The Jurassic Arabian Intrashelf Basin provides the setting for the world's greatest conventional oil reserves, including the world's largest oilfield, the supergiant Ghawar field. The stratigraphic interval corresponding to the development and infill of the Arabian Intrashelf Basin is from the uppermost Dhruma Formation to the top of the Hith Anhydrite Formation, spanning the late Bathonian–early Callovian to Tithonian. Many areas of the intrashelf basin have been well described in recent years and the stratigraphic succession has been defined in sequence concepts, but the regional development of the intrashelf basin has not been well synthesized. This Memoir builds on published data to give a regional interpretation of the geological evolution of the Arabian Intrashelf Basin. This introductory chapter reviews some of the earlier work, summarizes the key events and elements in the geological history of the Arabian Intrashelf Basin and gives a brief review of the history of petroleum exploration in this region. It is intended to serve as an extended abstract to introduce the general setting and summarize the contents of this Memoir, including some of the proposed revisions of depositional models, correlations and the sequence nomenclature, providing a context for considering and evaluating each subsequent chapter. The themes summarized in this chapter are documented and discussed in much greater detail in the subsequent chapters of this Memoir.
The Arab-C and -D reservoirs in the Qatif field are carbonate members of cycles which deposited shallow marine carbonate sediments capped by regressive sabkhah anhydrites. The impermeable capping anhydrites prevented subsequent extensive fresh water influx and thus prevented extensive late diagenetic cementation. As a result, much primary and early diagenetic porosity has been preserved. The Arab-D carbonates are mostly fine grainstones which were deposited in current-swept shelf environments and later extensively dolomitized. The dolomitized grainstones are the best Qatif Arab-D reservoir rock. The Arab-C carbonates consist of a lower transgressive oolite-mollusc grainstone facies (with high primary porosity and high permeability), a middle regressive lime mudstone – algal boundstone facies, (ranging from tight to moderately porous and permeable), and an upper slowly transgressive Clypeina algal and peloid facies (with high primary and moldic porosity and good permeability).
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