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Significance of reef limestones
as oil and gas reservoirs
in the Middle East and North Africa

Middle East
by H. Stewart Edgell



The oil reservoirs of the Middle East and North Africa contain some 70% of the world's known oil reserves and about 50% of the world's natural gas reserves. Most of these are contained in high-energy carbonate platform sediments, or in fractured limestones of large, doubly plunging anticlines. A considerable proportion of oil and gas reserves of the region also occur in Cretaceous sandstones of similar structures.

Oil exploration in the vast sedimentary basins of the Middle East and North Africa is still primarily at the stage of drilling simply folded surface, or seismically defined structures. It has not yet reached the stage of exploration for stratigraphic traps and reefs. Nevertheless, a significant number of reef and fore reef limestone reservoirs have been found by drilling, and a very few have been recognized in seismic profiles. 

Limestone reservoirs in fringing reef, barrier reef, fore reef and back reef shoal facies can be recognized in parts of the region, as well as open shoal reefs. Reef walls are rare in the subsurface of the area, either because they are too narrow or because they have been eroded. There are no known atoll-type fringing reefs and isolated reef bioherm reservoirs in Libya are best compared to buried platform reefs, or reef knolls.

Different organisms have acted as reef builders in epeiric seas of the geologic past, and most ancient reefs are not built of hexacorals as with present-day reefs.

A compound fringing reef belt some 800km long ranging in age from Middle Eocene to Middle Miocene extends along the foreland shelf of the Persian Gulf margin sag-interior sag basin from Iraq into Iran. In the giant Kirkuk oil field, there is a 610m oil column mostly in reef, fore reef and shoal reef limestones. The main fringing reefs cross the Kirkuk anticlinal axis in a 20km wide belt and are Oligocene limestones. They are composed of larger Foraminifera, calcareous algae and corals, such as Actinactis. Lower Miocene reef limestone reservoirs also occur in the Asmari Limestone of certain Iranian oil fields, such as Haft Kel and Gachsaran. The Ras Gharib oil field on the eastern edge of the Gulf of Suez also produces from a Middle Miocene 'Nullipore' fringing reef reservoir, really an algal Lithothamnion reef.

Lower Tertiary (Paleocene) buried platform reefs, or coral-algal bioherms form spectacular permeable limestone reservoirs of the Intisar (Idris) A, B, C, D and E oil fields in the Sirte Basin of eastern Libya.

Rudist reefs occur in the Middle Cretaceous (Cenomanian) Sarvak Formation limestones of oil fields in the productive Bangestan Group on the Karun Shelf of southwest Iran. The Middle Cretaceous Mauddud and Mishrif formations also contain rudist reef reservoirs on local structural highs of the offshore United Arab Emirates. The Augila oil field in eastern Libya contains a rudist reef reservoir of similar age.

In the Lower Cretaceous (Aptian) Shuaiba Formation limestone reservoirs of the U.A.E., there is a large rudist reef buildup with calcareous algae and orbitolinids. This forms a subcircular reef trend in eastern Arabia and the eastern Persian Gulf, including the large Bu Hasa, Shah, Sirri and Shaybah oil fields, bordered basinward by a belt of fore reef detritus.

Permian reef limestones with abundant fusulinids, algae and corals occur in the thick Dalan Formation exposed in the High Zagros Ranges of southern Iran. They are also believed to contribute to the large gas reservoirs of the Aghar, Dalan, Kangan, and Pars gas fields.

Reef Dalan complex limestone reservoirs of the Middle East and North Africa contain ultimate recoverable oil reserves estimated to be about 64 billion barrels.

This paper was published previously only as an abstract and was presented by the author at the 10th Edgeworth David Symposium, held at the University of Sydney, September 4-5, 1997. 


In the Middle East and North Africa, an area of more than 14 million square kilometres, shallow water carbonates are very common both in outcrop and the stratigraphic sequence. This immense, largely desert region is the largest marine carbonate province in the world (Fig. 1) extending over a maximum east-west width of 8370 km and up to 4180 km from north to south. In this vast area of dominantly limestones and dolomites, petroleum exploration has already revealed a number of significant reef, and reef related oil and gas reservoirs.



Figure 1: Map of the Middle East and North Africa.




These consist of fringing reefs in the Eocene and Oligocene of giant Kirkuk oil field in Iraq, where fore reef and back reef limestones are also productive, and fringing reefs in some of the oil fields of southern Iran. Known barrier reefs are limited to Middle Eocene limestone reservoirs of the western Kirkuk Field, and the only possible case of a reef wall reservoir to-date is in the Oligocene limestones of the Bai Hassan oil field of Iraq. A well-developed fringing reef buildup of rudists forms major oil reservoirs in the Lower Cretaceous of Bu Hasa and Shah fields of Abu Dhabi, the Shaybah-Zarrara fields of eastern Saudi Arabia and Abu Dhabi, as well as in fields in Oman, offshore Qatar and Iran. In the giant Bu Hasa oil field, fore reef facies of the Cretaceous rudist reef are also productive.



