Geology of Emirates
Geological setting of the Arabian Peninsula
The Arabian Peninsula is set off from Africa by the Red Sea from Iran by the Arabian Gulf and the Gulf of Oman and is bounded on the south by the Arabian Sea and the Gulf of Aden. The Arabian Peninsula can be divided into three main divisions: shield, shelf and mountains.
The Arabian Shield lies to the west of the Peninsula occupying about one third of its area. It is composed largely of Precambrian Igneous and metamorphic rocks.
Arabian platform (Shelf):
The Arabian platform lies to the east of the Arabian shield. It is represented by long, arcuate belts of escarpments in which Paleozoic, Mesozoic and Lower Tertiary rocks crop out. Resistant limestones cap each scarp and many of their rock units continue without significant interruption for 500 to nearly 1000 Km across the Arabian shelf. Beds reflecting buried basement configuration dip gently and uniformly away from the escarpment region into the Arabian Gulf and Rub' aL-khali basins.
A mobile zone of young mountains with thin foreland areas rims and butts against the stable Arabian Shelf in coastal Oman. These mountains include Oman and Zagros Mountains.
During the latest Precambrian and Early Cambrian the northwest trending left lateral Najd wrench fault system cut older north south folds and faults. This caused lateral movements of more than 100 Km on the southwestern fault zone. From the Cambrian on epicontinental seas moved back and forth across the lower part of the shield and buried it beneath a succession of nearly flat-lying strata which form the Arabian shelf.
Lithologically, these strata consist mainly of shallow water limestones as well as sandstones and shales. These units are thin and are of constant lithology over large areas. Gradual shifting in facies do occur.
The Arabian shelf can be subdivided into the three structural features: The Interior Homocline, the Interior Platform and several basinal areas.
The Interior Homocline:
This is a sedimentary belt bordering the shield from the east. It has a width of about 400 Km and dips basin-ward at about 0.5-1 degree. On this homocline and approximately along latitude 24 north, the central Arabian Arch affected all rocks from the basement upwards and strongly influenced the present surface distribution of sedimentary rocks in the interior escarpment.
The Interior platform:
This is a remarkably flat area bordering the homocline. The width at the platform varies between 100 km along the southern and western side of the Rub' al-Khali basin to 400 Km or more across the Qatar Peninsula. Several major north-south anticlinal trends rise above the general level of the platform and include the great oil fields of the Arabia. These areas are probably underlain by horst-like basement uplifts.
Several depressions or basins formed on the Arabian shelf. The largest of these is the Rub' al-Khali basin, which is an elongate trench plunging gently into the Arabian Gulf and extending almost as far as the Iranian coast. It is primarily a Tertiary feature with Paleocene, Lower and Middle Eocene and Late Tertiary sediments thickening towards the center.
Many oil fields in the western Gulf area are located on structural highs and gravity lows that may indicate salt flowage at depth. East of the Qatar peninsula diapiric salt structures occur frequently on islands. These salts one of Late Proterozoic age and its source is the infra-Cambrian Hormuz basin which underlies much of the Arabian Gulf geosyncline.
General Stratigraphic History of the Arabian Peninsula
The oldest rocks in the Arabian Peninsula are coarse clastic sediments of Cambrian-Lower Ordovician age. These rocks are overlain by the Tabuk Formation (Middle Ordovician - Lower Devonian) which consists of a thick section of shales alternating with micaceous sandstones. Within this formation there are two significant unconformities at the beginning and end of the Silurian. The Tabuk Formation is followed upward by a sequence of sandstone and shale with subordinate carbonates known as Jauf Formation (Middle - Upper Devonian). The Carboniferous rocks are called "Pre-Khuff clastics" and are overlain by the Khuff Formation (Permian). The Khuff Formation consists of an alternation of shallow water limestones and dolomites with red and green gypsiferous shale containing some evaporites. This Late Permian transgression marks a significant change in sedimentation.
The Triassic rocks in the Arabian Peninsula includes three formations known from base to top as Sudair Formation, Jilh Formation and Minjur Formation. The Sudair Formation consists of brick-red and green shale (Triassic red beds) whereas the other two formations are represented by sandstones. These shales and sandstones thin northeastward and are replaced in the subsurface by shallow water limestone known as Khail and Gulailah formations. The Triassic ends with a regional unconformity known as pre-Toarcian erosional unconformity.
The lower Jurassic rocks consist mainly of soft red shales with limestones and subordinate sandstones. These shales and limestones are known as Marrat Formation (Toarcian).
