Stratigraphic Framework and Reservoir Development of the Upper Jurassic in Abu Dhabi Area, U.A.E.
Published:January 01, 2000
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Mohamed R. Ayoub, Ibrahim M. En Nadi, 2000. "Stratigraphic Framework and Reservoir Development of the Upper Jurassic in Abu Dhabi Area, U.A.E.", Middle East Models of Jurassic/Cretaceous Carbonate Systems, Abdulrahman S. Alsharhan, Robert W. Scott
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The Upper Jurassic succession exhibits lateral variations in both thickness and lithology over the Abu Dhabi area. These variations are related to eustatic changes in sea level, structural growth of the Qatar Arch and Mender-Lekhwair high, spreading of the Neo-Tethys, and local diapiric movements.
The Upper Jurassic section in the eastern part of Abu Dhabi is subdivided, from bottom to top, into the following formations and reservoirs: Tuwaiq Mountain Formation with Hadriya reservoir, Hanifa Formation with Hanifa reservoir in the southeast area, Jubaila dense section, and the Asab-Asab equivalent with the Asab Oolite reservoir. In the western area, the section contains the Tuwaiq Mountain, Hanifa, Jubaila with Arab D reservoir at the top overlain by the Arab Formation with Arab A, B, and C reservoirs and the Hith anhydrite, locally overlain by the Manifa reservoir at the top of the Hith Formation.
The Late Oxfordian structural growth of the Mender-Lekhwair high and the Qatar Arch led to the development of the main intrashelf basin in between, where the Hanifa Formation was deposited. The Hanifa is the main source rock for the Arab and the Thamama hydrocarbons.
Approximately 75% of the present structural configuration of Abu Dhabi onshore area resulted from the late Cretaceous structural compressional movements due to the Oman Nappes overthrust zone. The Late Cretaceous subsidence in the Falaha trough in central Abu Dhabi enhanced organic and inorganic gas generation.
Many of the major hydrocarbon-bearing reservoirs in the Arabian Peninsula are found in the Upper Jurassic rocks. In addition the Upper Jurassic Hanifa Formation is considered to be the main source rock for the Upper Jurassic and lower Cretaceous hydrocarbons (Alsharhan and Kendall, 1986). Hydrocarbon occurrence in the Upper Jurassic reservoirs in Abu Dhabi is controlled by the presence of the Hith Anhydrite (cap rock), which passes laterally across central Abu Dhabi along a NNW-SSE trend into the carbonates of the Asab Formation. This lateral facies change has directed and controlled the migration of the Hanifa hydrocarbons to the Thamama Group reservoirs (Alsharhan, 1989).
The Upper Jurassic sequence exhibits lateral variations in both thickness and lithology across onshore Abu Dhabi area (Fig. 1). These variations are related to eustatic changes in sea level, structural growth of the Qatar Arch and the Mender-Lekhwair high, spreading of the Neo-Tethys ocean, and local diapiric movements. The Upper Jurassic succession unconformably overlies the Middle Jurassic Araej Formation and underlies the Lower Cretaceous Habshan Formation (Fig. 2) (de Matos, 1994). The objectives of this paper are to review the lithostratigraphy and reservoirs development of the Upper Jurassic succession in the onshore Abu Dhabi area (ADCO Concession).
The Abu Dhabi onshore area is part of the Arabian platform. Gentle and simple anticlinal folds are the common features encountered in the area, though complexly faulted structures are also present. These structures gave birth to a significant number of hydrocarbon accumulations. The north/northeast-south/ southwest direction is the main elongated trend of the onshore producing fields.
During the Middle Jurassic (Bajocian to Callovian time), a shallow to very shallow warm-water carbonate ramp prevailed across the Abu Dhabi area and gave rise to the deposition of the relatively clean carbonate section of the Araej Formation (Fig. 3). This formation exhibits poor reservoir characteristics because of subsurface diagenesis, especially in the deeply buried area (central and northeastern part of Abu Dhabi).
The thickness of the Araej varies from 870 ft (265 m) in the southeast area to 680 ft (207m) towards the northwest, reflecting the effect of structural growth of the Qater Arch and possible diapiric movements in the offshore areas. In general, the Middle Jurassic succession shows good lateral continuity in terms of lithology and thickness, which reflects a passive tectonic period and a steady structural growth during deposition.
Tuwaiq Mountain Formation
The Tuwaiq Mountain Formation is the lowest sequence of the Upper Jurassic section. It is Oxfordian in age, overlying the Middle Jurassic (Callovian) Araej Formation (Fig. 3). The contact is an unconformity surface placed at the first appearance of the clean, dense aphanitic lime mudstone interval directly above the slightly argillaceous packstone-grainstone section of the Araej Formation. The contact between the Araej and the Tuwaiq Mountain is marked by an irregular yellowish limoninc surface with some anhydrite-filled fractures and nodules marking the beginning of the first stratigraphic sequence within the Upper Jurassic section.
