Geochemistry and accumulation process of natural gas in the Shenmu Gas Field, Ordos Basin, central China

The chemical and stable isotopic compositions of 24 gas samples were investigated to determine the geochemistry and accumulation process of the Shenmu Gas Field (SGF), Ordos Basin, central China. Natural gas of the SGF includes both wet and dry gas with dryness coefficients (C-1/C1-5) between 0.907 and 0.958. delta C-13(1) and dD1 values range from -39.4 parts per thousand to -35.6 parts per thousand and -191 parts per thousand to -201 parts per thousand, respectively, displaying a positive isotopic series. Geochemical characteristics indicate that gases of the upper Paleozoic reservoirs of the SGF are typical coal-type gas, whereas gases of the lower Paleozoic reservoirs are primarily oil-type gas (occasionally containing a small proportion of coal-type gas). Gas-source correlation demonstrates that gases of the upper Paleozoic reservoirs of the SGF display near-source accumulation, primarily derived from the underlying Carboniferous-Permian humic source rocks, while gases of the lower Paleozoic reservoirs in the SGF are mainly sourced from the carbonate Majiagou Formation (O(1)m). SGF reservoirs were generally tight prior to gas accumulation. Integrated with the maturation process of source rocks, the accumulation process of gas in the SGF could be separated into four stages: prenatal (before early Jurassic), development (early-late Jurassic), prime (early Cretaceous), and formative (late Cretaceous-present).