In the uppermost Liassic epoch (Toarcian), Tethyan transgression began, spread widely during the Middle Jurassic (Bajocian and Bathonian) and continued without apparent interruption into the Late Jurassic.
The Middle Jurassic (Bajocian - Bathonian) rocks are mainly limestones with subordinate shales and sandstones. These limestones conformably overlie the Marrat Formation and were deposited in shallow water environment. The name Dhruma Formation is applied to these limestones (Bajocian - Bathonian).
The Late Jurassic witnessed a progressive change in sedimentation from pure limestone (Tuwaiq Mountain Limestone, Hanifa Formation and Jubaila Formation) through limestone and anhydrite (Arab Formation) to predominantly evaporitic deposits (Hith Formation). The limestones of the Arab Formation act as good reservoirs in many oil fields. Four reservoirs known as Arab-A, B, C and D are found in the Arab Formation, the lowermost reservoir (Arab-D) is of great importance as it contains the largest reserve of petroleum in the world.
The Cretaceous rocks are dominated by limestones with shales and sandstone interbeds. They are subdivided into three groups known from base to top as Thamama, Wasia and Aruma.
The Lower Cretaceous Thamama Group extends to the end of the Aptian time with a major erosional unconformity with the overlying Middle Cretaceous Wasia Group. This unconformity caused the absence of the uppermost Thamama Formation in some places.
The Middle Cretaceous rocks of the Arabia are represented by a continental facies mainly quartzose sands (Nahr Umr Fm.) which dominated during the Albian in Iraq, Kuwait and Saudi Arabia. These quartzose sands are overlain by a shale sequence followed by limestones and shales which deposited during the Cenomanian. In Qatar shallow water shales like the Nahr Umr dominated during the Albian but make up only minor constituents of the overlying Cenomanian limestone. A major regional unconformity ended the Middle Cretaceous. The Upper Cretaceous is represented by shales known as Laffan Shales in Qatar. These shales are overlain by the Halul, Sharghi and Simsima formations. The Upper Cretaceous sedimentation continued with but little interruption into the Paleocene, the hiatus apparently representing a disconformity where existing.
The Lower Tertiary (Paleogene) comprises two sedimentary cycles: Paleocene- Eocene and Oligocene. The Upper Tertiary (Neogene) also has two sedimentary cycles: Miocene and Pliocene.
Paleocene and Lower Eocene are represented by shallow water limestones known as Um er Radhuma and Rus formations respectively. The Middle Eocene is represented by limestone-dolomite sequence known as the Dammam Formation. At the end of the Middle Eocene, the Arabian Platform rose and reduced the Tethys to a remnant sea and initiated a long-lasting erosion interval. In the UAE sedimentation was continuous where the Upper Eocene rocks are represented by shallow water limestones overlain by shales and limestones of Oligocene age. In the Neogene continental conditions prevailed and evaporite precipitation took place in certain localities. Some intermittent floodings led to the deposition of limestones and shales especially in the areas of subsidence.
The United Arab Emirates is a part of Arabian Peninsula and lies at its northeastern part. This peninsula is limited by four major tectonic features, largely of post-Cretaceous age:
1. The Red Sea and Dead Sea rift system at the west and northwest.
2- The Thrust zone from the Alpine Orogeny at the north.
3- The mobile belt of Zagros and Oman Mountains at the east and southeast.
4- The wrench fault associated with Owen Fracture zone at the south .
The Arabian platform:
A vast area to the east of the Arabian shield including Qatar and U.A.E. It has undergone periodic subsidence, which resulted in accumulation of a sedimentary sequence ranging in age from Cambrian to recent.
The Arabian platform has remained a tectonically stable area resulting in the persistence of lithofacies over great distance through Permian to Upper Cretaceous. However, it has been affected by basement activated epeirogenic movements, which caused warping and formation of basins as in eastern Dubai and the Northern Emirates. The structural evolution of the Arabian platform has strongly influenced the sedimentation pattern in the U.A.E. After the Hercynian Orogeny (Upper Paleozoic) the prevailing sedimentary conditions changed drastically from mainly clastic to predominately shallow marine carbonates until the Eocene. During the Late Tertiary, the late paroxysms of the Alpine Orogeny (Miocene- Pliocene) re-established clastic deposition with subordinate carbonates.
