Shu-Zheng Yin,Guo-Ping Bai,Lin Gao,Chuan-Yang Xu

College of Geosciences,China University of Petroleum(Beijing),Beijing 102249,China

Abstract The United Arab Emirates(UAE)is the 8thlargest oil producing country and is rich in oil and gas resources.By the end of 2015,68 oil and 23 gas fields had been discovered.The initial proved and probable(2P)oil,gas and condensate reserves amount to 81,135.9 MMb(million barrels),192.09 Tcf(trillion cubic feet),and 6496.58 MMb respectively,which are mostly reservoired in the Jurassic and Cretaceous carbonates.With the latest field data,this study attempts to document the salient features of petroleum systems in UAE.Based on depositional facies of source rock intervals,pods of source rocks were delineated.On the basis of an oil and gas-source correlation, five known petroleum systems were identified and they are Lower Silurian-Upper Permian Khuff gas,northeast fore land Upper Jurassic-Lower Cretaceous gas,Upper Jurassic—Jurassic petroleum,Upper Jurassic/Lower Cretaceous-Lower Cretaceous composite petroleum,and Middle Cretaceous-Middle to Upper Cretaceous/Cenozoic petroleum systems.Of them,the Upper Jurassic/Lower Cretaceous-Lower Cretaceous composite petroleum system contains 73.2%of the total 2P reserves and thus it is the focus of this study.The Upper Jurassic and Lower Cretaceous source rocks consist of argillaceous limestone,mudstone and shale,which were deposited as intrashelf basin facies.The distribution of oil and gas in this system is controlled by the source kitchens and the regional evaporite seal.

Keywords UAE,Carbonate reservoir,Intrashelf basin,Petroleum system,Evaporite seal

1. Introduction

The United Arab Emirates(UAE)produced 182.4 million tonnes of oil(or 4.07 MMb/day)in 2016 and was ranked 8thin the world(BP,2017).It is rich in oil and gas resources.Since the discovery of its first oil field—the Bab Field in 1954,a total of 68 oil fields and 23 gas fields has been discovered in UAE by the end of 2015(Fig.1).They host a total of initial proved and

＊Corresponding author.

E-mail address:baigp@cup.edu.cn(G.-P.Bai).

Fig.1 Major structures and oil and gas fields in UAE(modified from Alsharhan,1993).Note:A—A′marks the location of the geological cross section shown in Fig.2.

Peer review under responsibility of China University of Petroleum(Beijing).probable(2P)reserves of 81,135.9 MMb of oil,192.09 Tcf of gas and 6496.58 MMb of condensate,which amount to 119,775.0 MMboe (million barrels oil equivalent).As UAE is a major oil producing country,its general petroleum geology has been extensively studied by a number of authors including Alsharhan(1989),Alsharhan and Kendall(1995),Alsharhan and Nairn(1997)and Glennie(2010).However,the petroleum systems in UAE have not been systematically documented in the public domain.With the latest data of oil and gas fields and results of previous studies of source rocks and depositional facies,this study attempts to document the distribution of petroleum systems and their salient features in UAE.

2. Synopsis of petroleum geology

Fig.2 Regional geological profile across UAE(modified from O'Donnell et al.,1995;Peterson and Wilson,1986).Refer to Fig.1 for its location.

Based on IHS Energy and our in-house field data base,we carried out statistical analysis.The results indicate that the structural trap is the dominant type in UAE.The structural traps,which consist of anticlines,host 94.6%of the discovered 2P reserves and the rest 5.4%are contained in structural—stratigraphic combination traps.In the Rub Al Khali Basin,the formation of anticlines is attributed to two mechanisms:basement uplifting and salt movement.The anticlinal structures including Mender and Zarrarah anticlines in the southeastern part of UAE were formed by basement uplifting.They took their initial forms in Early Cretaceous and grew into the current forms in Late Cretaceous and Cenozoic times(Alsharhan and Nairn,1997).As shown in Fig.1,these structures are mostly elongated NE—SW trending anticlines.In the central part of UAE,the anticlines including Shah,Asab,Sahil,Bab,Bu Hasa,Mubarraz,Ghasha,Zakum,Fateh and Huwaila are dome-shaped or brachy-formed anticlines.They trend in N—S,E—W,SW-NW and NW—SE directions and were formed by salt movements in the period of Late Cretaceous to present day(Alsharhan and Nairn,1997).

