Mechanisms for the accumulation of deep gas in the southern Songliao Basin, China

The Lower Cretaceous Yingcheng Formation in the southern Songliao Basin contains inorganic CO2, hydrocarbon, and mixed-gas reservoirs and is overlain by Denglouku Formation sandstone hosting secondary hydrocarbon gas reservoirs. We measured the diffusion coefficients of CH4 and CO2 in volcanic rock samples collected from five wells in the Yingcheng Formation. Our results show that the diffusion coefficient of CH4 is greater than that of CO2. Following the accumulation of hydrocarbon gas in the Yingcheng Formation, the region was affected by movement along deep-seated faults, resulting in the upward migration of mantle-derived inorganic CO2 into hydrocarbon gas reservoirs in volcanic rocks of the Yingcheng Formation, displacing pre-existing hydrocarbon gas. The hydrocarbon gas was transported upward into Denglouku Formation sandstone (which has favorable physical properties for gas storage) and was then enriched to form secondary gas reservoirs.