AAPG Middle East Region Geoscience Technology Workshop

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Structural, Geomorphological and Stratigraphic Controls on the Deposition of the Paleocene Shammar Formation of North Oman


The Shammar formation hosts the recently discovered first Cenozoic oil play in North Oman. It is composed of a thin, sub-seismic succession of clastic deposits overlying several important Cretaceous oil fields. The main reservoir is the basal Shammar sandstone, which is 1 to 5m thick. It is stratigraphically sealed by the Shammar shales and lies uncomformably on a folded and eroded succession of Aptian Shuaiba carbonates, Albian Nahr-Umr shales and Albian to Cenomanian Natih carbonates. Paleocene in age, the North Oman Shammar sediments were deposited in an incised valley system as part of an estuarine environment during an overall transgressive episode. The deposition took place at the end of the Alpine I regional phase of deformation, at the front of the Semail ophiolite and Hawasinah thrust. Observations suggest that at deposition time, the structure and geomorphology of the substratum were strongly controlled by earlier Mesozoic deformation phases. These included Triassic to Jurassic extension, Late Cretaceous compression and, finally, late Cretaceous transtension. The substratum structure, its paleo-geomorphology and its response to the pre- and syn-depositional erosion processes were major parameters affecting the depositional environment and the heterogeneous distribution of the Shammar sediments. This talk will delineate the paleo-geomorphology of the Pre-Shammar substratum as a result of the local structural evolution and subsequent erosion. A link will be made between these processes and the present-day subsurface distribution of the Shammar as sampled by a significant number of wells, and imaged by encouraging preliminary seismic attributes.