In the eastern part of UAE corresponding to the Oman Basin,anticlinal structures including Margham and Sajaa faulted anticlines trend N—S and started to form in Late Cretaceous but were largely formed in Miocene(Alsharhan and Nairn,1997).Their formation was closely related to the uplifting and compression of the present day Oman Mountains.The pre-exiting faults in the area also had a considerable impact on the formation of these anticlines.

The discovered oil and gas in UAE are almost all reservoired in carbonates in the Upper Permian to Neogene(Fig.3).They concentrate in three major reservoir intervals of Lower Cretaceous,Upper Jurassic and Upper Permian,which host 92.2%of the total proved and probable reserves.Gas tends to be in older reservoir intervals and oil in younger ones.

3. Petroleum systems

3.1. Geological elements

In petroleum system analysis,source rock is the key and its distribution is largely controlled by the depositional environment(He and Luo,2010;Tao et al.,2013).Based on previous studies including Gumati(1993),Alsharhan and Nairn(1997),and Sharland et al.(2001),oil and gas in UAE were considered to be derived from four sets of source rocks:shelf shales in the Qusaiba Member of the Lower Silurian Qalibah Formation,argillaceous lime mudstones in the Lower Jurassic Diyab Formation,argillaceous lime mudstones and shales in the Bab Member of the Lower Cretaceous Shuaiba Formation,and argillaceous lime mudstones in the Middle Cretaceous Shilaif Formation(Fig.3).The Jurassic and Cretaceous source rocks were all deposited in intra-shelf basin environments.

During the Late Permian to Middle Cretaceous,the Arabian Plate lay in the passive margin of the Neo-Tethys.A vast shelf was developed on the northeastern margin of the Arabian Plate.It was very extensive and had a width of several hundreds of kilometers(Murris,1980;Ziegler,2001).Within the shelf,differential intra plate subsidence led to the development of intra-shelf basins(Ziegler,2001),which had a water depth deeper than the surrounding shelf.The deeper restricted intrashelf basinal setting resulted in an anoxic environment so that organic rich source rocks were deposited in the intrashelf basins.

Fig.3 Composite stratigraphic column for UAE(modified from Alsharhan,1993;Alsharhan and Nairn,1997).Note:There exist two regional unconformities in the Cretaceous in the Middle East and they divide the Cretaceous into Lower,Middle and Upper Cretaceous,rather than the two-fold division in the international stratigraphic chart(Alsharhan and Nairn,1997).

Fig.4 Paleogeographic maps of UAE and surrounding areas.a-early Late Jurassic(modified from Ziegler,2001);b-late Middle Cretaceous(modified from Sharland et al.,2001).

In the Late Jurassic,the southern Gulf intrashelf basin was developed in the southern Arabian Gulf and western UAE(Fig.4a).The Diyab Formation,containing the most important source rocks in UAE,deposited in this basin.It is dominated by argillaceous lime mudstones in the intrashelf basin and changes eastwards to shallow water platform wackstones and packstones(Fig.4a).

In the Aptian of Early Cretaceous,an intrashelf basin was developed in the central part of UAE and it received the deposition of argillaceous limestone and shales,which comprise the source rocks in the Bab Member of the Shuaiba Formation.The intrashelf basin was surrounded by ruidist reefs and algal buildups of the Shuaiba Formation(Fig.4b),which constitute good quality reservoir rocks.

By the Albian time of Middle Cretaceous,an intrashelf basin was formed in the central UAE and it was more extensive than the one developed in the Aptian time.In the intrashelf basin,organic-rich argillaceous lime mudstones were deposited and they make up the source rocks in the Shilaif Formation.Due to their shallow burial,these source rocks are immature and marginally mature so that they only made a minor contribution to the total oil and gas reserves in UAE.Rudist-algal packstones to grainstones of the Mishrif Formation accumulated as shallow water shelf facies under high energy conditions on the basin margins,possibly forming reservoir rocks(Alsharhan,1989).

