Components and stable carbon isotopic fractionation mechanisms of over-mature coal-derived gas in the Daji gas field, Ordos Basin, NW China

Based on the natural gas composition and stable carbon isotope data from the Upper Paleozoic tight sandstone gas in the Daji gas field, Ordos Basin, and through a comparative analysis of the geochemical characteristics of typical overmature coal-derived gases in China and the world, this study clarified the geochemical features and the origins of stable carbon isotopic anomalies of overmature coal-derived gas, and revealed the components of overmature coal-derived gas and the mechanisms of stable carbon isotopic fractionation and their geological implications. The research shows that the Upper Paleozoic tight gas in the Daji gas field is dominated by methane, and its stable carbon isotopic compositions exhibit a large-scale full reverse sequence, suggesting that it was primarily originated from a mixture of kerogen, crude oil, and wet gas cracking gases during the over-mature stage of coal-measure source rocks. Vertically, with the thick limestone of the Permian Taiyuan Formation as a boundary, two gas-bearing systems are delineated in the upper and lower sections with gas respectively supplied by the source rocks of the second member of Permian Shanxi Formation and the Carboniferous Benxi Formation, which exhibit significant differences in migration and accumulation patterns and exploration directions. A three-stage evolution pathway for the stable carbon isotopic composition sequence in overmature coal-derived gas is proposed. This full reverse sequence is not only controlled by the mixed-source genesis effects during the overmature stage, but also influenced by the migration fractionation effects resulting from the preferential diffusion of natural gas generated at this stage. Both factors have, to some extent, enhanced the abundance of coal-derived gas resources in the area, although the enrichment effects of natural gas differ across the various gas-bearing systems.