Research progresses in geological theory and key exploration areas of coal-formed gas in China

Based on the research progress of the geological theory of coal-formed gas, the contributions of coal-formed gas to the natural gas reserves and production in China and to the development of natural gas in major gas-producing basins are analyzed, and the key favorable exploration zones for coal-formed gas in China are comprehensively evaluated. The following results are obtained. First, coal measures are good gas source rocks, and hydrocarbon generation from coal measure was dominated by gas, followed by oil. Second, a natural gas genetic identification index system based on stable isotopes, light hydrocarbon components, and biomarkers is established. Third, the quantitative and semi-quantitative factors controlling the formation of large gas fields, represented by the indicator of gas generation intensity greater than 20×10 8 m 3 /km 2 , are identified to guide the discovery of large gas fields in China. Fourth, coal-formed gas is the major contributor to the current natural gas reserves and production of China, both accounting for over 55%. The high proportion of coal-formed gas has enabled the Tarim, Sichuan and Ordos basins to be the major gas production areas in China. Fifth, coal rock gas is an important field for future exploration of coal-formed gas, and key zones include the Carboniferous Benxi Formation (Fm) in the Wushenqi-Mizhi area of the Ordos Basin, the Permian Longtan Fm in central-southern Sichuan Basin, the Jurassic Xishanyao Fm in the southern margin and Luliang uplift of the Junggar Basin. Sixth, tight gas is the main area for increasing reserves and production, and the favorable exploration zones include the Carboniferous–Permian in southern Ordos Basin and the Bohai Bay Basin, and the Triassic Xujiahe Fm in the transition zone between central and western Sichuan Basin. Seventh, the Jurassic in the southern margin of the Junggar Basin is a key favorable exploration zone for subsequent investigation of conventional coal-formed gas. These insights have valuable theoretical and practical significance for further developing and improving the theory of coal-formed gas, and guiding the exploration of coal-formed gas fields in China.