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The discovered oil and gas are reservoired in multiple intervals ranging in age from Permian to Neogene(Fig.3).The Lower Cretaceous Kharaib and Lekhwair Formations,Upper Jurassic Arab Formation and Lower Cretaceous Shuaiba Formation are the four major reservoir intervals,which host 36.1%,20.2%,13.6%and 13.0%of the total proved and probable oil and gas reserves in UAE respectively.The discovered oil and gas are largely sealed by three regional and semi-regional seals,consisting of tight carbonates in the Lower Triassic Sudair Formation,evaporites in the Upper Jurassic Hith Formation and shales in the Middle Cretaceous Nahr Umr Formation.In addition,there exist a number of local seals throughoutthe Permian to Neogene succession(Fig.3).

3.2. Identification of petroleum systems

As defined by Magoon and Dow(1994),a petroleum system is a natural system which encompasses a pod of active source rocks and all the essential elements and geological processes for the formation of a hydrocarbon accumulation.Its distribution is confined by the geographic extent of the source kitchen and the maximum distance of lateral and vertical migration.

With the latest data base for oil and gas fields in UAE and the results of oil-oil correlation and oil-source correlation,the discovered oil and gas are classified into five known petroleum systems:Lower Silurian-Upper Permian Khuff gas(simplified as S1gas),northeast foreland Upper Jurassic-Lower Cretaceous gas(NE J3gas),Upper Jurassic—Jurassic petroleum(J3petroleum),Upper Jurassic/Lower Cretaceous-Lower Cretaceous composite petroleum(J3/K1composite petroleum),and Middle Cretaceous-Middle to Upper Cretaceous/Cenozoic petroleum(K2petroleum)systems.The approach proposed by Magoon and Beaumont(1999)was applied to delineate the distribution of the five systems.These petroleum systems overlap each other geographically(Fig.5).The S1gas,J3/K1composite petroleum,andK2petroleum systems occur over most of UAE.In contrast,the J3petroleum system is limited to the western part of UAE and the NE J3gas system is confined to the northeastern part of UAE.

Of the five known petroleum systems in UAE,the J3/K1composite petroleum system hosts bulk of the discovered hydrocarbon reserves containing 73.2%of the total proven and probable reserves.It is followed by the J3petroleum system hosting 15.7%of the total,S1gas system(4.8%),K2petroleum system(4.0%),and the NE J3gas system(2.1%).In terms of the discovered reserves,the J3/K1composite petroleum system is the most important one in UAE and thus will be discussed in details in the following section.No further discussions will be made for the other four systems.

4. Upper Jurassic/Lower Cretaceous-Lower Cretaceous composite petroleum system

4.1. Salient features

The Upper Jurassic/Lower Cretaceous-Lower Cretaceous petroleum system is distribute dover most of UAE(Fig.5).128 oil/gas accumulations from 46 fields are classified into this system.Their proven and probable reserves amount to 67,300.5MM bofoil,97.64Tcfof gas and 4097.8 MMb of condensate,or 87,670.8 MMboe.

The system is a composite petroleum system as it has two source rock intervals:Upper Jurassic Diyab Formation and the Bab Member of the Lower Cretaceous Shuaiba Formation.The Lower Cretaceous Thamama Group,which consists of Habashan,Lekhwair,Kharaib,and Shuaiba Formations from the base to top(Fig.3),comprises the reservoir interval.High-energy grainstones and packstones as well as rudist reefs form reservoir rocks.The Middle Cretaceous Nahr Umr shales constitute the regional seal for the underlying Thamama reservoirs(Fig.6).

Fig.5 Distribution map for five known petroleum systems in UAE.

Fig.6 Events chart showing the relationship between essential elements and geological processes as well as preservation time for the Upper Jurassic/Lower Cretaceous-Lower Cretaceous petroleum system.