Some of the clearest examples of reef limestone reservoirs are found in the subcircular Paleocene reef knolls of the Sirte Basin in Libya, which were probably originally platform reefs. Atolls are as yet unknown in the subsurface of the region. 



A primary reason that reef or reef related limestone reservoirs are so far relatively few in the Middle East and North Africa is that most exploratory drilling is still at an early stage with most emphasis on structure drilling. In addition, most known oil and gas fields are caused by basement uplift, deep-seated diapirs, or fold fractured limestones. The majority of carbonates in the region appear to have accumulated as shallow water, low-energy deposits of the open shelf, carbonate ramp type with a reef margin being rarely formed. An analogue for past carbonate depositional environments is the present-day Persian Gulf, a remnant of the Tethys, where carbonates are currently being widely accumulated, although reefs account for only a very small percentage of the area. 



Despite these apparent limitations, there is considerable geological knowledge of various reef facies in the stratigraphic successions of the Middle East and North Africa. The pioneering studies have been those in Iraq on Kirkuk and nearby anticlinal structures by geologists of the former Iraq Petroleum Company, especially Henson (1950), van Bellen (1956) and Dunnington (1958). Much of this early work was based on detailed petrographical and paleontological studies of thin sections from cores and well cuttings. For example, van Bellen examined over 20,000 thin sections from 45,000 feet of drilled wells in the Kirkuk oil field and vicinity to reach his conclusions on Eocene and Oligocene reef facies. In southern Iran, Thomas (1950, 1952) and Kent et alii (1951) have made reef studies of the major Oligo-Miocene Asmari Limestone reservoir Formation, while Edgell (1977) has studied the shoal reef facies of the Permian Dalan Formation. In Abu Dhabi, Harris (1968), Wilson (1975) and Alsharhan (1985) have evaluated the buried Cretaceous rudist reef reservoirs. In North Africa, studies by Terry and Williams (1969) of the platform reef Paleocene Intisar "A" (Idris) oil field and of productive Cretaceous rudist reef facies in the Augila oil field (Williams, 1968) have proved that buried reef limestones constitute major oil reservoirs. 



The petroleum potential of buried reef facies in the Middle East and North Africa is now beginning to be better understood. Seismic profiles of reef knolls, or platform reefs, are now known in the Sirte Basin of Libya and are actively followed up as attractive new petroleum prospects. Oligo-Miocene reef trends have now been established in Iraq and Iran and have proved to be very productive where crossed by anticlinal traps. An example is the Alborz No.1 Well drilled on an anticline 10km north of Qum, northern Iran, which encountered a narrow, very permeable Oligocene reef and blew wild at 110,000 BOPD in 1958 before blocking itself with a natural obstruction. Knowledge of Lower Cretaceous rudist reef trends in the eastern Persian Gulf Basin is now well established and contributes to the development of giant oil fields, such as Bu Hasa (Abu Dhabi), Shaybah (Saudi Arabia), and Sirri (offshore Iran).



An estimated 64 billion barrels of ultimate recoverable oil reserves are now known in the Middle East and North Africa, comprising about 7.5% of the presently known oil reserves in the region. Although reef limestones and related reef facies are never likely to account for more than 10% of the region's oil reserves, they are still very significant oil producers and future petroleum prospects.



Types of ancient reef facies

Exploration and exploitation wells, as well as outcrops in the dominantly carbonate province of the Middle East and North Africa, indicate the stratigraphic relationships of distinct reef facies. The principal types distinguished are fringing reefs, barrier reefs, reef walls, shoal reefs, reef knolls (platform reefs), and bank reefs formed over submerged highs (Fig. 2a).

Figure 2a: Reef facies and sedimentary environments (Henson, 1950).


Back reef and fore reef facies are also associated with these reefs, and the whole combination of facies in any given case is referred to as a reef complex. Wilson (1975) recognizes 9 facies belts on a rimmed platform (Fig. 2b). From sea to shore, these are 1) basin facies, 2) open marine neritic facies, 3) toe of slope carbonates (to storm wave base), 4) foreslope talus, 5) organic build-up (reef), 6) platform lime sand belt, 7) open platform (wackestone, mudstone), 8) restricted platform, and 9) platform evaporites (sabkha). Reef wall facies are rare, being observed in the Lower Cretaceous of Bu Hasa and the Oligocene of Bai Hassan, having elsewhere been either been eroded, or missed as very narrow belts due to the low density of drilling. Detrital limestones associated with reefs are much more important than the latter both in relative bulk and as oil carrier beds and reservoirs. 

