Tectono-sedimentary Evolution and Petroleum Systems of the East Gobi Basin in Mongolia,East Asia

The East Gobi Basin is the most promising petroliferous rift basin in Mongolia, East Asia. Its tectono-sedimentary and hydrocarbon conditions are controlled by the subduction of the Indian plate and the Pacific plate. Based on... [ view full abstract ]

The East Gobi Basin is the most promising petroliferous rift basin in Mongolia, East Asia. Its tectono-sedimentary and hydrocarbon conditions are controlled by the subduction of the Indian plate and the Pacific plate. Based on the analysis of structural, stratigraphic, seismic and well logging data, the structural evolution can be divided into the basement, foreland, rift and depression periods. At the end of the Paleozoic, Mongolian sea died out due to the collision between the Siberian plate and the Sino Korean plate, accompanied by the formation of the basement. Sediments are predominantly marine siliciclastic rocks and carbonate rocks. The basin entered into the foreland stage during Triassic to Middle Jurrasic, which is characterized by intense structural activities. Alluvial fan sand, conglomerate and shale was developed in the Middle Jurrasic whose thickness was 1500 meters. During the late Jurrasic to early Cretaceous the crust extended, and the basin went into the rift period, which can be subdivided into early expansion, deep depression and shrinking periods. Alluvial fan, fan delta and fluvial sand, conglomerate, variegated mudstone with some andesite were developed in the early expansion stage which were 1000 meters thick. The lake was deepest at the deep depression stage. The sediments were mainly composed of lacustrine dark shale, braided river delta and near-shore submarine sandstones which were 300 meters thick. The lake became shallow at the shrinking stage. Fan delta sand, conglomerate and some coal seams were developed which were 670 meters thick. The rift period consisted of a completed lacustrine transgressive-lacustrine regressive cycle. With the release of tectonic stress in the late Cretaceous, the crust cooled down and contracted. The basin developed into the depression stage. Sediments consisted of alluvial fan and fluvial sand, conglomerate and shale which were 750 meters thick. Affected by compression from the late Yanshan to Himalayan movement. The basin uplifted and denuded at the end of the Cretaceous. The rift period formed the most hydrocarbon potential of the sedimentary sequence. The basin developed a set of Cretaceous petroleum system. The source rock of this system is semi-deep to deep lake dark shale from the lower Cretaceous whose kerogen type is I. Total Organic Hydrocarbon is 1.74%～16.65%. Reservoirs are fan delta, braided river delta and near-shore submarine sandstones whose porosity is 13%～18%. Permeability is 2～474 mD. Cap rock is composed of intraformational shale of the lower Cretaceous and regional shale in the upper Cretaceous.