Gas geochemistry and hydrochemical analysis of CBM origin and accumulation in the Tucheng syncline in western Guizhou Province

Abstract

Coalbed water plays an important role in the process of coalbed methane (CBM) generation, storage (adsorption) and production. Because integrated analysis of hydrochemical characteristics and gas geochemical characteristics can offer fresh insights into CBM origin and accumulation, this paper presents an in-depth analysis of the gas geochemistry and hydrogeochemistry characteristics of CBM coproduced water to trace in-situ CBM accumulation in the Tucheng syncline. The main gaseous geochemical parameters (C₁/∑C_1–5: 0.97~0.99; C_HC: 38.3~72.6; CDMI: 5.1~10.5%; δ¹³C-CH₄: –40.7~–43.2‰; δD-CH₄: –185.8~–169.2‰; δ¹³C-CO₂: –13.4~–9.3‰) indicate that thermal gas is the primary genetic type, and hydrogeochemical analysis reveals strong traces of microbial activity. Based on carbon isotope fractionation calculation of DIC-CO₂-CH₄ in CBM and coproduced water, we established a quantitative method for calculating biogenic methane production in CBM coproduced water, finding that production can reach 0.002~0.104 m³/t. Accordingly, we identify a mixed pattern of thermogenic and secondary biogenic CH₄. Taking into account the region’s tectonic evolution, we conclude that primary biogenic gas was generated from the Permian to the Early Triassic, thermogenic gas from the Middle Triassic to the Middle Cretaceous, and secondary biogenic gas from the Late Cretaceous to the Quaternary. Secondary biogenic methane’s contribution to the gas pool is limited (20.8%~27.6%), whereas thermogenic methane accounts for 72.4%~79.2%.