Figure 2b: Reef and associated facies belts on a rimmed shelf (Wilson, 1975).



Fringing reefs are found along the foreland shelf part of the Persian Gulf Basin (a margin sag-interior sag basin) in limestones of the Miocene, Oligocene, Eocene, and Cretaceous (Maastrichtian, Cenomanian and Aptian). The best known fringing reef in the Middle East extends through the giant Kirkuk oil field of Iraq (Fig. 3) ranging in age from Middle Eocene to Oligocene. The Kirkuk reefs are really compound fringing reefs.



Figure 3: Fringing reef (Oligocene) of Kirkuk oil field (Dunnington, 1958).


Reef wall limestone reservoirs are seen in the higher part of the Lower Cretaceous of the Bu Hasa oil field of Abu Dhabi where 170m of rudist reef forms the core of this giant field (Harris, 1968; Wilson, 1975). They are also known in the Oligocene of the Bai Hassan oil field, where 44m of coral reef wall were encountered (van Bellenr, 1956). 



Barrier reefs are as yet only rarely known despite the huge North African-Middle East Permian to Cenozoic carbonate province. In the area between Kirkuk and Mosul in northern Iraq, Middle Eocene reefs are classed as barrier reefs and pass northeastward into chemical limestones of a wide, back reef lagoon (van Bellen, 1956). The Middle Eocene (Lutetian) limestones of the Khurmala and Avanah domes in the Kirkuk oil field have developed as barrier reefs.



Bank reefs have developed over submerged tectonic uplifts and occur in the Upper Cretaceous (Maastrichtian-Campanian) of northern Iraq and in the Lower to Middle Eocene and Oligocene of western Syria (Henson, 1950).



Open reef shoals occur where patches of larger Foraminifera, such as Peneroplidae, Amphisteginidae, Alveolinidae, and larger rotalids mix with Mollusca and Echinoidea to form shell banks typically in the Tertiary carbonate formations of the Middle East, especially in Iran and Iraq. They are also found in the fusulinid-coral shoal reef build-ups of the Upper Permian in southern Iran (Fig. 4).



Figure 4: Shoal reef biofacies (fusulinid-coral) Upper Permian, Iran (Edgell, 1977). Diagrammatic biofacies cross-section of the Upper Permian from the Hejaz - Arabia - to the Qashqai Sarhad - Iran -.


Fore reef shoals are situated in shallow areas seaward of the fringing reef and the reef talus (Facies 2 of Wilson, 1975), where there are high concentrations of larger Foraminifera. In the Tertiary limestones of the region, major shoal reef forming organisms are Nummulitidae, Lepidocyclinidae, Operculinidae, Miogypsinidae, and Alveolinidae, while in the Cretaceous Orbitoididae and Orbitolinidae are the principal constituents of fore reef shoals. In the United Arab Emirates and its offshore, a northerly belt of permeable Lower Cretaceous shoal grainstone with abundant fragmented orbitolinids forms important oil reservoirs in the Bab and Zakum fields. 



Figure 5: Fore reef rudist detritus, Aptian, Bu Hasa oil field (Wilson, 1975).



Talus slope deposits consist of accumulated reef debris and broken shell fragments. Due to their high porosity and permeability talus slope deposits probably account for more oil production in the Middle East and North Africa than reef limestones. An example is the extensive fore reef rudist detritus of the Bu Hasa oil field of Abu Dhabi (Fig. 5). The trend of this rudist reef build-up and its fore reef facies is now well established in the eastern Persian Gulf and Arabia (Fig. 6).



Figure 6: Trend of rudist reef build-up in the Persian Gulf and Arabia (Alsharhan, 1985).


Atolls, similar to those of the South Pacific, such as Funafuti and Kanton islands, are as yet unknown in the stratigraphic sequences of the Middle East and North Africa.



Reef knolls, probably better described as platform reefs, are known from the large Sirte Basin of Libya, where they form the subcircular Paleocene coral-algal bioherms of the productive Intisar (Idris) "A", "B", "C", "D", and "E" oil fields. Intisar "A" Field (Fig. 7a) was described by Terry and Williams (1969). The Intisar "D" oil field is a typical example (Fig. 7b), being 5km in diameter with an initial oil column of 291m, so that the buried reef was full to spill point. When discovered in 1967 the initial well yielded 75,000 BOPD due to permeability as high as 500 millidarcies and original stock tank oil in place was estimated at 1.8 billion barrels (Brady et alii, 1980). Similar productive platform reefs occur in the Paleocene of discovery wells in the A1-NC 29B and A1-NC 29C concessions of the northwestern Sirte Basin. They contain build-ups of scleractinian corals and encrusting calcareous algae, solenoporoid algal reef growth of Parachaetetes asvapatii Pia, and colonial madreporarian corals, such as Porites.