I. Geology of Abu Dhabi
The surface geology of Abu Dhabi is concealed under a cover of sands which form dune ridges reaching heights of about 150 m inland. The coastal plains are dominated by evaporitic flats, sabkhas, which extend more than 80 Km southwards into the sand deserts.
Outcrops in Abu Dhabi are confined to Al Ain area to the east where the Oman Mountains form part of the region. Important outcrops include Jabal Hafit, a prominent feature in Al Ain City with a marine succession ranging in age from Lower Eocene to Oligocene (Rupelian). Jabal Malaqet and Jabal Mundassa are two other important outcrops with a marine succession ranging in age from Upper Cretaceous (Campanian) to Upper Eocene .
The structural development of Abu Dhabi is influenced by two main structural features: the central Arabian arch and Rub’ Al Khali basin. These two structures plunge gently towards the Arabian Gulf with a general thickening of sedimentation in this direction indicating an increasing rate of subsidence.
Folds are common features in the area, these folds are simple folds with flank dips of less than 5 degrees. They trend in different directions as N-S, E-W/NE-SW and NW-SE. The most dominant folds are the "Arabian Folds" which are clearly visible in western Abu Dhabi. These folds trend N-S and are probably related to deep-seated basement tectonic .
East of the Falaaha syncline, in the south –east of Abu Dhabi, the fold axes trend NE-SW (Shah, Asab, Sahil) while in the north-east the trend becomes NW-SE (Jarn Yaphour). This suggests conformity with the central Arabian arch and may lead to the conclusion that the Abu Dhabi area is located on the southern flank of this arch.
In central offshore area E-W aligned folds (Zakum, Ghasha) related to the Zagros folding system are superimposed on the older N-S trending "Arabian Folds". This Zagros system together with the Tertiary salt diapirism has greatly modified the structure.
Diapiric salts are believed to underlie most of the structures of Abu Dhabi
In some wells e.g. (Maundous), salt has been found intruding the Tertiary formations. Furthermore, in western Abu Dhabi islands and the Jabal Dhanna Peninsula are salt piercement structures surrounded by Tertiary and Quaternary deposits associated with Cambrian sediments carried up by salt plugs.
The first non-metamorphosed sediments that cover the Arabian shield were formed during the infra –Cambrian. These rocks are mainly clastics
(Sandstones and shales) with subordinate carbonates and evaporites. They belong to the so-called Huqf Group.
The oldest rocks in Abu Dhabi are exposed at Jabal Dhanna and in a number of islands (Das, Dalma, Arzana, Zirkouh, etc.), located in the west of the area. These rocks consist of a mixture of shales, dolomites and volcanics that appear to correlate with Cambrian Hormuz series of Iran. They have been brought to the surface as a result of salt tectonics.
In the Late Carboniferous the middle East craton was affected by the Hercynian Orogeny that led to considerable upwarping and erosion. Late Carboniferous to Early Permian clastics were deposited unconformably over the older rocks, these clastics are mainly sandstones, shales and siltstones with thin beds of dolomites and anhydrites reflecting deposition in a terrestrial to coastal marine environment.
Marine transgression took place in the Middle Permian leading to the deposition of a thick sequence of shallow –water carbonates (micro crystalline dolomites with subordinate limestones) with subordinate evaporites (anhydrites) known as Khuff Formation (2800-2970 ft. thick.
In the Early Triassic the Sudair Formation (800-900 ft. thick) was deposited, it is lithologically similar to the Khuff Formation but with more argillaceous material. It consists of dolomite and argillaceous limestone at the base, followed by dolomite and thin anhydrite with some shale interbeds at the middle, the upper part is composed of interbedded shales and dolomites. This formation was deposited in peritidal conditions than those prevailed during the Middle Permian.
The same conditions prevailed during the Early Triassic continued during the Middle Triassic leading to the deposition of micro crystalline dolomite interbedded with subordinates anhydrite, minor shales and argillaceous limestones. These rocks are known as Gulailah Formation (881-1023 ft thick) in offshore Abu Dhabi or Jilh Formation (1360 ft thick ) in onshore Abu Dhabi .
Regression took place at the end of the Triassic leading to the deposition of continental clastic beds known as Minjur Formation, This formation is only present in onshore Abu Dhabi at Minjur but absent in offshore Abu Dhabi which indicates a large unconformity marking the end of the Triassic period.
The onset of the Jurassic evidenced a major marine transgression that produced a vast carbonate-evaporite platform which still received substantial clastic influx from the west.