4.2. Characterization of source rocks

The Diyab Formation in onshore UAE correlates to the Dukhan Formation in offshore UAE.The formation and its stratigraphic equivalent intervaloccur throughout UAE and vary from 240 m to 480 m in thickness(Al-Suwaidi et al.,2000).Based on lithology and GR logs,the Diyab Formation is subdivided into the Lower,Middle and Upper Members.It was deposited as the intrashelf basin,slope,shelf margin and shelf facies from west to east(Fig.7).Source rocks were developed in the intrashelf basin and reservoirs in the shelf margins.

In the western Abu Dhabi,lime mudstones in the Lower Member of the Diyab Formation and correlative stratigraphic interval are rich in organic matter with an average TOC content of 1.5%.As part of the organic matter has been converted to hydrocarbons,the measured values are actually lower than the original values.Thermal simulation indicates the original TOC abundance should be 30%—50%higher.Source rocks in the Lower Member are up to 200 ft(61 m)in the central intrashelf basin(Al-Suwaidi et al.,2000).

Fig.7 Depositional model of the Lower Member of the Diyab Formation(slightly modified from Al-Suwaidi et al.,2000).Note:Wells A,B,BH and S are located in the Asab,Bab,BhHasa and Sahil Oil Fields,which are shown in Fig.1.

In the eastern Abu Dhabi,lime mudstones in the Lower Member have low S2values of only 0.2—0.5 gm/kg and low TOC contents of less than 0.3%.The low values indicate they hardly have a significant potential for hydrocarbon generation,which is consistent with the facies variation shown in Figs.4a and 7.

In comparison with the Lower Member,the Middle and Upper Members of the Diyab Formation have a much lower hydrocarbon generation potential as their source rocks have TOC contents of less than 0.8%and the source rocks in the Middle and Upper Members are mostly 25-50 ft(8-15 m)and 25-100 ft(8-30 m)thick respectively(Al-Suwaidi et al.,2000).

By superposing the isopach map and isoline map of TOC abundances for the source rocks,the active pods of source rocks were delineated by circumscribing the“0”isopach line and “0.25%” TOC isoline(Fig.8).The active source kitchens for the Middle and Upper Members lie within the source kitchen of the Lower Member so that the pod of active source rocks in the Diyab Formation is the same as that for the Lower Member.The pod lies in the western Abu Dhabi(Fig.8).

Low mature Diyab source rocks have higher HI values of 200—250 mg HC/g TOC while high mature Diyab source rocks lower HI values of around 50 mg HC/g TOC.According to Lijmbach et al.(1992),the organic matter in Diyab source rocks is characterized by oil-prone type II kerogens.The Diyab source rocks in onshore UAE entered the oil window at 90 Ma,were at the peak oil generation at 85 Ma,and then evolved into the gas window at 15 Ma.Currently,they are overmature.As the Diyab source rocks were buried to shallower depths in offshore parts than onshore parts,they entered oil window later than the onshore ones and they are still within the oil window at present day(Al-Suwaidi et al.,2000).

The Bab Member of the Lower Cretaceous Shuaiba Formation is the other source rock interval for the composite oil system.It consists of argillaceous lime mudstones.Both Alsharhan(1989)and Lijmbach et al.(1992)considered that the intrashelf basinal lime mudstones in the Bab Member acted as good source rocks,which have TOC contents of 1%—6%and a genetic potential(S1+S2)of up to 16 mg HC/g rock.Based on the depositional facies and geochemical features,the distribution of the Bab source rocks is delineated and shown in Fig.9.Basin modeling by Azzam and Taher(1995)indicates that oil and gas in the reservoirs in the upper Thamama Group were largely derived from the Bab source rocks,which entered the oil window in Late Cretaceous(～70 Ma)and are still at the peak oil generation stage.

Fig.8 Distribution map of the Diyab source rocks.TOC isolines and source rock isopach contours are from Al-Suwaidi et al.(2000).

Fig.9 Map showing distribution of source kitchens and hydrocarbon accumulations in the composite petroleum system as well as the pinchout line of evaporites in the Upper Jurassic Hith Formation.Note:source kitchens are delineated based on facies maps shown in Fig.4 and geochemical study results shown in Fig.8.The pinchout line of the Hith evaporites is from Alsharhan and Kendall(1994).

