Geological Conditions of Oil Occurrence in Middle East Fields

The Middle East geological provinces are: (1) the Arabo-Nubian and Arabo-Somali massifs of pre-Cambrian igneous and metamorphic rocks in western and southern Arabia; (2) the foreland shelf, north and east of the massifs; and (3) the orogenic Taurus-Zagros-Oman mountain belt, peripheral to the foreland. Interpretations of the tectonics are controversial, some authors emphasizing the possible role of taphrogenesis and vertical movements in the basement, while others postulate a long history of intermittent compression (Alpine orogenies) from the north and northeast. All commercial oil so far discovered occurs in anticlinal traps in the foreland shelf and the orogenic mountain belt; though the possible presence of other types of accumulation is not excluded. The sedimentational history of the region was conditioned by slow progressive subsidence of the Arabian foreland, interrupted intermittently by epeirogenic uplift movements which are reflected in thickness and facies variations of the sediments. Deposits of successive marine cycles tongue into a flange of continental sands surrounding the Arabo-Nubian and Arabo-Somali massifs. Exclusive of non-productive pre-Mesozoic and post-Miocene rocks, the stratigraphy of the oil-field belt consists of: (1) Triassic to Lower Cretaceous—mainly chemical limestone-dolomite deposition; prolific oil producer in eastern Arabia; (2) Middle Cretaceous to Oligocene—(a) globigerinal limestone, chalk, and marl, with oil in associated reef complexes and in fractured limestone in northern Iraq, southwestern Iran, and southeastern Turkey; (b) sands and shales from western shorelines; prolific production in Bahrein, Kuwait, and Basrah; and (3) Miocene—rapidly alternating evaporites, limestones, and clastics; major oil pools in southwestern Iran and Iraq. Fracturing and availability of plastic or non-fractured cover are important factors in the migration and accumulation of Middle East oil.