The structural growth of the Mender-Lekhwair high and Qatar Arch played an important role in controlling the stratigraphy (Fig. 4). Sea level rose rapidly during deposition of the Tuwaiq Mountain Formation. The carbonate production rate was high in the southeast section because of the structural growth in the Mender-Lekhwair high, whereas the rate of sedimentation was very low in the western and northern areas of Abu Dhabi, thus giving rise to a starved basin (Fig. 5). The thickness of the Tuwaiq Mountain section ranges from less than 50 ft (15 m) in the northern offshore area to over 700 ft (210 m) in the southeast area.
The structural growth in the Mender area resulted in deposition of a clean aphanitic dense lime mudstone interval (lower part), which gradually thickens towards the southeastern area, and it ended by the deposition of the Hydria reservoir at the top of the formation. This porous section pinches out basinward (Fig. 6).
The upper part of the Tuwaiq Mountain attests to a drop in sea level, which resulted in subaerial exposure (unconformity surface) in the southeast area and an arid hypersaline environment in the restricted intrashelf basin to the west. As a result, anhydrite streaks are the lateral equivalent of the unconformity surface in the western and northwestern area (Figs. 7, 8). However, locally, because of the increase in water depth (semi-restricted), a marly to highly argillaceous lime mudstone streak was developed as a lateral equivalent to the anhydrite streaks, thus marking the end of the first stratigraphic sequence in the Upper Jurassic succession.
The Late Jurassic intra-shelf basin was structurally active during the Late Oxfordian to Early Kimmeridgian time because of the structural growth of the Qatar Arch and the Mender-Lekhwair High. This intrashelf basin was short-lived as it was filled during the subsequent sea-level rise, post top Tuwaiq Mountain unconformity surface. During Oxfordian-Early Kimmeridgian time organic rich sediments accumulated in the intrashelf basin to form the main source-rock intervals in the Upper Jurassic section (Fig. 8). The depocenter is located in the western and southwestern areas of Abu Dhabi and east of the Qatar Arch. The thickness of the Hanifa Formation ranges from about 1000 ft (304 m) in the southwestern are to less than 700 ft (213 m) in the northern offshore, with gradual thinning towards the southeastern area.
The Hanifa reservoir was developed in the southeastern area (Fig. 6) as a result of upward shallowing associated with continued emergence of the Mender High. In the Mender area however, the Hanifa Formation is missing because of either nondeposition or erosion (Fig. 8). In the offshore area, because of contemporaneous salt doming, the section becomes less argillaceous and more porous and locally dolomitic toward the northeastern area. The top of the Hanifa Formation could be an unconformity surface associated with a drop in sea level and indicating a second stratigraphic sequence.
The Jubaila Formation is the upper part of what was previously called the Dukhan Formation. Its thickness ranges from 220 ft (67 m) in the southwest to 550 ft (168 m) in central Abu Dhabi (Fig. 9). After deposition of the Hanifa Formation a rise in sea level led to the deposition of slightly argillaceous and aphanitic deep-water lime mudstone facies throughout the area (Fig. 4) Because of shallowing to the northeast, toward the edge of the Hanifa intrashelf basin, however, the lithofacies passed into dolomitic limestone. The top of the Jubaila Formation has been picked at the base of the Arab Formation in the western area and at the base of the Asab Formation or Asab equivalent in the eastern area. The GR/CNL response shows some cyclicity and development of porosity in the offshore area which may be related to local structural growth (salt diapirs). The top of the Arab D reservoir might locally be considered to be an unconformity surface marking the end of the third sequence in the Upper Jurassic section.
In the central and western part of Abu Dhabi as well as most of Arabia, an arid climate prevailed during early Tithonian time and resulted in extensive evaporite deposits interbedded with carbonate beds (Fig. 5; Murris, 1980).
The Arab Formation ranges in thickness from zero beyond the pinchout of the Hith in central Abu Dhabi to more than 400 ft (122 m) in the western and northwestern area (Figs. 4, 10). It was deposited as a major regressive sequence that accumulated in a peritidal to supratidal environment. In general the Arab Formation consists of a series of depositional cycles each consisting of a lower bioclastic lime mudstone-wackestone and an upper thin tidal-flat dolomitic mudstone that grades upward into supratidal anhydrite units. It is dominated by porous dolomite and dense limestones interbedded with several streaks/beds of anhydrite. A relative drop in sea level in the western area resulted in deposition of the Arab-D oolitic packstone-grainstone section, and to the east (towards the deep marine area) a subtidal environment prevailed (Fig. 11). Three porous members of the Arab C, B, and A (in ascending order) are separated by anhydrite intervals (Fig. 12). The three zones show porosity development with a thinning trend towards the east (the Hith Anhydrite edge). It becomes difficult, therefore, to differentiate and correlate among the three Arab members toward the zero isopach.
