Modeling hydrocarbon accumulation based on gas origin and source rock distribution in Paleozoic strata of the Ordos Basin, China

Paleozoic gases in the Ordos Basin, China, are produced from upper Paleozoic tight sandstones, Ordovician weathered-crust, and 'pre-salt' (beneath gypsum deposits) dolomite reservoirs. Their origins have been debated, particularly as to whether Carboniferous-Permian (C-P) or Ordovician source rocks contributed more gas to the lower Paleozoic gas reservoirs. The expanded exploration of lower Paleozoic gas to the eastern, western, northern, and southern regions of the basin has re-initiated the debates. In this study, geochemical characteristics of gas samples from newly drilled wells in different regions of Ordos Basin in combination with the source rock evaluation were used to determine the gas origin. The carbon and hydrogen isotope of alkane indicate that gas mixing occurred in both upper Paleozoic and Ordovician weathered-crust gases, and that TSR occurred in pre-salt gases. A modified delta C-13-1/C-n plot was suggested to be useful to determine the carbon isotopic patterns of gases generated from the different source rocks. The light hydrocarbon characteristic shows that oil-type gas mainly distributes in the eastern basin. A study of source-rock distributions also indicates that the most favorable Ordovician source rock is located in the eastern basin. Based on the distribution of C-P coal measures, limestones and lower Paleozoic carbonate source rock, a gas accumulation history was established. The results of this study provide applicable method for identifying gas origin in Ordos Basin. Moreover, the accumulation history proposed in this study is useful for further exploration of the lower Paleozoic gas in Ordos Basin.