The Upper Cretaceous Shilaif intra-shelf basin at the eastern margin of the Arabian platform contains rich source rock sequences in the hydrocarbon generation window, which are targets for ongoing exploration work. Time stratigraphy and sedimentology are key components to unravel the complex development of the hydrocarbon system. This paper presents detailed bio-, chemo- and sequence-stratigraphic data in combination with sedimentological analysis from core, well, seismic and outcrop data to support an analysis of basin architecture and regional time stratigraphic correlations. The Late Albian to Turonian saw the final intra-shelf basin development and infill on the eastern margin of the Arabian plate Cretaceous carbonate platform. Basin development began with the deposition of the Late Albian Mauddud Formation. Mauddud cycles are laterally continuous over large distances but thin in western and central Abu Dhabi in the area of the future intra-shelf basin. Differential aggradation is likely to have been accentuated by unfavorable climatic and water-quality conditions related to the Albian/Cenomanian Oceanic Anoxic Event. The Mauddud is overlain in the west and east of Abu Dhabi by the shallow marine aggrading & prograding carbonates of the Mishrif Formation with a SSW-NNE elongated intrashelf basin forming in central Abu Dhabi. This basin is filled by the Shilaif, Tuwayil and Ruwaydah Formations. Four depositional sequences with organic rich intervals are recognized in the Shilaif Fm corresponding in time to pulses of aggradation, progradation and retrogradation of Mishrif shallow-water platform margin sediments. The Tuwayil is a shaly lowstand/transgressional sequence onlapping onto the Mishrif, which was at this time partially exposed based on erosional channel incisions on the platform top. The Ruwaydah Fm is composed significantly of calcispheres recording continued transgression, increased accomodation space and an increase of carbonate production. The Coniacian Laffan shales unconformably overlie the Cenomanian/Turonian sedimentary sequences. Basin-fill sequences have been age dated with detailed chemo- and biostratigraphy. Based on carbon isotopes the Mauddud predates/coincides with the OAE at the Albian/Cenomanian boundary and is hence late Albian. Other carbon peaks, ammonites, calcispheres and planktonic foraminifera align the Shilaif with the Cenomanian reaching near its top the Cenomanian/Turonian boundary. The Tuwayil and Ruwaydah Fms are Turonian.
A sequence stratigraphic study of the Cenomanian-early Turonian Mishrif Formation has been carried out in the SW of the United Arabian Emirates (UAE) in order to understand the evolution of the reservoir distribution and occurrence of potential stratigraphic and diagenetic traps in the UAE. The study has used an integrated reservoir approach that includes regional geology, seismic stratigraphy together with well data. The use of the core data and logs has allowed identification of facies changes, key stratigraphic surfaces and a number of a few feet thick dense intervals that show a relatively high lateral correlatibility, which appear to be below the seismic resolution. The use of seismic attributes together with the well calibration has been crucial to define seismic facies and their lateral and vertical distribution. Results from this study allowed recognition of several smaller scale 3rd order cycles, which are comprised in a larger 2nd order genetic sequence. Each small-scale cycle is bounded by a sequence boundary overlain by a flooding event made up of a thin dense interval which shows an increase in argillaceous content and locally chert. These sequences are characterised by an eastward progradation from muddy inner ramp facies passing upwards to rudist-rich patch reef/mixed skeletal shoal complexes developed in an inner to mid-ramp depositional setting. This latter setting appears to be best developed in the uppermost large-scale sequence, below the regional unconformity that separates the Mishrif from Ruwaydha and Tuwayil formations. Highly porous facies appear to decrease seismic velocity. The integrated approach presented here captures the variety in depositional environments, architecture, geometry and reservoir heterogeneity, which results in higher likehood to identify stratigraphic and diagenetic traps. Furthermore, the recognition of seismic characteristics of the Mishrif reservoir may be used as a tool to predict the distribution and evolution of units that create velocity variations in the Mishrif Formation and which affect the structural definition of deeper reservoirs as the Shuaiba Formation.