Figure 7a: Platform reef, or reef knoll, of the Paleocene coral-algal Intisar "A" field, Libya (Terry and William, 1969).


Figure 7b: Platform reef, or reef knoll, of the Paleocene coral-algal Intisar "D" field, Libya (Brady et alii, 1980).



These Paleocene platform reefs of Libya can be distinctly recognized on seismic profiles (Figs. 8a and 8b). Features are the convex shaped reef top, overlying drape, break-up of reflectors at the reef edge, almost no continuity of reflectors through the reef mass, and velocity sag under the reef due to lower velocity for reef limestones than surrounding rocks. 



Figure 8a: Seismic cross-section of the Intisar "A" reef, Libya (Terry and William, 1969).



Figure 8b: Seismic cross-section over crest of Intisar "D" platform reef, Libya (Brady et alii, 1980).


Development of reefs responds to eustatic sea level changes. With regressive or constant sea levels the reef tends to prograde or builds out over its own talus deposits (Fig. 9a). With gradually rising sea levels, reefs are trangressive and backstep or build shoreward over earlier reef accumulations. An example of a regressive reef is the Lower Cretaceous (Shuaiba Formation) rudist reef in Bu Hasa Field, Abu Dhabi, which has gradually built northward, progading over its fore reef detritus. A transgressive reef building shoreward over its earlier back reef lagoonal facies is seen in Kirkuk oil field where Middle Oligocene reef and fore reef deposits have built shoreward over the earlier Lower Oligocene reef.



Figure 9a: Regressive and transgressive reef growth patterns (Henson, 1950).



Kendall and Schlager (1981) have recognized a number of different types of reef response to eustatic changes of sea level under terms such as "give-up", "catch-up", "back-step", "keep-up", "prograde", and "spillout" (Fig. 9b). 



Figure 9b: Reef growth response to eustatic sea-level changes (Kendall et alii, 1991).


Reef builders of the past in the Middle East and North Africa

Whereas the framework builders of present-day and Tertiary reefs are corals, and to a lesser degree calcareous algae, those of earlier eras have been quite different.



The earliest framework builders of shallow shoal to intertidal reefs in the Middle East were stromatolites of the Proterozoic. These occur in the Khufai, Buah and Ara formations of the up to 4000m thick Hugf Group in Oman, where they form domal and columnar branching bioherms. In the Buah Formation linked domal stromatolites are up to 5m wide and 3m high (Fig. 10), as well as narrow erect columnar stromatolites (Gorin et alii, 1982; Wright et alii, 1990).



Figure 10: Stromatolitic domes, Proterozoic Buah Formation, Oman. The earliest reef builders (Wright et alii, 1990).



In dolomites interbedded in the dominantly evaporitic Ara Formation, columnar stromatolites have been obvious framework builders (Fig. 11), and show considerable oil staining in intercrystalline pores of the stromatolite laminae (Hughes-Clarke, 1988), indicative of the Proterozoic origin of oil in Oman (Edgell, 1991, 1996). These columnar stromatolites and their entrapped sediment can be classified as the reef forming rock bafflestone (Embry and Klovan, 1972), where stalked organisms have grown up trapping sediment between them.



Figure 11: Columnar oil-stained Proterozoic stromatolites, Ara Formation, Oman as reef framework builders (Hughes-Clarke, 1988).



Siliciclastic deposits represent the Lower Paleozoic in the Middle East and most of North Africa and there is a regional hiatus throughout the Arabian Platform and southern Iran, whereby Devonian and Carboniferous strata are rarely present.



Permian limestones and dolomites of the Khuff Formation in Arabia are mainly platform carbonates but their much thicker lateral equivalent in southern Iran is the Dalan Formation. This thick carbonate formation thickens towards the northeast to over 1000m and extends as reefoid or shoal reef limestone in a belt over 2000km long and 75km wide from Kuh-e Gahkum behind Bandar Abbas to Kermanshah and the Iran / Iraq border and beyond into northeast Iraq and southeast Turkey. This belt reaches a thickness of 1074m in Ushtaran Kuh and 1075m in Zard Kuh in the High Zagros Ranges and has been described by Edgell (1977) as the fusulinid-coral shoal reef facies of the Dalan Formation. It contains abundant rock-forming fusulinids, such as Parafusulina, Neoschwagerina, Afghanella, and Eopolydiexodina (Fig. 12), together with the tetracorals Iranophyllum, Stylidophyllum, Waagenophyllum and Wentzelella, as well as numerous calcareous algae like Mizzia and Permocalculus. Although the thick reef facies of these folded Permian carbonates in Iran and Iraq offer good hydrocarbon reservoir potential, wells have not been drilled in them because of difficulty of access in the rugged terrain of the High Zagros Ranges. However, the restricted, back reef Permian carbonate shelf of southern Iran contains many long, narrow, doubly-plunging anticlinal folds. These have been found to contain immense quantities of natural gas in the Kangan, Aghar, Nar, Varavi, Mand, and Dalan gas fields, as well as oil in the latter. The large, offshore, diapiric Pars Field also contains very large amounts of gas in Permian reservoirs. Reserves of these Permian gas fields are conservatively estimated at 13.5 trillion cubic feet, or oil equivalent of 19 billion barrels (Carmalt and St. John, 1986). The huge North Field of offshore Qatar and offshore Iran is estimated to contain 300 trillion cubic feet of gas in the same Upper Permian reservoir carbonates of restricted to marine, neritic environments. This is the oil equivalent of approximately 50 billion barrels.