The Lower Jurassic is represented by two formation:
Lower dolomite overlain by argillaceous limestone and subordinate shales. The thickness varies from 172 - 442 ft.
It overlies conformably the Hamlah Formation. It consists of argillaceous limestone bounded on the top and the bottom by lime-mudstone and wackestone. The thickness varies from 489-768 ft.
In the Middle Jurassic shallow shelf conditions with warm and clear water prevailed, this led to the deposition the carbonates of the Araej Formation . This formation is divided into three units:
1- Upper Araej: argillaceous limestone overlain by grainstones /pack stones.
2- Uweinat Member: wackestones and packstones with minor grainstones.
3- Lower Areaj: packstones/ grainstones and lime-mudstones.
At the end of the Middle Jurassic a very extensive shallow epicontinental sea was developed. Consequently sedimentation changed gradually from relatively deep- water shelf conditions grading through shoal-lagoonal and culminating in subaerial supratidal conditions. As a result, a thick sequence of carbonates and evaporites was developed.
The upper Jurassic is represented by three formations namely Diyab (or Dukhan in onshore fields), Arab and Hith formations.
The Diyab Formation consists of deep-water carbonates mainly argillaceous lime-mudstones and wackestones. The deposition of these carbonates was a result of active subsidence of the western and south-western part of Abu-Dhabi. This resulted in the development of an intra-shelf basin with a subsequent marine transgression from east to west.
Gradual shallowing of the depositional environment after the deposition of the Diyab Formation led to the deposition of a carbonate/anhydrite sequence which is called the Arab Formation (Qater-Fahahil) in onshore oil fields.
Continuous shallowing led to the prevalence of supratidal conditions and the deposition of a thick sequence of anhydrites and dolomites which is called the Hith Formation. The formation consists mainly of anhydrite with subordinate dolomite in the western part of Abu Dhabi while to the east the anhydrite content of the formation decreases gradually.
The overall picture is that we have two contrasting environments during the Upper Jurassic, the supratidal anhydrite/subordinate dolomite in the west and the oolite/ dolomite shoal environment (Asab Formation) in the east .
The end of the Jurassic was marked by a period of non-deposition, perhaps erosion in some areas.
The Jurassic-Cretaceous boundary is placed at the top of the Hith Formation, which marks the cessation of the evaporitic phase and the resumption of the normal marine shelf carbonate sedimentation.
The Lower Cretaceous rocks are known as Thamama Group, this group is subdivided into four formations namely Habshan, Lekhwair, Kharaib and Shuaiba formations.
This formation represents the first marine transgression which started at the Lower Cretaceous. Conditions were largely restricted or semi-restricted and led to the deposition of lagoonal lime-mudstone and wackestone.
To the east of Abu Dhabi, the Habshan Formation is represented by two phases of sedimentation, an early phase consists of oolitic grainstone interbedded with lagoonal and supratidal dolomite and anhydrite. The later phase consists of lagoonal and intertidal limestone and subordinate dolomite.
This formation rests unconformably on the Habshan Formation and represents the beginning of normal unrestricted marine shelf conditions.
It consists of a number of cyclic sequences, each cycle starts with argillaceous limestone (lime-mudstone and wackestone) which grades upwards into porous clean limestone (packstone with some grainstone). The argillaceous limestone represents the transgressive phase of an open-shelf subtidal environment whereas the clean limestone represents regressive shallow water shelf conditions, occasionally above wave-base level.
This formation rests with minor unconformity on the Lekhwair Formation. It also exhibits the cyclic pattern of carbonate sedimentation shown in Lekhwair Formation. Trangressive cycles are represented by argillaceous limestones (lime-mudstone and wackestone) whereas the regressive cycles are represented by porous clean limestone (packstone and grainstone). The Kharaib Formation is distinguished from the underlying Lekhwair Formation by the dominance of packstones and grainstones (regressive cycles ), in Lekhwair Formation transgressive cycles (lime-mudstone and wackestone ) dominate.
This formation represents the continuation of shelf deposition shown in Lekhwair and Kharaib formations. It consists of clean wackestone and packstone (regressive phase) followed by deeper water transgressive argillaceous limestone (lime-mudstone and wackestone). The deeper-water shelf conditions persisted over most of offshore Abu Dhabi until the end of the Lower cretaceous.
In some localized areas, thick shallow water rudistid carbonates are encountered in the Shuaiba Formation. These rudistid build-ups resulted from the rise of salt plugs at depths causing the development of submarine topographic highs, which were sufficiently shallow for the limestone build-ups to develop.
