Middle East--Stratigraphic Evolution and Oil Habitat: ABSTRACT

Abstract

The post-Hercynian sequence of the Middle East is dominated by carbonate sedimentation on a stable platform flanked on the northeast by the Tethys ocean. Two principal types of depositional systems alternated in time: (1) ramp-type mixed carbonate-clastic units and (2) differentiated carbonate shelves. The first type was deposited during regressive conditions, when clastics were brought into the basin and resulted in layer-cake formations. The second type was formed during transgressive periods and is dominated by carbonate cycles separated by lithoclines, time-transgressive submarine lithified surfaces. Differentiation is marked, with starved euxinic basins separated by high-energy margins from carbonate-evaporite platforms. The tectonic development of the Middle East can be divided into several stages. The first stage, which ended with the Turonian, was characterized by very stable platform conditions. Three types of positive elements were dominant: (1) broad regional paleohighs; (2) horsts and tilted fault blocks trending NNE-SSW; and (3) salt domes. All three influenced deposition through synsedimentary growth. The subsequent stage, from Turonian to Maestrichtian, was one of orogenic activity, with the formation of a foredeep along the Tethys margin and subsequent ophiolite-radiolarite nappe emplacement. From the Late Cretaceous to the Miocene, the platform regained its stability, only to lose it again at the close of the Tertiary, when the last Alpine orogenic phase affected the region, creating the Z gros anticlinal traps. Source rocks were formed in the starved basins during the transgressive periods. Marginal mounds, rudist banks, oolite bars and sheets, and regressive sandstones form the main reservoirs. Supratidal evaporites and regressive shales are the regional seals. The spatial arrangement of these elements and the development of source maturity through time explain the observed distribution of the oil and gas fields. The Middle Jurassic to Albian sequence in the central part of the Gulf, around the Qatar Peninsula, provides a well-studied example of the control on oil distribution by the distribution in space and time of mature source beds, effective regional seals, and reservoirs. Oil, tar, and extract typing as well as maturation studies show that the Upper Jurassic Hanifa bituminous limestone is the source for the oil and tar in the Jurassic as well as the Lower Cretaceous reservoirs, the latter reservoirs being only charged where the intervening regional Hith seal is either absent through nondeposition or is breached through faulting. Growth structures draining mature Hanifa kitchens contain sizable accumulations, whereas the Jurassic and Cretaceous reservoirs of the large Qatar dome contain o ly minor amounts of oil, which can be ascribed to insufficient source maturity and too late closure. Geochemical and geologic evidence indicates that the tar mats present at the base of many oil accumulations are not the result of biodegradation or early, immature expulsion from the source, but probably the product of gas deasphalting of reservoired oil. End_of_Article - Last_Page 1358------------