Hydrocarbon occurrences and exploration suggestions in the foreland basins of central western China

The foreland basins in NW China have multi cycle composite features: (1)the Mesozoic (including the late period of late Paleozoic in local areas) sediments of the foreland basins are main bodies of basin fillings, which are completely composed of nonmarine sequences; (2)the sedimentary sequences underlying the Mesozoic foreland basins lack passive continental margin sediments of typical foreland basins and mainly consist of the late Paleozoic or even earlier folded basement or foreland basins; (3)the Mesozoic foreland basins are generally overlain by the Cenozoic rejuvenated foreland basins. The Triassic and Jurassic source rocks (including the Carboniferous Permian source rocks in some regions) are major source rocks in the foreland basins of central western China, whose formations are controlled by depressions or faulted sags developed in the early stage of the foreland basins and warm humid climate. These source rocks have big thickness, high organic richness, and predominant typeⅢ kerogen. Hydrocarbon generating peak appeared late and oil/gas pools formed late as well, which were controlled by the stacked Neogene sediments deposited in rejuvenated foreland basins. Natural gas accumulation is dominated in the region of the thick sediments of the stacked rejuvenated foreland basins because buried depth is very deep and hydrocarbon generation is at the high matured to over matured stage, while oil accumulation is dominated in the region of the thin, stacked sediments. Hydrocarbon occurrences are obviously controlled by basin architecture of foreland basins. There are different play types within different structural units. The thrust belt: the formation of oil and gas pools is related to thrust faulting, and hydrocarbon migration is vertically dominated, showing the feature of short distance close to source rocks. The foreland slope belt: the formation of oil and gas pools is of stratigraphic facies pool type and fault block pool type, displaying features of long distances and lateral directions of hydrocarbon migration. Richness of hydrocarbon accumulation in northwestern Chinese foreland basins is controlled by basin prototype development, stacking and reformation. The early formed, long evolutionary history, continuous developed or stacked foreland basins have good hydrocarbon potential, while relict foreland basins have poor hydrocarbon potential. On the basis of analyzing hydrocarbon geological conditions of foreland basins and their folded thrust belts, further exploration fields are proposed. These favorable fields contain the Kuqa depression of the Tarim basin, the southern margin of the Jiuquan basin, the southwestern margin of the Tarim basin, the nouthern margin of the Jiuquan basin, the southwestern margin of the Qaidam basin, the western Sichuan depression and the northern margin of the Qaidam basin.