Figure 12: Fusulinid shoal facies from the Permian Dalan Formation, Iran, with Eopolydiexodina and Neoschwagerina (Sampo, 1969).



Although Jurassic limestones form some of the most important oil reservoirs in the Middle East, they do not seem to be developed in reef facies but owe their productivity to areas of highly porous, high-energy grainstones covering much of the Arabian Shelf.



In the Lower and Middle Cretaceous of the Middle East and North Africa the dominant framework reef builders in reefs have been the rudists. These peculiar, asymmetrical pelecypods, with a coral-like appearance, grew in colonies in the shallow, warm waters of the Cretaceous, especially during the Lower and Middle Cretaceous (Albian-Turonian). They built reefs and formed oil reservoirs, such as that of the giant Bu Hasa oil field in Abu Dhabi (Fig. 5), where a rudist reef in the Lower Cretaceous (Aptian) Shuaiba Limestone reaches a thickness of 170m. This rudist reef extends in a subcircular pattern through the eastern Persian Gulf and eastern Arabia (Fig. 6). It also includes the oil fields of Sirri (offshore Iran), Idd al Shargi (offshore Qatar), Safah (Oman), Wadi Rafashi (Oman), Al Huwaisah (Oman), Yibal (Oman), Shah (Abu Dhabi) and the giant Shaybah-Zarrara Field (mostly in Saudi Arabia but extending into Abu Dhabi).



Large rudists, such as Radiolites (Fig. 13), Durania, and Sphaerulites were also important reef builders in the Cenomanian Bangestan Group of the Karun Shelf in southwest Iran and contribute to the Cretaceous oil reservoir of Hakt Kel oil field. 



Figure 13: Reef-forming rudist (Radiolites) from Sarvak Formation, Cenomanian, south Iran.



Rudists have also built up shoal banks or reefs in the Lower Cretaceous (Aptian-Barremian) of the eastern Kirkuk oil field, where they form the so-called 'Second Pay'. In the Augila oil field of the eastern Sirte Basin in Libya, Cretaceous fringing reefs flank granitic basement highs (Fig. 14) and their fore reef debris forms significant oil reservoirs.



Figure 14: Fringing Upper Cretaceous rudist reef reservoirs flanking basement highs, Augila oil field, Sirte Basin, eastern Libya (Williams, 1968).


Upper Cretaceous deposits throughout most of southern Iran, Iraq and North Africa are predominantly marly. Neritic to shoal reef limestones of this age occur in western Iraq with the rudist Hippurites and other framework builders, such as the larger Foraminifera Orbitoides, Omphalocyclus, and Loftusia. These contribute to the reservoir limestones of the Qaiyarah oil field of Iraq, which contains large quantities of heavy sulphurous oil.



In the lowermost Tertiary (Paleocene) of Libya, local build-ups of scleractinian and madreporarian corals, together with abundant calcareous algae (Fig. 15), form subcircular reef knolls, or platform reefs, which act as excellent oil reservoir rocks.



Figure 15: Palaeocene coral-algal reef facies of reservoir limestones in A1-NC 29B field, Sirte Basin, north central Libya.



Middle and Upper Eocene limestones are widespread throughout the Middle East and North Africa. They are often developed as thick shoal reef limestones composed mainly of Nummulites (Fig. 16), with other larger Foraminifera, such as Discocyclina, and Asterigerina. In the northeast part of the Kirkuk oil field (Khurmala Dome) such a shoal reef facies occurs forming a lower part of the productive "Main Limestone". Similarly, in North Africa, the Middle Eocene nummulitic shoal limestones of the Upper Gialo Limestone form the principal oil reservoir in the Gialo Field of the southeastern Sirte Basin (Barr and Weegar, 1972).



Figure 16: Middle Eocene nummulitic shoal reef facies with Nummulites gizehensis, N. aturicus, and Discocyclina sp. Oil reservoir in Kirkuk and Bushgan fields (Sampo, 1969).