4.3. Model of hydrocarbon accumulation

In the composite petroleum system,hydrocarbons are reservoired in the Lower Cretaceous Thamama Group.They accumulated in two different types of traps:anticlinal traps and structural/stratigraphic combination traps with the latter occurring along the margin of the Bab source kitchen(Fig.9).The distribution of these hydrocarbon accumulations is closely related to the occurrence of anhydrites in the Upper Jurassic Hith Formation.They are mostly distributed along the pinchout line of the Hith anhydrites which thin eastwards from over 450 ft(137 m)in the far western UAE to 0 ft in the eastern UAE(Alsharhan and Kendall,1994).Due to the absence of anhydrites in the eastern UAE,hydrocarbons sourced from the Diyab source rocks could migrate vertically to the reservoirs in the Thamama Group and then migrated laterally within the carrier beds in the group until they accumulated in suitable structural or structural—stratigraphic combination traps.The Diyab source rocks supplied oil to the Thamama reservoirs in the early and middle stages of hydrocarbon generation and then gas in the late stage due to the increasing source rock maturity.As a result,the traps along the anhydrite pinchout line were initially filled with oil.With adequate oil charging,they might be filled to the spill point and then oil was gradually replaced by the subsequent migrating gas.The replaced oil remigrated and accumulated in traps further away from the pinchout line(Fig.10).This is supported by the finding that gas generally makes up a larger proportion of the total reserve in the accumulations along the pinchout line whereas it does not exist or constitutes only a small portion of the total reserve in the traps in western Abu Dhabi further away from the pinchout line.

Fig.10 Oil and gas accumulation model for the Upper Jurassic and Lower Cretaceous reservoirs(modified from Al-Suwaidi et al.,2000).

Along the margins of the Bab source kitchen,the Bab source rocks are adjacent to the reservoirs in the Shuiaba and underlying Kharaib Formations so that hydrocarbons in these reservoirs are most likely to be derived from the Bab source rocks.Based on the distribution of the Thamama oil/gas accumulations,Diyab and Bab source rocks,and the Hith evaporites,this study suggests that the gas and most of the oil in the Thamama reservoirs were generated by the Diyab source rocks.The oil in the Shuaiba and Kharaib reservoirs in the shelf margins surrounding the Bab intrashelf basin was at least partly derived from the Bab source rocks.Thus,the Diyab source rocks comprise the major source and the Bab source rocks the minor source in the Upper Jurassic/Lower Cretaceous-Lower Cretaceous composite petroleum system.

5. Conclusions

1)The facies and palaeogeographic approach could be effectively applied to delineate the source kitchens in UAE.The three sets of Mesozoic source rocks were all deposited as intra shelf basin facies while the Lower Silurian source rocks as marine shelf shales.

2)There are 5 known petroleum systems in UAE,including the Lower Silurian-Upper Permian Khuff gas,northeast foreland Upper Jurassic-Lower Cretaceous gas,Upper Jurassic—Jurassic petroleum,Upper Jurassic/Lower Cretaceous-Lower Cretaceous composite petroleum,and Middle Cretaceous-Middle to Upper Cretaceous/Cenozoic petroleum systems. Of them, the composite petroleum system contains 73.2%of the total proved and probable reserves in UAE.

3)Source kitchens and evaporites are the two main controls for the generation-migration-accumulation of hydrocarbons in the composite system with oil and gas generated largely by the Upper Jurassic Diyab source rocks and subordinately by the source rocks in the Bab Member of the Lower Cretaceous Shuaiba Formation.

Acknowledgements

This study was financially supported by National Science and Technology Major Project of the Chinese Ministry of Science and Technology(No.2017ZX05005001-006)and Strategic Pioneering Project of Chinese Academy of Sciences(No.XDA14010306).We would like to thank Prof.G.Shanmugam and Prof.Keyu Liu for their constructive comments and suggestions,which greatly improved the manuscript.