The Middle Cretaceous rocks are knows as Wasia Group. This group is subdivided into three formations namely Nahr Umr, Salabikh and Mishrif formations.
Nahr Umr Formation
After deposition of the Thamama Group in the Lower Cretaceous, a regional uplift and erosion affected most of Abu Dhabi especially in the south-eastern part at Mender and Agrab regions which caused the absence of the Middle and Upper Cretaceous formations.
In the Middle Cretaceous subsidence took place again leading to the deposition of Nahr Umr Formation.
The Nahr Umr Formation Consists of a sequence of variegated shales with some rare sand lenses, glauconitic silts and occasional beds of limestones mostly packstone and wackestone.
After the clastic deposition of Nahr Umr Formation, Influx of terrrigeneous sediments ceased and a transgression phase resulted in the deposition of normal marine clear water sediments of the Mauddud Member, which is the first member in the Salabikh Formation. The Salabikh Formation includes also the Shilaif, Tuwayil and Ruwaydha members which lie above the Mauddud Member.
This member is thinly developed in Abu Dhabi and consists of bioclastic and foraminiferal wackestone and packstone.
This member deposited in a deep-water basin resulted from the continuous subsidence, which started in the Middle Cretaceous. It consists of a succession of argillaceous limestone, mostly fine-grained packstone and wackestones together with subordinate calcareous shales in the lower part.
The remaining two members (Tuwayil and Ruwaydha members) are found in central Abu Dhabi and probably deposited synchronously with the Mishrif Formation
The Mishrif Formation consists of three Units: a lower Unit composed of fine – grained bioclastic packstones and wackestones, which represent distal shelf slope environment. The middle unit consists of medium- grained packstones indicative of proximal shelf slope environment. The upper Unit consists of coarse – grained bioclastic and shelly packstones and grainstones (rudist shell and debris) indicative of shoal of environment.
The Middle and Upper Cretaceous are completely absent in the southeast of Abu Dhabi (Mender-Lekhwair region) due to the Uplift and erosion which took place after the deposition of Nahr Umr Formation (Early Middle Cretaceous).
The Upper Cretaceous rocks are Known as Aruma Group, this group is subdivided in to four formations namely Laffan, Halul, Fiqa And Simsima formations.
Towards the end of the Turonian, a large unconformity terminated the Middle Cretaceous.
In the Upper Cretaceous (Campanian) renewed subsidence led again to the deposition of a shale sequence with minor argillaceous limestones which represent the Laffan Formation. The lower part of the formation was deposited in a deltaic environment whereas the main bulk was deposited in an open marine shelf environment.
This formation overlies conformably the Laffan Formation. It consists of calcareous shales and lime-mudstones at the base, passing upward into bioclastic and foraminiferal wackestone and subordinate packstone. This indicates a gradual regression and both Halul and Laffan formations can represent one transgressive – regressive cycle.
This formation consists of soft marls and calcareous shales passing upwards into argillaceous lime-wackestones. These rocks were deposited in a deep open marine – shelf environment, representing a regressive phase.
This formation consists of limestones, mainly packstones, which were deposited in a shallow water environment, representing a regressive phase. Both Fiqa and Simsima formations can represent a second transgressive – regressive cycle. Consequently in the Upper Cretaceous there was two transgressive – regressive cycles, the lower is represented by the Laffan and Halul formations and the upper is represented by the Fiqa and Halul formations.
Both Fiqa and Simsima formations are replaced in eastern offshore Abu Dhabi by the Gurpi Formation.
At the end of Cretaceous regression took place especially in southeastern Abu Dhabi (Mender area), Where the Aruma group is absent.
In the Lower Tertiary, the Qatar arch was a rather Stable area and Abu Dhabi lay at the margin of two basin: the Rub Al Khali basin to the south and Pabdeh-Gurpi basin centered in the northern Emirates.
The Lower Tertiary rocks Known as the Hasa Group, this group includes three formations: the Umm Er Radhuma (Paleocene), Rus (Lower Eocene) and the Dammam (Middle Eocene).
Umm Er Radhuma Formation (Paleocene)
This formation consists of shelly and bioclastic limestone and dolomites with thin shales at the base.
The formation was deposited in shallow marine conditions and indicates a widespread transgression at the beginning of the Paleocene.
In some localities such as Zakum and Um Shaif, the basal shales are overlain by Sabkha cycles, each cycle is terminated by thin anhydrite.
