Cratonization of the Arabian Shield: The Basement Grain is Imprinted
The evolution and consolidation of the Arabian eraton has been the subject of considerable investigation and research over the past two decades most of which is outside the scope of this book. However, the early evolution created prominent basement grain directions and shear fracture systems (most notably on north-south and northwest-southeast trends) that later had varied effects on younger structuring and sediment patterns. This in turn controlled major hydrocarbon occurrences. Therefore, a brief summary is essential to understanding hydrocarbon occurrence.
The evolution of the Arabian basement and consolidation of the Arabian shield have been discussed by many authors whose work was reviewed by Beydoun (1988, 1989b). Stoesser and Camp (1985) and Brown et al. (1989) provided excellent recent succinct summaries. Husseini (1988, 1989) also provided recent reviews that addressed some of the contradictions in different basin evolution models and attempted to reconcile them.
The primary crust of the western Arabian shield is composed of a combination of several constructional units, each of which is an intra-oceanic island arc terrane consisting of an andesitic assemblage of metavolcanic rocks and a dioritic suite of plutonic rocks. There are at least three such terranes. The ensimatic island arc development and progressive cratonization by coalescence of arc terranes took place over a long time span that began about 950 Ma with formation of the neocraton by accretion of arcs. Each closure and arc collision led to deformation and ophiolite obduction now preserved as cryptic sutures that segment the shield in approximately north-south units 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.