Applying isotopic geochemical proxy for gas content prediction of Longmaxi shale in the Sichuan Basin, China

Numerous studies have been conducted on the isotopic geochemical characteristics of shale gas. However, a systematic study of the relationship between the isotopic geochemical characteristics and gas content has not been addressed to date. In this study, we investigated the geochemical and isotopic compositions of shale gas samples, and the gas content of the Lower Silurian Longmaxi marine shale in the Huangjinba area, southern Sichuan Basin, China. In addition, we analyzed the quantitative relationship between the geochemical parameters and the gas content. The Longmaxi shale gas is a dry gas with a low wetness (0.12%–0.67%), dominated by methane (89.49%–99.00%) with a small amount of ethane (0.11%–0.61%). The average values of δ13Cmethane, δ13Cethane and δ13Cpropane are −29.8‰, −35.0‰, and −33.2‰, characterized by a partial isotopic reversal (δ13Cmethane > δ13Cpropane > δ13Cethane) with a maturity level (Ro ≈ 2.30%–2.51%). The total gas content of the Longmaxi shale varies widely from 0.42 m3/t to 3.88 m3/t. Correlation analyses revealed that the ethane isotope (δ13Cethane) and the magnitude of isotopic reversal (δ13Cmethane - δ13Cethane) exhibit strong correlations with the total gas content and the free gas content. This study demonstrates that carbon isotopic reversal of shale gas largely depends on the secondary cracking, and thermal maturity and residual liquid hydrocarbons content are important factors affecting the magnitude of carbon isotopic reversal. Based on the quantitative study conducted in this paper, we provide a new proxy for predicting the gas content of high maturity shale using an isotopic method. •Carbon isotopic reversal of shale gas largely depends on the secondary cracking.•Sufficient residual liquid hydrocarbon is necessary for the carbon isotopic reversal.•Wetness of 1.4% in high maturity shale gas indicates the peak of secondary cracking.•δ13Cmethane–δ13Cethane can be used to predict the gas content of high maturity shale.