Geochemical Characteristics and Origin of Shale Gases From Sichuan Basin, China

Natural gases from the Taiyang (shallow), Jiaoshiba (middle), and Weirong (deep) shale gas fields in the southern Sichuan Basin were analyzed for molecular and stable carbon isotopic compositions to investigate the geochemical characteristics and gas origins. All the gases belong to shale gas from the Upper Ordovician-Lower Silurian shale and are dominated by methane with gas wetness generally less than 0.83%. The delta C-13(1) values are -28.5 parts per thousand, -30.3 parts per thousand, and -35.2 parts per thousand in Taiyang, Jiaoshiba, and Weirong shale gas fields, respectively. The extremely high thermal maturity is the controlling factor for the enrichment of C-13 in methane, with a minor contribution from the heavy carbon isotope of the organic matter in the Ordovician Wufeng Formation. Fischer-Tropsch-type synthesis of hydrocarbon gas from CO2 and H-2 contributes to the increase of wet gas, which results in the offset from the delta C-13(1)similar to wetness linear trend in the Taiyang and Jiaoshiba gas fields. Methane, ethane, and propane in the Taiyang shale gas field have increasing delta C-13 values with increasing burial depth, which is mainly caused by diffusive migration. All gases are characterized by a complete carbon isotopic reversal trend (delta C-13(1) > delta C-13(2) > delta C-13(3)), and it is mainly caused by the reversible free-radical reactions with the conversion from alkane to alkyl groups, with some contribution from the Fischer-Tropsch-type synthesis. The results of this study will improve our understanding of the geochemical characteristics of shale gases from different burial depths and have important implications for future shale gas exploration in the deep and shallow layers.

Automated summary

This study analyzed gases from shale gas fields in the southern Sichuan Basin to investigate their geochemical characteristics and origins. The gases belonged to shale gas from the Upper Ordovician-Lower Silurian shale and were dominated by methane. The isotopic compositions of carbon indicated high thermal maturity and contributions from organic matter. The study found a complete carbon isotopic reversal trend and highlighted the importance of understanding the geochemical characteristics of shale gases for future exploration.