Gas compositional and isotopic variation during desorption of middle-upper Permian shales in western Hubei, Southern China

Abstract

Significant progress has been made in exploring shale gas resources in the middle-upper Permian strata of western Hubei. However, the unclear geochemical characteristics and origins of the gas have hindered a comprehensive understanding of reservoir evolution and have constrained integrated resource development. This study involved a 28-h desorption experiment in which gases released from shales of the Dalong and Gufeng Formations were systematically collected for compositional and carbon–hydrogen isotope analyses. Comparing the compositional and geochemical characteristics of the desorbed gases and investigating their genetic origins resulted in four main findings: (1) Gas composition during the middle stage of desorption, excluding H₂S, best represents the original reservoir fluid, with component variations linked to adsorption and diffusion capacities. (2) Both shales display similar gas compositions with trace H₂S; the Dalong shale contains lower CH₄, CO₂, and H₂S but higher C₂H₆ and N₂. (3) Carbon and hydrogen isotopes indicate oil-type thermogenic gases, with carbon isotope reversal reflecting tectonic uplift, depressurization, and the mixing of primary kerogen-derived and secondary oil-/condensate-derived gases. (4) N₂ likely originates from ammonium-bearing silicate decomposition, CO₂ is mainly inorganic, from thermal carbonate decomposition with minor organic contribution, and H₂S may result from oil cracking or thermochemical sulfate reduction. This study provides a comprehensive record of the fluid characteristics of middle-upper Permian shale gas reservoirs and enhances the understanding of their formation and evolution.