Origin and diffusion of the over-mature transitional natural gas in multiple lithologic reservoirs: A case study of Carboniferous-Permian strata in the southeastern margin of Ordos Basin

To investigate differences in gas origin and diffusion among various lithologies in the continent–marine transitional strata of the Lower Permian Shanxi Formation (P1s) and Upper Carboniferous Benxi Formation (C2b) in the southeastern margin of Ordos Basin, 31 natural gas samples were collected from the existing lithologies (including mudstone, sandstone, and coal) and their gas components and carbon and hydrogen isotope compositions were measured. Remarkable differences in the ethane (δ13C2) and carbon dioxide isotope values (δ13CCO2) within natural gas were found between the Shanxi and Benxi Formations: the δ13C2 value of natural gas in the Shanxi Formation is generally less than −30‰, whereas that of the Benxi Formation is generally greater than −28‰; and CO2 in the Shanxi Formation is mainly of organic origin while that in the Benxi Formation is mainly inorganic. The unusually depleted δ13C2 values found in the Shanxi Formation were caused by the addition of oil-type gas generated from the carbonate rock in the Taiyuan Formation, which resulted in partial reversal of the carbon isotope values (δ13C1 > δ13C2). However, this incorporation of oil-type gas did not occur in the Benxi Formation, which is most likely caused by the tight mudstone at the bottom of Taiyuan Formation. The differences between the Shanxi and Benxi Formations reveal that the gases in the two Formations belong to two separate gas systems, and this could be attributed to the tight mudstone and limestone existing between the reservoirs, which acts as an effective barrier preventing diffusion of gases.