Geochemical characteristics of Longtan transitional shale gas in the western Hubei Basin, northwest Middle Yangtze Block: Implications for the origin and carbon isotopes

Abstract

Shale gas geochemical characteristics are critical for genetic identification and enrichment mechanism analysis. Current research focuses predominantly on marine shale gas, whereas studies on transitional shale gas (particularly the Permian Longtan Formation in South China) remain relatively limited, which constrains our understanding of the geochemical characteristics and genesis of transitional shale gases. The geochemical characteristics of transitional shale gas from the upper Permian Longtan Formation in western Hubei Province indicate that the organic matter is predominantly sapropelic, with some humic organic matter. The gas composition of the Longtan Formation consists primarily of CH₄, with a low content of C₂H₆. The δ¹³C_CH4 values range from −25.40 ‰ to −21.70 ‰, the δ¹³C_C2H6 values range from –32.00 ‰ to −27.02 ‰, and the δ²H_CH4 values range from −124.01 ‰ to −119.46 ‰. These findings imply that shale gas is composed mainly of oil-type gas of thermal origin, with a potentially minor presence of mixed gas. Isotope analysis of the shale gas reveals that it has undergone reversal (δ¹³C_CH4 > δ¹³C_C2H6), which is attributed to the cracking of liquid hydrocarbons under overmature conditions. The CO₂ content ranges from 0.21 % to 2.33 % and the δ¹³C_CO2 values range from −21.80 ‰ to −19.00 ‰, suggesting that the CO₂ in the study area is of organic thermal origin. Additionally, a geochemical evolution pattern suggests that the gas composition of different sedimentary phases is controlled by the type of kerogen and the degree of thermal evolution. The multistage cracking of organic matter thermal evolution products caused the dynamic changes in carbon isotopes.