Natural gas accumulation in the Upper Paleozoic of Ordos Basin, China

The natural gas pools developed within the Upper Paleozoic of Ordos Basin are of low porosity, low permeability and low pressure. The gas accumulation characters and processes were analyzed through the study of fluid inclusion homogeneous temperature, fluid inclusion laser-Raman spectrum, bitumen distribution and genesis, and rock slices, together with the experimental data of the composition of the natural gases, light hydrocarbons from C_ 5 to C_ 8 and individual carbon isotope. Three gas plays were identified as inside source play, P_ 1 s_ 2 -C_ 3 t; top source play, P_ 1 s_ 1 -P_ 1 x_ 8 ; and outside source play, P_ 2 s. Four gas pool-forming stages developed in Upper Paleozoic, including CO_ 2 injection, light liquid hydrocarbon injection, extensive gases injection and the dynastic stage between the gain from coal-absorbed gas and the loss from cumulative natural gas. The accumulation processes and key factors of different types of gas pools were also studied.