The occurrence of major hydrocarbon prone Mesozoic source rock sequences of the eastern Arabian plate is directly tied to the generation of intra-shelf basins within the giant carbonate platforms that formed during this time period. This paper investigates the driving forces behind the formation of intra-shelf basins and related source-rock/seal sequences. Results impact topics such as exploration, reservoir distribution, regional tectonics and climate modelling. A combination of large scale regional stratigraphic correlations, age dating, geochemical indicators and global climatic/tectonic events are investigated to explain differences and commonalities in basin formation and their impact on source rock seal pairs. In the Mesozoic major intra-shelf basins existed during three time periods: in the Late Jurassic during the Oxfordian to Tithonian, and in the Cretaceous during the Aptian and the Cenomanian. The late Jurassic basin is predominantly generated by plate margin tectonism possibly in conjunction with the rejuvenation of major WNW-ESE and N-S basement structures. Uplift at the eastern plate margin during the late Jurassic caused exposure at the eastern plate margin towards the Neo-Tethys, which in combination with sea level fluctuations resulted in the deposition of several large scale cycles. In central Abu Dhabi westward progradation of the Tuwaiq Mountain, Hanifa and Jubaila Sequences into an intrashelf basin are key evidence for the eastern uplift as are large scale collapse features reported from the eastern margin itself. The widespread deposition of the Arab and Hith anhydrites in the interior of the eastern Arabian plate are taken as further evidence for tectonically driven basin isolation leading to restricted evaporitic conditions. In contrast, the Cretaceous Aptian and Cenomanian intra-shelf basins formed mainly as a consequence of environmental/climatic disturbances associated with global oceanic anoxic events. During these times of global climate stress carbonate sedimentation was unable to keep up in areas with relatively high subsidence rates and laterally segregated providing the impulse for a switch from flat-topped platforms to a nascent basin topography (Hawar & Thamama A/Shuaiba during the latest Bar/Early Aptian; AP Apt 1), and Mauddud FM (latest Albian). Subsequent differential aggradation in combination with continued subsidence led to the full development of the basin topography in the early Aptian (AP Apt2–4; Bab Basin) and early Cenomanian (Shilaif Basin). Significantly, detailed carbon isotopic data indicate that climatic disturbances and the onset of oceanic anoxic events correspond to the generation of the initial topography and not to the onset of the deposition of organic rich basin fill sediments. Subsequent to deposition of organic rich sequences Cretaceous intra-shelf basins are dominated by argillaceous limestones and siliciclastics (Bab basin: Upper Bab Member - AP Apt5 and basal Nahr Umr AP-Apt6; Shilaif Basin: lower & upper Tuwayil FM). Thus, basin fill differs significantly between the late Jurassic tectonic basin featuring evaporate seals (Arab and Hith FM) and the Cretaceous climatic/constructional intrashelf basins being covered by clastic-rich sequences. Finally, it is postulated that the formation of another intra-shelf basin associated with the Valenginian Oceanic Anoxic Event (OAE1) was prevented by a regional tectonic uplift and platform exposure during the Late Valenginian.