Oligocene reef, back reef, and fore reef limestones are the major oil reservoir beds of the "Main Limestone" in Iraq and are well-known in the giant Kirkuk oil field. The Lower Oligocene back reef and reef sequence was succeeded by transgressive Middle Oligocene back reef, reef and fore reef facies, so that the later reef complex was deposited on the older reef complex. The main framework builders in the reef and back reef were the larger Foraminifera Archaias kirkukensis, and Praerhapidionina delicata, with the miliolids Austrotrillina howchini and Heterotrillina hensoni together with peneroplids like Peneroplis evolutus in the back reef facies (Fig. 17). In the Oligocene fore reef, numerous Lepidocyclina and Nummulites intermedius occur. There is 44m of real Oligocene coral reef in the Bai Hassan oil field of Iraq, taken to represent the reef wall (van Bellen, 1956). However, in the Kirkuk area only fragments of reef organisms were found, since the reef apparently moved forward to the southwest, crumbling the reef material behind.



Figure 17: Oligocene back reef facies reservoir limestone, Kirkuk oil field with miliolids and peneroplids (van Bellen, 1956).


The Asmari Limestone of Oligo-Miocene age occurs widely throughout southern Iran where it is the major oil reservoir. Although Hull and Warman (1970) claim that this formation contains no reef material, a coralline Upper Oligocene reef facies of the Asmari occurs in the High Zagros Ranges (Sampo, 1969) in the vicinity of the Shurom oil field (Fig. 18). 



Figure 18: Upper Oligocene reefal coral biolithic dolomite, Asmari Formation, High Zagros, southern Iran. Reservoir rocks in Shurom oil field (Sampo, 1969).



Also, Thomas (1950) has shown that reef development has taken place in the lower and middle part of the Asmari Limestone in the prolific Gachsaran oil field. In this field, bryozoan-algal shoal reef facies occurs, with Lithophyllum and Lithothamnion as the most important reef builder (Fig. 19), together with Cycloclypeus and Lepidocyclina. Extending over at least 2000km, the Asmari Limestone is time-transgressive, being entirely Oligocene just north of the Strait of Hormuz, Oligo-Miocene in the central part of southern Iran and Lower to Middle Miocene on the Iran-Iraq border. It passes into Iraq, and is the main oil producing horizon in the Jambur oil field, south of Kirkuk, where it is known as the Euphrates Limestone. Limestones of the Asmari Formation are also productive in eastern Syria, where they are known locally as the Jeribe Formation. They also extend into western coastal Lebanon with abundant reef algae, such as Archaeolithothamnium, Corallina, Lithoporella, Lithothamnion, and Mesophyllum (Edgell and Basson, 1975). Southeastward, the Asmari Limestone is found underlying the eastern Persian Gulf and in the U.A.E. It also occurs in the downfaulted part of coastal Dhofar, Oman, as an Oligocene shoal facies, nummulitic limestone (Edgell, 1955).



Figure 19: Miocene algal reef facies with Lithothamnion, southern Iran.



Miocene algal reef limestones of the so-called 'Nullipore Limestone', actually a Lithothamnion reef accumulation, also form productive oil reservoir rocks in the Ras Gharib oil field (Fig. 20) in an upfaulted block on the western side of the Gulf of Suez beneath a seal of the Miocene Evaporite Group.



Figure 20: Miocene Lithothamnion algal reef limestone reservoir, Ras Gharib oil field, western Gulf of Suez (modified from Morgan - Barkouly, 1956).



Contribution of reef limestones to oil and gas reserves in the Middle East and North Africa

More than two-thirds of all the world's oil reserves are found in the Middle East and North Africa. This amounts to some 830 billion barrels of ultimate recoverable oil, counting the reserves of Saudi Arabia as being substantially larger than those officially given.



In attempting to assess the contribution of reef limestones in this region, recoverable oil reserves in reef facies of all kinds have been considered, including fringing reef, barrier reef, platform reef, reef wall, back reef, fore reef, and shoal reef facies.



The total ultimate recoverable oil reserves from carbonate reservoirs of all types of reef facies are estimated to be about 64 billion barrels. Thus, only a little over 7.5% of recoverable oil in the Middle East and North Africa is contained in reef or reef related limestone reservoirs. To keep this in perspective, it should be added that oil reservoirs of the Middle East and North Africa contain more than 18 times the known recoverable oil reserves of Australia and its offshore areas, and considerably more than the known oil reserves of the Far East and Australasia.



Gas reserves from reef or reef related carbonates in the Middle East and North Africa are difficult to estimate. This is because gas reserves are rarely given independently and all oil produced yields a large amount of associated gas. All carbonate reservoirs in the Middle East and North Africa contain huge gas reserves estimated at 1255 trillion cubic feet. If, as with oil reserves, only a little over 7.5% are from ancient reef facies, this would still mean that over 96 trillion cubic feet of gas reserves in the region are from reef, or reef related reservoirs. This is more than the very substantial gas reserves known in Australia and its continental shelf.




