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.