Isotopic and hydrochemical evidence for the origins of unconventional gases in the Middle and Lower Jurassic coal measures in the Fukang area of Junggar Basin, NW China

Extensive coalbed gas exploration has been conducted in the Fukang area of the Junggar Basin in China, but no significant breakthroughs have yet to be achieved, partly due to the unclear origin of coalbed gas. In this study, the compositions and isotopes of coalbed, shale, and tight sandstone gases, along with the chemical characteristics of formation water are reported. The results indicate that the coalbed, shale, and tight sandstone gases are dominated by CH₄. Based on the carbon isotopes of CH₄ and CO₂, the coalbed gas in the Fukang area is primarily biogenic gas via CO₂ reduction pathway, whereas the shale and tight sandstone gases are mainly biogenic gas via acetate (methyl-type) fermentation pathway. The formation water is chiefly derived from glacial meltwater with the chemical type of Na-HCO₃. High concentrations of Na⁺, Cl⁻, and HCO₃⁻ and low concentrations of Mg²⁺, Ca²⁺, and SO₄²⁻ indicate that the study area is distant from the water recharge area. In the contact area between the recharge runoff and the stagnant area, microbial modification of thermogenic gas is enhanced, leading to an enrichment of secondary biogenic gas. The findings of this study contribute to the geological understanding of coalbed, shale, and tight sandstone gases, providing valuable insights for future exploration efforts in the Fukang area.