Controls on gas storage characteristics of Upper Paleozoic shales from the southeastern Ordos Basin

Potential shale gas resources in China are not only of marine but also of marine-terrigenous transitional and continental origins. To analyze the controls on gas storage capacity of shales from the Carboniferous Benxi and Permian Shanxi formations, high-pressure/temperature methane sorption isotherms (up to 25 MPa and 120 °C) and porosity have been measured on 12 samples from the Yishan slope. Total organic carbon (TOC) content of Shanxi and Benxi shales are low to moderate from 0.1% to 80%) is stored on clay minerals (illite/smectite mixed layer and kaolinite) controlling both sorption capacity and porosity. TOC content, due to its low values, is of minor importance. Benxi and Shanxi shales possess comparatively low gas storage capacities compared to producing marine shale gas systems. Combined with the large amount of clay minerals that make hydraulic fracturing more difficult they are regarded unfavourable for shale gas exploitation. Although a substantial amount of gas can be stored sorbed in excess of the free gas in gas shales, methane sorption has a negligible effect on gas production at pressures above 10 MPa. This has major implications for deep reservoirs where pore pressures in the matrix may not be reduced sufficiently before production ceases. •Competitive production and conversion between the excess sorbed gas and free gas during gas production has been described.•Sorption capacity and porosity are controlled by clay minerals rather than TOC content.•For shales with TOC