embrace carbonate source rocks, which contain mainly Type I/IIorganic matter with total organic carbon contents (TOC) of up to 7%. Silicilyte source rocks are foundin intrasalt settings. They have variable TOCs ranging up to 10%, and may occur in massive sectionsup to 1,750 m thick. They are considered world class source rocks that are characterized by anomalouslylow activation energies.

Finally, a group of hydrocarbons have been defined in Oman, the so-called ‘Q’-oils that have distinctgeochemical characteristics (Grantham et al., 1987). The exact source of these hydrocarbons remainsto 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) containthe main reservoirs of the Silurian petroleum system. They are sealed by intraformational claystoneand shale, or by tight carbonates and evaporites. The regional seal in Saudi Arabia and Oman isTriassic shales of the Sudair Formation, which completely separate the Silurian hydrocarbon systemfrom the overlying Mesozoic systems. Lack of seals renders little prospectivity to the Permian sequencesin northern Arabia.

The Carboniferous to Devonian sequences may include excellent reservoirs, especially the Devonianof 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 thereservoirs 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 dominatedouter 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.

Konert et al.

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

The pre-Silurian section becomes an important target in addition to the Permian in those regionsunderlain by Precambrian source rocks. The trapping potential in the Cambrian-Ordovician basinmargin sections, made up of massive coarse clastics, depends on truncation, which juxtaposes themagainst Permian-Carboniferous or younger seals. Seal potential increases basinward in parallel withchanges 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 reservoirsbecomes highly dependent on the diagenetic history.

ESTABLISHED PLAYS

The established frontier Paleozoic plays include the Permian Khuff, Permian-Carboniferous Unayzahand 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 in1949. Subsequent gas discoveries were made in deeper pool tests of the major structures in AbuDhabi, Iran, Oman and Saudi Arabia. In 1971, the world’s largest gas field, the North Dome Khuffreservoir 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 ofhydrocarbons 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.

50 km Precambrian Basement

WAQR GHAWAR TINAT

Dhruma

Minjur

Sudair

Unayzah

JaufTawil

Sharawra

Qusaiba

Hercynian Unconformity

Dhruma

Sudair

Unayzah

Qusaiba

TawilB

Dhruma

West East

Figure 19: Schematic E-W structural cross-section across the Waqr-Ghawar-Tinat fields, showingthe major Paleozoic plays. A=Khuff play; B=Unayzah structure play; C=Hypothetical Unayzahstratigraphic play; D=Jauf truncation play; E=Ordovician play.

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

The quality of the reservoirs varies from excellent to poor, with abrupt lateral and vertical variationsin porosity and permeability. These are controlled by dolomitization, leaching, fracturing, andcementation (particularly by anhydrite). Leached zones often form the better portion of the reservoir.Reservoir porosity types range from primary intergranular to secondary oomoldic. Reservoirpermeability 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 hamperedby uncertainties. On the other hand, these factors suggest that the Khuff has considerable potentialfor stratigraphic traps, as yet unexplored. 3-D seismic data has proven to be a good approach fordelineating 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 theunderlying 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 inflanking regions (Wender et al., 1998).

Unayzah/Gharif Play

Oil in the Permian Gharif sandstones was first discovered in 1972 in the Ghaba North structure inOman, and the subsequent campaign demonstrated the economic viability of the play throughoutOman. In Saudi Arabia, the potential of the Permian Unayzah was confirmed in 1979 by a gas discoveryin the Qirdi field. The Unayzah play became much more significant in 1989, when super light oil wasdiscovered in Hawtah-1 in central Saudi Arabia. Another 16 Unayzah fields have been discovered inthe past decade. The fields are structural closures along Hercynian basement-cored uplifts, that maybe transpressional in origin (Simms, 1995). Moreover, the stratigraphic variability of the Unayzah,influenced by paleotopography and the continental environments of deposition, lends a stratigraphiccomponent 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 drivenby influx of meteoric water from outcrops along the western edge of the basin (Figure 14). The Siluriansource rocks in central Saudi Arabia are immature, and the Unayzah oils were evidently generated inthe deeper parts of the basin and migrated about 200 km westwards towards the basin margin (AbuAli 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 areseparated by silt- and mudstone (McGillivray and Husseini, 1992). The sandstone reservoirs are laterallydiscontinuous, 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 onedarcy, particularly in the eolian sandstone facies. The top seal are transgressive shales at the base ofthe 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 ofsix additional Unayzah gas/condensate fields near Ghawar field, such as Waqr and Tinat (Figure 19).

Konert et al.

The Unayzah deep gas play presents additional challenges, which include poor seismic imaging ofthe 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 theacquisition 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 northGhawar field. Subsequent tests showed that the Devonian section was mostly eroded from the crestof the structure. The discovery in 1994 of Jauf gas in a combination structural-stratigraphic trap alongthe flank of the Ghawar structure was a major exploration success, especially in light of the poorseismic 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 haveundergone extensive silica cementation, the Jauf reservoir is weakly cemented with authigenic illitethat 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 cationexchange capacity of illite. This can cause pessimistic water saturation estimates and lead to potentiallybypassed low-resistivity pay zones (Wender et al., 1998).