The Hith Anhydrite is the top seal to the Arab hydrocarbon-bearing zones in the western part of Abu Dhabi. It is Late Tithonian in age and represents the final regressive supratidal stage of the major Late Jurassic depositional cycle. Toward Late Jurassic time, the climatic conditions became arid (restricted basin or sabkha) in western Abu Dhabi and shoal to deep marine conditions prevailed towards the eastern area and Oman.
The massive Hith Anhydrite thins gradually eastward across central Abu Dhabi and passes laterally into intertidal carbonate sediments of the Asab Formation (Fig. 13). The Hith seal plays an important role in the distribution of the Arab and Thamama group hydrocarbons. Most of the Thamama zones are hydrocarbon bearing in the absence of the anhydrite, and the Arab zones are hydrocarbon bearing where the anhydrite seal is present. Both the Arab and the Thamama are hydrocarbon bearing in the vicinity of the Hith Anhydrite edge (Alsharhan and Kendall, 1994).
The isopach map of the Hith Formation shows thickness variations from zero in central Abu Dhabi to 480 ft (146 m) in the western area. Also, differentiation between the Hith and the Arab Formations becomes less clear towards the Hith Zero line (Fig. 14). The top of the Hith (Jurassic) marks the fourth stratigraphic sequence in the Upper Jurassic section of western Abu Dhabi. East of the Hith anhydrite edge, the depositional environment ranged from shallow open marine to shoal, grading eastward to outer-shelf deeper open marine. The Asab Formation or Asab equivalent strata were deposited in this area. The formation is encountered in a north-south trend updip of a carbonate ramp, and it is interpreted as the eastern lateral equivalent of the Hith and the Arab formations (Fig. 14).
The lower part of the Asab Formation is an argillaceous, hard and dense lime mudstone grading upward into dolomite with scattered anhydrite nodules, which in turn is overlain by the Asab Oolite near the shoreline margin. East of the Asab Formation, toward the relatively deeper marine environment, the Asab equivalent section was deposited as lime mudstone facies. In the southeastern area, a deeper marine environment prevailed where glauconitic lime mudstone facies was deposited as a lateral equivalent of the Asab Formation (Fig. 13). The isopach map of the Asab Formation and its equivalent ranges from zero near the Hith anhydrite edge to 350 ft (107 m) in the onshore area (Fig. 15). The top of the Asab Formation (top Jurassic) could be an unconformity surface, which marks the fourth stratigraphic sequence in the eastern area of Abu Dhabi.
The Upper Jurassic section contains several reservoir intervals. The eustatic changes in sea level and the structural growth of the Mender-Lekhwair High together with local offshore salt diapirs played an important role in reservoir development (see de Matos, 1994; Alsharhan, 1989). Eight reservoirs are developed in the Upper Jurassic succession in onshore Abu Dhabi (Fig. 1): the Hadriya at the top of the Tuwaiq Mountain; the Hanifa at the top of the Hanifa Formation in the southeast area; the Asab Oolite in the central area (at the top of the Asab Formation); the Manifa reservoir at the top of the Hith Formation; the Arab Members A, B, and C in the Arab Formation; and Arab D at the top of the Jubaila Formation in the western area (west of the Hith Anhydrite edge) (Fig. 10).
Ten to sixteen relatively porous limestone or dolomite streaks/beds can be recognized throughout the Arab Formation (Fig. 12). Three porous streaks are identified in the Arab Member A, and Members B and C are variable in cyclicity and reservoir development. Therefore the differentiation between the Arab members is less clear towards the Hith anhydrite edge. The Arab zones are hydrocarbon bearing in most of the Abu Dhabi developed anticlines.
The top of the Arab D is picked at the base of the lowest anhydrite streak or dense limestone bed of the Arab Formation. The lithology is mainly oolitic packstone-grainstone of variable reservoir development in the western area. The base, although not clear, is picked at the base of the oolite section (Fig. 12). The Arab D is the uppermost part of the Jubaila Formation.
In the eastern area, the facies equivalents of the Arab D were deposited in open-shelf, deep marine conditions. The thickness ranges from 10 to 90 ft (3 to 30 m) near the Hith anhydrite edge. The Arab D porosity is variable, from fair to good, and the permeability ranges from 10 to 30 md.
Asab Oolite and Manifa Reservoirs
The Asab oolite reservoir, 15 to 170 ft (5 to 52 m) thick, constitutes the uppermost part of the Asab Formation. The section consists of well cemented oolitic packstone-grainstone underlain by dolomite, with scattered anhydrite nodules. The Asab oolite section exhibits good porosity (17–19%), and low permeability (0.9–3 md). The oolitic limestones were deposited in a north-south trend in a high-energy setting parallel to the Hith anhydrite edge (Fig. 14). The hydrocarbon generated from the Hanifa source rock in the presence of Hith anhydrite cap rock migrated vertically through the Asab oolite and laterally westward to the Lower Cretaceous Thamama Group (Fig. 16).