A review of carbonate reservoir rocks in the oil fields of the Middle East and North Africa shows that almost all types of reef facies can be recognized in the subsurface, although barrier reefs are rare, and atolls as yet unknown. Most of the oil in various reef facies is found in reservoirs of Cretaceous and Tertiary age. However, oil has been found as far back as the Late Precambrian in Proterozoic stromatolitic dolomites of Oman, where they form a few small oil fields, such as Birba, Amal South and Athel fields (Edgell, 1991), and some of the earliest bioherms.



Framework builders of ancient reefs have often been quite different from those of the present-day. They have included stromatolites, fusulinids, tetracorals, rudists, larger Foraminifera, and various types of calcareous algae.



Slightly more the than 7.5% of the ultimate recoverable oil reserves of this immense carbonate province is contained in reef or reef related limestones. This is estimated to be nearly 64 billion barrels of ultimate recoverable oil reserves, considerably more than that known from the entire Far East and Australasia. Gas reserves in limestones of reef facies in the region are comparable to the very large gas reserves known from Australia and its offshore fields.



A low density of exploratory drilling and inadequate knowledge of subsurface reef reservoir facies and their distribution may partly explain the relatively low percentage of oil reserves known in reef facies in the Middle East and North Africa. Other factors are the emphasis on structure drilling, and the large amounts of oil found in diapir, or basement induced anticlines, as well as in folded and fractured limestone reservoirs. Perhaps the most compelling reason is that the majority of carbonates in the region appear to have been formed as shelf, carbonate ramp deposits without bordering reefs, or with very narrow reef environments.



Nevertheless, subsurface reef facies offer very attractive oil prospects in future oil exploration in the Middle East and North Africa. This can be judged from the giant Kirkuk oil field of Iraq with 17 billion barrels of reserves and the Bu Hasa and Shaybah-Zarrara oil fields of Abu Dhabi and eastern Saudi Arabia, each with reserves of at least 8 billion barrels. Even small buried platform reefs provide very rewarding exploration prospects, as shown by the approximately 5km diameter Paleocene Intisar "D" platform reef in Libya with an initial 1.8 billion barrels of oil reserves. Reef limestone reservoirs generally have good porosity and permeability, thick oil columns and prolific production.




Alsharhan, A.S. 1985. Depositional Environment, Reservoir Unit Evolution, and Hydrocarbon Habitat of Shuaiba formation, Lower Cretaceous, Abu Dhabi, United Arab Emirates, Amer. Assoc. Petrol. Geologists, Bull., v. 69, no. 6, pp. 899-912. 

Alsharhan, A.S., and Nairn, A.E.M. 1986. A review of the Cretaceous formations in the Arabian Peninsula and the Gulf: Part 1. Lower Cretaceous (Thamama Group), stratigraphy and paleogeography, Journal of Petroleum Geology, v. 9, no. 1, pp. 365-392. 

Barr, F.T., and Weegar, A.A. 1972. Stratigraphic Nomenclature of the Sirte Basin, Libya, Petroleum Exploration Society of Libya, 179 p., Tripoli, Libya. 

Brady, T.J., Campbell, N.D.J., and Maher, C.E. 1980. Intisar 'D' oil field, Libya, in Halbouty, M.T. (ed.), Giant oil and gas fields of the Decade 1968-1978, Amer. Assoc. Petrol. Geologists, Tulsa, pp. 543-564. 

Carmalt, S.W., and St. John, B. 1986. Giant oil and gas fields, in Halbouty, M.T. (ed.), Future Petroleum Provinces of the World, Amer. Assoc. Petrol. Geologists, Mem. 40, pp. 11-53. 

Dunnington, H.V. 1958. Generation, migration, accumulation, and dissipation of oil in northern Iraq, in Weeks, L.G. (ed.), Amer. Assoc. Petrol. Geologists Symposium, Tulsa, pp. 1194-1251. 

Edgell, H.S. 1955. Stratigraphy and Paleontology of Dhofar, Oman (unpublished report). 

Edgell, H.S. 1977. The Permian System as an oil and gas reservoir in Iran, Iraq and Arabia, Second Iranian Geological Symposium, pp. 161-195, Tehran. 

Edgell, H. S. 1991. Proterozoic salt basins of the Persian Gulf and their role in hydrocarbon generation, Precambrian Research, v. 54, pp. 1-14, Elsevier, Amsterdam. 

Edgell, H.S. 1996. Salt tectonism in the Persian Gulf Basin, in Alsop, G. I., Blundell, D.J., and Davidson, I. (eds.), Salt Tectonics, Geological Society Special Publication No. 100, pp. 129-151. 