Rus Formation (Lower Eocene)
In the Lower Eocene a wide evaporitic platform existed over most of the Abu Dhabi region giving rise to the evaporitic /carbonate sequence of the Rus Formation.
The formation consists of massive anhydrites representing subaerial supratidal conditions. The argillaceous limestone at the base represent lagoonal conditions.
Dammam Formation (Middle Eocene)
In the Middle Eocene transgression took place and normal marine shallow shelf conditions prevailed.
The formation consists of nummulitic and foraminiferal packstones and grainstones representing very shallow high-energy shoal conditions.
The above mentioned formations lose their characteristic lithology at the east and extreme eastern offshore Abu Dhabi and are replaced by the Pabdeh Formation. This formation consists of argillaceous limestones and shales, reaching up to 2200ft thick and was deposited in a basin centered east of Abu Dhabi.
Asmari Formation (Upper Eocene-Lower Oligocene)
In the upper Eocene and Lower Oligocene times, Uplift and erosion affected the western part of Abu Dhabi causing some erosion of the Eocene platform. Transgression then followed causing the deposition of shallow-water fossiliferous limestones of the Asmari Formation.
In Al Ain city ( Gabal Hafit), the Asmari Formation consists of reefal limestone rich in corals, algae and bryozoa together with planktonic and benthonic foraminifera as well as pelecypods, gastropods and echinoids.
Gachsaran Formation (Early Miocene)
This formation consists of three units:
1-upper unit: interbedded anhydrites, shales, marls and carbonates.
2-Middle unit: dolomite and limestone.
3- Lower unit: mainly anhydrite with minor clastics.
The Gachsaran Formation is of Early Miocene age and was deposited in a restricted environment.
In the Upper Miocene-Pliocene times uplift and erosion occurred over most of Abu Dhabi and adjacent area as a result of the Alpine Orogeny. Zagros and Oman Mountains developed, severe erosion took place from the Oligocene to the Maastrichtian exposing the Jebels which outcrop in the Oman mountain area.
The Quaternary is represented by continental sands and gravels forming sand sheets and sand dunes. On the coastal areas of Abu Dhabi Sabkha deposits are common and widely distributed.
Khuff Formation (Permo - Triassic)
The formation contains large volumes of gas which is produced at Um Shaif field and Zakum field.
Araej Formation (Middle jurassic)
The formation produces light oil which occurs in all the three members of the formation (lower Araej,Middle Oweinate and upper Araej) especially at Um Shaif field.In Zakum and other structures only non-associated gas is present.
Arab Formation (upper Jurassic)
The Arab formation is one of the most essential reservoirs especially in off shore Abu Dhabi. The most important Hydrocarbon occurrences are in Um Shaif field where oil is found in the four Arab zones (A, B, C, D). The lower most zone C,D are the most important reservoirs.
In the Bab field, the upper Jurassic Hith / Qatar Formation and the Fahahil Member have proved to be gas bearing so far.
Thamama Group (lower Cretaceous)
The Thamama Group contains the largest oil accumulation in Abu Dhabi. Hydrocarbon - producing zones occur in the onshore fields of Bu Hasa, Asab, Bab and Sahil and in the off shore fields of Zakum, Mubarraz and Um Shaif. Onshore Abu Dhabi, oil is produced mainly from zones A,B and C and from the Shuaiba reef facies in the case of Bu Hasa field. Offshore Abu Dhabi in Zakum field, oil occurs in all the six zones of the Thamama Group. The Nahr Umr shales act as a regional sealing formation for the Thamama reservoirs.
Mishrif Formation (Middle Cretaceous)
The Mishrif Formation is an oil-bearing unit in the eastern offshore areas at Umm Addalkh field and Fateh field of Dubei. It forms a reefal build-up which pinches westwards against the dense limestone of the Shilaif and the overlying Laffan cap rock.
Halul Formation (Upper Cretaceous)
Small accumulation of heavy oil in the Halul Formation occur only in Mandous and AL Khair, northeast of offshore Abu Dhabi.
Simsima Formation (Uppermost Cretaceous)
The Simsima Formation is only oil-bearing in the Shah field.
Asmari Formation (Oligocene)
The Asmari Formation is the only oil -containing formation in the Tertiary of Abu Dhabi, where oil occurs in the upper most part of the formation.