The Manifa reservoir is relatively porous wackestone-pack-stone interval, about 40 ft (12 m) thick, locally developed at the top of the Hith Formation, with average porosity of 6% and permeability of less than 4 md. The Manifa tested hydrocarbons in a few wells in the Abu Dhabi onshore area.
The Hanifa reservoir is the uppermost porous section at the top of the Hanifa Formation. It consists of a peloidal to oolitic packstone interval restricted to parts of the southeastern area of Abu Dhabi. It is predicted to form an updip stratigraphie pinchout along the western flank of the west Mender High. The thickness of the Hanifa reservoir reaches approximately 165 ft (50 m) in the vicinity of the Mender-Lekhwair High, and the average log porosity is 15%.
The Hadriya reservoir forms the uppermost porous section of the Tuwaiq Mountain in the southeast area. The interval is composed of medium- to coarse-grained, cemented pelletal packstones and grainstones. The Hadriya reservoir is up to 400 ft (122 m) thick; it has an average porosity of about 18%, and permeability ranges between 10 to 50 md.
The Upper Jurassic section contains a triplet of source rocks, reservoirs, and seals in the western area of Abu Dhabi. Based on the available geochemical information, the Hanifa is the main source rock for the Upper Jurassic and Lower Cretaceous hydrocarbons in the area. The Hanifa source rock entered the oil-generation window some 100 million years ago during middle Cretaceous time (Fig. 17).
During the Late Cretaceous the depth of burial of the Hanifa source rock was about 9000 ft (2743 m) in the Falaha trough, with average reservoir temperature of 140°C (peak of oil generation). The gradual temperature increase in the Falaha and Ghurab troughs is considered the main factor for gas generation. The Late Cretaceous to Early Tertiary is the possible time for oil migration from the Hanifa source rock, and the gas-migration phase took place possibly during Tertiary time as a second phase of hydrocarbon generation. Souring occurred in a subsequent phase of geothermal processes when H2S was generated in the Falaha and Ghurab troughs, in the presence of the Hith Anhydrite and the Arab Formation. The Hanifa source rock [about 260 ft (79 m) thick] may have generated more than 150 x 109 barrels of oil from the Central Abu Dhabi area (40,000 km2), assuming source rock potential (SI + S2) equal to 10 kg per ton, 75% conversion factor, 2.55 g/cm3 density, and 50% oil-migration efficiency (Taher, 1997; Hawas and Takezaki, 1995).
The Hith Anhydrite is a major hydrocarbon seal for the Arab Zones. It represents the final regressive supratidal stage of the last major Jurassic depositional cycle in the western area of Abu Dhabi. In the eastern area, east of the Hith edge, dense limestone of the Habshan Formation forms the seal for the Asab Oolite reservoir.
The Upper Jurassic section exhibits lateral variation in thickness, lithology, and reservoir development because of the eustatic changes in sea level and the structural growth of the Mender-Lekhwair High and the Qatar Arch. The Upper Jurassic witnessed a relative subsidence of the intrashelf basin between the the Mender High and the Qatar Arch, which resulted in deposition of the source rocks of the Hanifa Formation. The Late Cretaceous to Early Tertiary is the main phase of hydrocarbon (oil) generation and migration from the Hanifa source rock to the Upper Jurassic and Lower Cretaceous reservoirs. The Hith Anhydrite forms a major seal for the Arab zones.
The authors wish to thank the Management of ADNOC and ADCO for permission to publish this paper. Also the authors thank R.W. Scott, A.S. Al sharahan, I.A. Al Jalal, and B. Granier for their fruitful comments on editing this paper.
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Middle East Models of Jurassic/Cretaceous Carbonate Systems
This volume will interest tectonic modelers, stratigraphers, sedimentologists, and explorationists. It is the product of the international conference of “Jurassic/Cretaceous Carbonate Platform-Basin Systems, Middle East Models” that was convened in December 1997 jointly by SEPM (Society for Sedimentary Geology) and the United Arab Emirates University in Al Ain, United Arab Emirates. The twenty-three papers present new data and interpretations arranged in three sections: 1) sequence stratigraphy, cyclostratigraphy, chronostratigraphy, and tectonic influences, 2) depositional and diagenetic models of carbonate platforms, and 3) hydrocarbon habitat and exploration/development case studies. New tectonic models of the Arabian Basin, new stratigraphic and sequence stratigraphic reference sections, new geochemical and source rock data, and new reservoir data are presented. New geologic models make this set of papers relevant to geoscientists working outside of Arabia also.