Chemical and stable isotopic evidence for water/rock interaction and biogenic origin of coalbed ethane, Fort Union Formation, Powder River Basin, Wyoming and Montana USA

摘要

Significant amounts (>36 million m~3/day) of coalbed methane (CBM) are currently being extracted from coal beds in the Paleocene Fort Union Formation of the Powder River Basin of Wyoming and Montana. Information on processes that generate methane in these coalbed reservoirs is important for developing methods that will stimulate additional production. The chemical and isotopic compositions of gas and ground water from CBM wells throughout the basin reflect generation processes as well as those that affect water/rock interaction. Our study included analyses of water samples collected from 228 CBM wells. Major cations and anions were measured for all samples, δD_(H2O) and δ~(18)O_(H2O) were measured for 199 of the samples, and δD_(CH4) of gas co-produced with water was measured for 100 of the samples. Results show that (1) water from Fort Union Formation coal beds is exclusively Na-HCO_3-type water with low dissolved SO_4 content (median