Source of Oil
There are two principal source rocks of Abu Dhabi oil, the first is the Diyab Formation (Upper Jurassic) which is the offshore equivalent of the Dukhan Formation and the Shilaif Member of the Salabikh Formation. The Mauddud Member shows some source rock characteristics onshore Abu Dhabi.
The Diyab Formation consists of intra-shelf argillaceous limestone and is believed to be the major source for the Arab and possibly the Thamama oil.
The Dukhan Formation (the onshore equivalent) of the Diyab Formation has poor to moderate TOC content and its hydrocarbon-generation potential is very small.
The Shilaif Formation is a high quality source rock across the whole Abu Dhabi. It is immature in most areas but has the potential to generate large quantities of oil if present at the sites where regional dip or salt tectonics have taken the formation deep enough to become mature. The Shilaif is believed to be the source of the oil in the Mishrif reservoirs of Umm Addalkh, others minor source rocks include Jilh, Minjur and Izhara formations.
There are two principal sealing formations, the Hith Anhydrite and the Nahr Umr shales. The Hith is the cap rock for Arab reservoirs. The absence of oil in the Arab formation in eastern offshore Abu Dhabi is due to the absence of the Hith formation. The presence of the Hith formation on the other hand prevent oil discharge in to the overlying Thamama reservoirs as in western Abu Dhabi except when there is faulting.
II. Geology of Dubei
The stratigraphic names of Dubai rock units are similar to those of Abu Dhabi with the exception of some little variation resulted from the lateral facies changes of strata due to the general tectonic settings in both areas.
Upper Permian – Lower Cretaceous
During the upper Permian marine transgressian led to the deposition of a thick sequence of carbonate rocks above the pre–khuff clastics. These carbonates include Khuff Formation. (Upper Permian), Sudair and Gulailah formations (Lower-Middle Triassic), Hamlah and Izhara formations (Bajocian), Araej Formation (Bathonian), Diyab, Darb and Arab formations (Oxfordian- Kimmeridgian), Hith equivalent (Tithonian) and Thamama Group (L. Cretaceous).
The above-mentioned formations were described in the chapter dealing with the geology of Abu Dhabi, with the exception of the Gulailah Formation and Hith equivalent.
Gulailah Formation (Middle Triassic)
The Gulailah Formation is the offshore equivalent of the Jilh Foramtion. It overlies the Sudair Formation (L. Triassic) and underlies the Hamlah Formation (L. Jurassic).
The Gulailah Formation consists of dolomite with anhydrite inclusions at the base overlain by shales and thin salt beds.
The Upper Triassic Minjur Formation is absent in Dubai as well as offshore Abu Dhabi due to the major unconformity at the end of the Triassic.
Hith equivalent (Asab Formation)
The Hith Formation as mentioned before is an Upper Jurassic evaporitic sequence, which acts as a cap rock for the Arab Formation. These evaporites are absent in Dubai and eastern offshore Abu Dhabi where they are replaced by fine dolomite with anhydrite inclusions known as Asab Formation.
At the close of the Lower Cretaceous, a transgression occurred and the whole area of Northern Emirates was covered by the Nahr Umr Shales deposited on top of the Thamama.
A semi-regressive phase then followed causing the deposition of the Mauddud shallow-water limestones above the Nahr Umr Shales. The Mauddud is of uniform thickness in the offshore fields but thickens to the east and south of the fields.
Subsidence of the Shilaif basin occurred to the west of Dubei and the Shilaif limestone shale sequence was deposited over the Mauddud limestone. Concurrently the Mishrif Formation began to deposit on the margins of the Shilaif basin.
At the end of the Middle Cretaceous movement of deep-seated salt plugs (Cambrian) led to uplift and erosion of some of the Mishrif high blocks. Consequently a major unconformity marked the end of the Middle Cretaceous.
During the Upper Cretaceous, transgression led to the deposition of the Laffan Shale above the Middle Cretaceous unconformity. The Laffan Shale forms the cap rock of the Mishrif reservoirs of Dubei. It is succeeded by the shallow-water limestones of the Halul Formation (Ilam Formation). The Halul is overlain by shales and mudstones.
At the end of the Cretaceous Period Plate movements led to major tectonic rearrangement in the Dubei area, new basins were formed as a result of renewed upward movement of deep-seated salt plugs. The climax of this crustal instability was the Zagros (Laramide) Orogeny, which caused nappe - folding and faulting in the Oman Mountains. These began to form at the end of the Middle Cretaceous.