In early Aptian times, subtle tectonic movements may have been activated along the NW-SE strike-slip faults and have resulted in a vertical displacement along these faults. The displacement would have allowed the carbonate-producing organisms to colonize along the shallower southern margin and generate well developed reservoir facies. The basinal facies were deposited to the north of the shelf margin, which is known to be the Bab Basin. Significant oil was discovered in the Shuaiba shelf facies. However, the lagoonal and basinal facies have potential for discovering a significant volume of hydrocarbon, especially in the fields that are located in the Upper Thamama hydrocarbon migration pathways. This potential is supported by the absence of an effective seal separating Thamama Zone-A from Shuaiba basinal facies above, which allowed for the Zone-A hydrocarbon to migrate vertically into the Shuaiba basinal facies. In addition, this potential was supported by the hydrocarbon shows while drilling and by the interpreted well logs, which confirm the presence of movable hydrocarbon in the Shuaiba lagoonal and basinal facies. The Shuaiba Formation is comprised of two supersequences (Azzam and Taher, 1995, van Buchem et al., 2010 and Lyndon et al., 2010) that were deposited during the Aptian time. The lower sequence started at the early Aptian time and mainly included Hawar Member, Thamama Zone-A, the Shuaiba shelf facies and its time equivalent Shuaiba basinal facies sediments. The second sequence was deposited after the drop in sea level below the Shuaiba shelf break. This drop caused subaerial exposure of the Shuaiba shelf platform and allowed for the clastics material to influx the fine-grained over the Shuaiba basinal facies area. The clastic influx and the carbonate rich claystone of the Late Aptian sequence sediments may represent the effective top seal for the Shuaiba basinal facies and Zone-A oil accumulation. The Shuaiba basinal facies were deposited in an intrashelf basin that was enclosed by the Shuaiba shelfal facies sediments. This resulted in restricted water circulation, anoxic condition and deposition below the wave base. Such depositional environment is favourable for source rock preservation. Lithologically, Shuaiba basinal facies consist of pelagic lime-mudstone, wackestone and packstone with abundant planktonic microfossils. These facies are characterized by low permeability values, but their porosity can reach up to 20%. The lagoonal sediments consists of a deepening sequence of carbonate sediments, with shallow marine algal deposits at the base and fine hemipelagic to pelagic carbonates in the upper section. The differences between the Shuaiba Shelf and the Shuaiba Basin are mainly in permeability values. By applying the latest technology in horizontal drilling and hydraulic fracturing, the Shuaiba basinal facies will produce a significant volume of hydrocarbon.
ADCO has drilled a number of wells and evaluated hydrocarbon potential of the Middle Cretaceous Wasia Group Unconventional reservoirs onshore Abu Dhabi since 2012. Latest logging and core analysis technologies were applied to assess key parameters such as Total Organic Carbon (TOC), source rock maturity, mineral compositions and fluid saturations, as well as geomechanical parameters such as Young's modulus, Poisson's ratio and minimum horizontal stresses. To date more than 2500ft of core has been extracted, described and analysed. We understand that ADCO has, by far, the largest data base among all the OPCOS with regards to unconventional play and has gained a significant local learning curve ahieved over the last 3 years. In addition to triple combo data, log data acquired consists of NMR, high resolution lateralog, dielectric logs, and mineral spectroscopy, cross dipole sonic, borehole imaging and sonic scanner, routine and advanced mudlogging. Core analysis consists of crushed rock analysis, pyrolysis and rock mechanics testing. Significantly high mud gas readings, observation of oil staining, odor, and fluorescence under UV light indicate presence of oil over large sections of the acquired cores and side wall cores from various geographically spread wells. Recently, we have conducted 5000 TOC and 1500 Pyrolysis measurements from core and cuttings samples in 110 wells spread all over Abu Dhabi. This data clearly demonstrate that large sections of Wasia Group bear excellent source rock quality in several wells located in large segments of onshore Abu Dhabi. Visual inspection of core and TOC measurements indicate that high frequency jet black centimeter scale organically rich lamellae often exceeding 20% TOC in several wells are not uncommon. The unconventional reservoir rocks are composed of predominantly clean and tight matrix carbonate mudstones and wackestones. Preliminary results of the current exploration campaign within the ADCO concession are very encouraging, showing indications of hydrocarbon presence. Formation testing is planned in the near future to confirm the Unconventional play in Abu Dhabi. The paper focuses on an integrated multi-disciplinary approach covering petrophysical, geological, geochemical and geomechanical assessment with the ultimate goal to determine optimal parameters for formation testing and production.
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