Edgell, H.S., and Basson. P.W. 1975. Calcareous algae from the Miocene of Lebanon, Micropaleontology, v. 21, pp. 166-184. 

Embry, A.F., and Klovan, J. E. 1972. Absolute water depth limits of Late Devonian paleoecologic zones, Geol. Rundschau, v. 61, pp. 672-686. 

Gorin, G.E., Racz, L.G., and Walter, M.R. 1982. Late Precambrian-Cambrian sediments of the Hugf Group, Sultanate of Oman, Amer. Assoc. Petrol. Geologists, Bull. 66, pp. 2609-2627. 

Harris, T.J., Hay, C., and Twombley, B.N. 1968. Contrasting limestone reservoirs in the Murban field, Abu Dhabi, Second Regional Technical Symposium, Society of Petroleum Engineers, Dhahran, pp. 149-187.  



Henson, F.R.S. 1950. Cretaceous and Tertiary reef formations and associated sediments in the Middle East, Amer. Assoc. Petrol. Geologists, Bull. 34, pp. 215-238. 

Hughes-Clarke, M.W. 1988. Stratigraphy and rock-unit nomenclature in the oil-producing area of Oman, Journal of Petroleum Geology, v. 11, pp. 5-60. 

Hull, C.E., and Warman, H.R. 1970. Asmari oil fields of Iran, in Halbouty, M.T. (ed.), Geology of Giant Petroleum Fields, Amer. Assoc. Petrol. Geologists, Mem. 14, Tulsa, pp. 428-437. 

Kendall, C.G.St.C., and Schlager, W. 1981. Carbonates and relative changes in sea-level, Marine Geology, v. 44, pp. 181-212, Elsevier, Amsterdam. 

Kendall, C.G.St.C., Bowen, B., Alsharhan, A., Cheong Dae-Kyo and Stoudt, D. 1991. Eustatic controls on carbonate facies in reservoirs, and seals associated with Mesozoic hydrocarbon fields of the Arabian Gulf and the Gulf of Mexico, Marine Geology, v. 102, pp. 215-238, Elsevier, Amsterdam. 

Kent, P.E., Slinger, F.C. and Thomas, A. N. 1951. Stratigraphic exploration surveys in south-west Persia, Third World Petroleum Congress, Proc., Section 1, pp.141-161.

Morgan, D.E., and Barkouky, A.E. 1956. Geophysical history of Ras Gharib field, Geophysical Case Histories, v. 2, pp. 237-247. 

Sampo, M. 1969. Microfacies and microfossils of the Zagros Area, southwestern Iran (from pre-Permian to Miocene), International Sedimentary Petrographical Series, v. 12, 74 pp., 105 pls., E. J. Brill, Leiden. 

Slinger, F.C.P., and Crichton, J.G. 1959. The geology and development of the Gachsaran field, southwest Iran, Fifth World Petroleum Congress, Proc., section 1, pp. 349-375, New York. 

Terry, C.E., and Williams, J.J. 1969. The Idris "A" Bioherm and Oilfield, Sirte Basin, Libya - its Commercial Development, Regional Palaeocene Geologic Setting and Stratigraphy, in Hepple, P. (ed.), The Exploration for Petroleum in Europe and North Africa, pp.31-48, The Institute of Petroleum, London. 

Thomas, A.N. 1950. Facies Variations in the Asmari Limestone, Int. Geol. Congress, Report of Eighteenth Session, 1948, Part 11, pp. 74-82. 

Thomas, A.N., 1952. The Asmari Limestone of South-west Iran, Int. Geol. Congress, Report of Eighteenth Session, 1948, Part 6, pp. 35-44. 

van Bellen, R.C. 1956. The Stratigraphy of the "Main Limestone" of the Kirkuk, Bai Hassan, and Qarah Chauq Dagh structures in north Iraq, Journal of the Institute of Petroleum, v. 42, No. 393, pp. 233-263. 

Williams, J.J. 1968. The Stratigraphy and Igneous Reservoirs of the Augila Field, Libya, in Barr, F.T. (ed.), Geology and Archaeology of Northern Cyrenaica, Libya, pp. 197-205, Petroleum Exploration Society of Libya, Tripoli. 

Wilson, J.L. 1975. Carbonate Facies in Geologic History, Springer-Verlag, Berlin, 471 p. 

Wright, V.P., Ries, A.C., and Munn, S.G. 1990. Intraplatformal basin-fill deposits from the Infracambrian Hugf Group, east Central Oman, in Robertson, A.H.F., Searle, M.P., and Ries. A.C. (eds.), The Geology and Tectonics of the Oman Region, Geological Society Special Publication No. 49, pp. 601-616.



The author 

H. Stewart Edgell is a well known geologist and educator, for more than 40 years in the Middle East and North Africa. He is also an Editor of the GMEOP site.

H. Stewart Edgell

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