Post-Paleozoic Development: Formation of the Arabian Northeast Passive Margin
The northeast Arabian plate margin initially formed as a result of the Late Permian extension and rifting. However, in the Triassic, this led to the breakup, subsidence and separation of former fragments of marginal Gondwana (represented by central Iran and other blocks, such as the Sanandaj/Sirjan magmatic arc, northwest Iran and part of Turkey) from the Arabian margin as part of the Cimmerian continent. The initial opening of the neo-Tethys ocean can perhaps be considered a back-arc basin (Sengor, 1990). Although compressed by Tertiary collision, this margin now extends from northwest Iraq, east of the north-trending Ha'il-Ga'ara (-Khleisia) arch of Paleozoic or older origin (Figure 1), to Oman.
An extraordinarily wide, shallow, marine (epeiric) shelf developed on this Mesozoic passive margin. The vast areal extent of this shallow sea (no comparable modern analog on this scale exists), engendered a remarkable uniformity of deposits and the great lateral extent of potential source, reservoir, and seal lithofacies. The southeast (Oman-Somali) margin and the north and northwest (Levant) margins evolved either simultaneously with the northeast margin or very shortly after. Present plate reassemblies require that the southeast margin formed when the Afghan and perhaps part of central Iran block separated from between Arabia and India along transform fractures, as the breakup of western and eastern Gondwana proceeded. The north and northwest margins formed by propagation of the neo-Tethys spreading center northwestwards, and by drift of the main Turkish blocks from the Levant northwards (Figure 11).
Arid and semi-arid conditions characterized the Triassic, and
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Arabian Plate Hydrocarbon Geology and Potential—A Plate Tectonic Approach
Reported proven hydrocarbon reserves of the Arabian plate region at the start of 1991 totaled 663.2 billion barrels (B bbl) of oil and 1,325.4 trillion cubic feet (tcf) of gas (66.4% and 31.5% of the world's oil and gas reserves, respectively). More than 98% of these are concentrated in the northeast margin region between northwest Iraq and Central Oman and lie in reservoirs ranging in age from late Paleozoic to early Neogene. Additional reserves, however, increasingly are being established along the other Arabian plate margins and in intra-plate basins. Occurrence of reserves, age and distribution of the sediments that generated or preserved them, and the formation of the mainly large structural (and other) traps are linked intimately to differing histories of plate margin evolution. The proper understanding of these histories could lead to additional reserves being established. The Arabian plate margins evolved at different times, the first being the northeast passive margin. This permitted the almost uninterrupted accumulation of thick sediments over a vast area including areally extensive organic-rich source rock deposits as well as good reservoir and seal units. The north/northeast margin(s) became a col-lisional boundary and a new Levant margin became a transform boundary in the Neogene.
Consolidation of the Afro-Arabian craton in the latest Proterozoic and Early Cambrian created a prominent north-south basement “grain” and a northwest-southeast (Najd) shear fracture system. Rejuvenations (affecting structures/sediment patterns) occurred in later periods and have controlled major hydrocarbon occurrences.
From latest Proterozoic to late Paleozoic time, the present north/northeast Arabian plate margin region, Anatolia, central Iran and the Afghan and Indian plates formed part of the long and very wide northern passive margin of Gondwana. This region was intermittently covered by shallow epeiric seas and bordering lowland.