Abstract

The post-Hercynian sequence of the Middle East is dominated by carbonate sedimentation on a stable platform, flanked to the northeast by the Tethys ocean. Two principal types of depositional systems can be recognized, which alternate in time: 1. “ramp”-type mixed carbonate-clastic units and 2. “differentiated carbonate shelves”. The first type represents regressive conditions, when clastics were brought into the basin resulting 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 dominated the scene: 1. broad regional paleo-highs, 2. horsts and tilted fault blocks trending north-northeast - south-southwest and 3. salt domes. All three influenced deposition through synsedimentary growth. The subsequent stage, from Turonian to Maastrichtian, 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 Zagros anticlinal traps.

Source rocks were formed in the starved basins during the transgressive periods. Marginal mounds, rudist banks, oolite bars and sheets and regressive sands form the main reservoirs. Supratidal evaporites and regressive shales are the regional seals. The spatial arrangement of these elements, together with the development of source maturity through time, go a long way in explaining the observed distribution of the oil and gas fields.