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

Mass Flow Sands

Eroded Deltaic Complex

Braid Delta Sands

Potential Trap

Basal Transgressive Sands

PERMIAN-CARBONIFEROUS

Marine Mudstone

Figure 20: Conceptual diagram showing the Ordovician mass flow sand play in Oman. Themass flow sands were deposited in outer-shelf environments within active salt withdrawalbasin. The reservoirs occur at an average depth of 3000m and the sandstones exhibit excellentreservoir qualities, with porosities of up to 32%.

Cambrian-Ordovician Plays

Saudi Arabia

Several discoveries have been made in Upper Ordovician structural traps, including Dilam and AbuJifan in central Saudi Arabia, Kahf and Jalamid in northern Saudi Arabia, and Wadi Sirhan and Rishain 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 lowporosities and permeabilities due to compaction and extensive cementation by quartz overgrowthsduring burial. Petrographic studies indicate that any significant porosity is secondary, due primarilyto dissolution of early intergranular carbonate cement. The early carbonate cementation was localizedin 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 sandstonesunderwent significant compaction. The subsequent dissolution of carbonate cement precededhydrocarbon migration into the reservoirs.

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

Figure 21: Conceptual diagram of the Cambrian-Ordovician Haima play in Oman.

Ordovician Sandstones

Existing FieldsMESOZOIC

TERTIARY

Massive

Cambrian Sandstones

Main Objective

Ara Salt

Regional Seal

Deep Gas Play

Sub-Salt

PERMIAN-CARBONIFEROUS

Konert et al.

Since the first Upper Cambrian Haima gas/condensate discovery in 1989, Oman has booked about17.6 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 considerablechallenge 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 latterresult in the deposition of a non-reservoir heterolithic, shallow-marine faces. Eustatic changes gaverise to eight stacked flow units. Reservoir characteristics vary with overall position within thedepositional 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 thepresence of an early oil charge that inhibited quartz overgrowth and dolomite cementation. Thepresence of higher quality thief zones complicates reservoir management through introducing a riskof 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 itsproduction is optimized by reservoir models that account for fractures. Hydraulic fracture stimulationof 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 Permiancarbonates. Hydrocarbons were mainly derived from the prolific Silurian ‘hot shale’ that extendsover most of the basin. Tectonostratigraphic relationships indicate that the platform southwest of theZagros Suture was generally stable until the Hercynian Orogeny that started in the latest Devonianand 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 basementcored uplifts (e.g. Ghawar field). In the Early Permian, rifting along the eastern margin led to theopening of the Neo-Tethys Ocean.

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

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

ACKNOWLEDGMENTS

This paper is based on the work of numerous individuals who cannot all be justly mentioned. Specialthanks 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., theOman Ministry of Oil and Gas, and the Saudi Arabian Ministry of Petroleum and Mineral Resourcesfor 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 tothe work of GeoArabia Special Publication 2 by Sharland et al. (2001). We thank Moujahed Al-Husseiniand Joerg Mattner of GeoArabia, and Peter Sharland from Lasmo for their assistance in preparing therevised version. The design and drafting of the final graphics was by Gulf PetroLink.

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Abdulkader M. Al-Afifi is Chief Explorationist, Southern AreaExploration, with Saudi Aramco. He received a BSc degree in Geologyfrom King Fahd University of Petroleum and Minerals, Dhahran,an MSc from the Colorado School of Mines, and a PhD degree fromthe University of Michigan, Ann Arbor. Abdulkader workedpreviously with the US Geological Survey Mission in Jeddah priorto joining Saudi Aramco in 1991. He is a member of the AmericanAssociation of Petroleum Geologists and Society of PetroleumEngineers.

Geert Konert is Principal Geologist for Shell InternationalExploration and Production B.V. in Research and Technical Services,Rijswijk Netherlands. He graduated from the University ofAmsterdam in Geology, Structural Geology and Geochemistry in1981 and joined Shell the same year. Geert has worked on variousexploration assignments in Brunei, The Netherlands and Oman, andhas been involved in E&P projects in the Middle East. His main areaof interest is the tectonic evolution of the Middle East.

Sa’id Al-Hajri is Chief Geologist of the Regional Mapping andSpecial Studies Division of the Saudi Aramco. He holds a BSc inGeology 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 andstratigraphy of northern Gondwana. He is a member of CIMP, AASP,BMS and the DGS, and has published several papers on geologicaland palynological subjects.

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Khuff) hydrocarbon geology of the Ghawar Area, Eastern Saudi Arabia. GeoArabia, v.3, no. 2,p.273–302.

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Konert et al.

Henk H.J. Droste joined Shell in 1984 after receiving his MSc inGeology from the University of Amsterdam. He worked as a Car-bonate Geologist with Shell Research in The Netherlands and as aSedimentologist in the Regional Studies Team of Shell Expro in Lon-don. He was transferred to PDO Oman in 1992 where has beenworking as a sedimentologist in the Exploration Laboratory, Geolo-gist/Seismic Interpreter in Exploration, Production Geologist of theYibal Field and as a Team Leader of the Regional Studies and Geo-logical Services Team. In 2001 he was posted to the Carbonate Re-search Centre located in the Sultan Qaboos University of Oman.

Paper presented at 4th Middle East Geosciences Conference and Exhibition,GEO 2000. Bahrain, March 27-29, 2000.

Manuscript Received September 6, 2000

Revised March 10, 2001

Accepted March 15, 2001

Konert Paleozoic Stratigraphy

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