From the Upper Cretaceous until Miocene time the Tertiary basin was the major feature of the Northern Emirates area, other than the continued growth of the Oman Mountains.
In Late Miocene renewed movement of deep -seated salt plugs resulted in piercement to surface forming the islands of the Arabian Gulf.
The main reservoirs in Dubei include Thamama reservoirs (Lower Cretaceous), Mishrif reservoirs (Middle Cretaceous) and Ilam (Halul) reservoirs (Upper Cretaceous).
III. Geology of the Northern Emirates
Pre-Lower Cretaceous data on the geology of Northern Emirates are not available due to absence of drilled wells that reached the Jurassic-Paleozoic formations. Consequently the following description considers only post-Lower Cretaceous formations of which data are available.
The Lower Cretaceous rocks in the Northern Emirates are also known as Thamama Group, this group is composed of cyclic carbonate deposits reflecting low-and high energy conditions which resulted from transgression and regression. The high-energy carbonates are characterized by the high porosity and consequently act as reservoir units and are sealed by the low energy open marine carbonates (lime-mudstones).
At the end of the Lower Cretaceous and during the deposition of the Shuaiba Formation, the broad Arabian shelf area developed a shallow basin with minor ramifications extending into Dubai to the northeast and to Oman in the south. Along this basin a peripheral shelf mostly accumulating quiet-water lagoonal sediments was developed and is separated from the basin by a narrow belt of higher energy sediments. Along this well-oxygenated high-energy zone rudists proliferated forming banks and bioherms reaching up to 450 ft thick.
The Shuaiba high-energy shelf edge may run through central off shore Dubai and the northeast of the Abu Dhabi (Fig.). The evidences for that assumption indicated from the drilled wells is that the sediments in the Northern Emirates are represented by quiet-water lagoonal wackestones whereas in central and western offshore Dubai they are represented by basinal lime-mudstone (Bab Mb.) and in central offshore Dubai Rudist bioherms developed.
The Middle Cretaceous rocks in the Northern Emirates are called Wasia Group which includes the following formations:
Nahr Umr Formation
As mention before in the geology of Abu Dhabi and Northern Emirates, the Nahr Umr Formation is represented by deep marine shales as a result of the transgression, which started in the Middle Cretaceous. The thickness of the formation is controlled by the form of the underlying Shuaiba Formation. It has a thickness of about 600 ft in the basinal area of Abu Dhabi but in Dubai, Northern Emirates and southeast Abu Dhabi the thickness ranges from 150-300 ft.
The Natih Formation is a shelf carbonate sequence that is laterally equivalent to the Mauddud, Shilaif and Mishrif formations. In the Northern Emirates only the mauddud and Mishrif are represented, where the Mishrif overlies directly the Mauddud. On the other hand the Shilaif is absent where its eastern limit lies in central offshore Dubai. As a result of regional unconformity, only the Mauddud is preserved in many parts of the Northern Emirates and Mishrif is mostly eroded.
The upper Cretaceous rocks in the Northern Emirates are also known as Aruma Group, this group is represented by a thick sequence of deep-water shales resulted from down warping.
The lower part of the Aruma Group is called Laffan Shale whereas its upper part is represented by high-energy limestone known as Ilam Formation (Lateral equivalent of the Halul Fm. in the Abu Dhabi). The Ilam limestone is thin or absent in the Northern Emirates and the Aruma shale is interbedded or replaced by a flysch sequence of carbonates and silicified shales containing abundant radiolarians and indicating a deep bathyal depositional environment.
At the end of the Upper Cretaceous, uplift led to the deposition of Maastrichtian limestones similar to the Simsima Formation of Abu Dhabi.
The Lower Tertiary in the Northern Emirates is represented by shale (Pabdeh basinal shale) which is equivalent to the limestone\evaporitic sequence of Abu Dhabi and Dubai (Um Er Radhuma, Rus and Dammam formations).
The Upper Tertiary rocks in the Northern Emirates are known as Gachsaran or Fars Formation which is represented by deep marine salts deposited in the remnant pabdeh basin. These salts consist of halite and anhydrite with occasional dolomites, and represent the end phase filling of the pabdeh basin.
Also in the Upper Tertiary, periodic closing of the Strait of Hormuz took place in reaction to the opening of the Red Sea during the Miocene and the subsequent rotation of the Arabian plate towards the northwest.
As a result of this movement, the Qatar arch plunged northward causing a southward uplift where fluviatile deposition prevailed with gravel, sand and conglomerates.