Orderliness of hydrocarbon distribution and main controlling factors of hydrocarbon accumulation in the Persian Gulf Basin

Based on the achievements and research advances in oil and gas exploration in the Persian Gulf Basin, this study analyzes the orderliness of oil and gas distribution and main controlling factors of hydrocarbon accumulation with reservoir-forming assemblage as the unit. In the Persian Gulf Basin, the hydrocarbon-generating centers of source rocks of different geological ages and the hydrocarbon rich zones migrate in a clockwise direction around the Ghawar Oilfield in the Central Arabian Subbasin. Horizontally, the overall distribution pattern is orderly, showing “oil in the west and gas in the east”, and “large oil and gas fields dense in the basin center and sparse at the basin edges”. Vertically, the extents of petroleum system compounding and sources mixing increase from west to east, the pattern of tectonic strength (weak in the west and strong in the east) forming the distribution characteristics of “gas rich in the Paleozoic, oil rich in the Mesozoic, and both oil and gas rich in the Cenozoic”. The large scale accumulation and orderly distribution of oil and gas in the Persian Gulf Basin are controlled by three factors: (1) Multiple sets of giant hydrocarbon kitchens provide a resource base for near-source reservoir-forming assemblages. The short-distance lateral migration determines the oil and gas enrichment in and around the distribution area of effective source rocks. (2) The anhydrite caprocks in the platform area are thin but have experienced weak late-stage tectonic activities. Their good sealing performance makes it difficult for oil and gas to migrate vertically to shallow layers through them. The thrust faults and high-angle fractures formed by intense tectonic activities of the Zagros Orogenic Belt connect multiple source-reservoir assemblages. However, the Neogene Gachsaran Formation gypsum-salt rocks are thick and highly plastic, generally with good sealing performance, so large-scale oil and gas accumulations are still formed beneath the salt; (3) Each set of reservoir-forming assemblages is well matched in time and space in terms of the development of source rocks and reservoir-caprock assemblages, the maturation and hydrocarbon generation of source rocks, and the formation of traps, thus resulting in abundant multi layer hydrocarbon accumulations. At present, the Persian Gulf Basin is still in the stage of structural trap exploration. The pre-salt prospective traps in effective hydrocarbon kitchens remain the first choice. The areas with significant changes in Mesozoic sedimentary facies have the conditions to form large scale lithologic oil and gas reservoirs. The deep Paleozoic conventional oil and gas reservoirs and the Lower Silurian Qusaiba Member shale gas have great exploration potential and are expected to become important reserve growth areas in the future.