Controls on Gas Domains and Production Behaviour in A High-Rank CSG Reservoir: Insights from Molecular and Isotopic Chemistry of Co-Produced Waters and Gases from the Bowen Basin, Australia

摘要

This paper uses hydrochemical and multi-isotope analysis to investigate geological controls on coal seam gas (CSG) saturation domains and gas well production performance in a high-rank (vitrinite reflectance (Rv) 1.1) CSG field in the north-western Bowen Basin, Australia. New hydrochemical and stable isotope data were combined with existing geochemical datasets to refine hypotheses on the distribution and origins of CSG in two highly compartmentalized Permian coal seams. Stable isotopic results suggest that geographic variations in gas content, saturation and production reflect the extent of secondary microbial gas generation and retention as a function of hydrodynamics. δ 13 C and δ 2 H data support a gas mixing hypothesis with δ 13 C-CH 4 increasing from secondary biogenic values to thermogenic values at depth (δ 13 C ?62.2‰ to ?46.3‰), whereas correlated methane and carbon dioxide carbon isotope compositions, Δ 13 C(CO 2 –CH 4 ) values and δ 13 C DIC /alkalinity trends are largely consistent with microbial CO 2 reduction. In addition, below 200 m, the majority of δ 13 C-CO 2 values are positive (δ 13 C: ?1.2‰ to 7.1‰) and δ 13 C DIC shows an erratic increase with depth for both seams that is characteristic of evolution via microbial activity. The progression of carbon isotope values along the CO 2 reduction fractionation line suggests progressive depletion of the CO 2 reservoir with increasing depth. Faults clearly segment coal seams into areas having significantly different production, with results of geochemical analysis suggesting that pooling of biogenic gas and waters and enhanced methanogenesis occur north of a faulted hinge zone.