Secondary accumulation of hydrocarbons in Carboniferous reservoirs in the northern Tarim Basin, China

Many oil and gas fields have been discovered in Carboniferous in the northern Tarim Basin. The oils and gases produced from the fields show great differences in their physical and geochemical properties. However, their origin and accumulation process have not been well studied even as they become a focus of exploration. Through comprehensive geological and geochemical analysis of the oils and gases, it is recognized that the oils were generated from Mid-Upper Ordovician source rocks, migrated from paleo-oil-accumulations in Ordovician, and were biodegraded to produce different heavy oils before the late Hercynian orogeny (about 250 Ma), so that the oils in the Carboniferous are secondary oil accumulations. Since the Yanshan orogeny (about 86–94 Ma), along with the quick formation of the Kuqa foreland basin, Carboniferous structures were reversed gradually from south dip to north dip, resulting in the Carboniferous nose-shaped traps. During the reversed, faults acted as migration paths connecting underlying Ordovician reservoirs to Carboniferous reservoirs. During the late period of the Himalayan orogeny (20-2 Ma), the Ordovician source rocks were buried so that this achieved level high maturity and generated gas in the eastern part of North Tarim, which resulted in gas accumulations in the eastern part of the studied area, where gas and condensate oil were discovered. The accumulation process of the secondary Carboniferous oil and gas was demonstrated by biomarker compositions, homogeneration temperatures of fluid inclusions and authigenic illite K–Ar dating analyses. Therefore, this conclusion of secondary accumulations of oil and gas in the Carboniferous reservoirs is significant for oil and gas exploration in the northern Tarim.