Stable and radiogenic isotopic analysis of aquifer systems, Atlantic Rim, Carbon County, Wyoming: Implications for production of coalbed natural gas

Coalbed natural gas (CBNG) production requires the extraction of considerable volumes of water from target formations. This process can dynamically alter local aquifers and affect the larger hydrologic systems of a producing area. An analytical method that provides immediate, cost-effective quantitative information on both resource (methane) and habitat (coalbed aquifer) would help to optimize gas production.This study used a combination of field measurements, water chemistry analysis, and isotopic analysis, both stable (delta13C DIC, deltaO, deltaD) and radiometric (87Sr/ 86Sr), to analyze and characterize the CBNG aquifers and hydrogeologic systems of Wyoming's Atlantic Rim. Waters were sampled and analyzed from streams, springs, and CBNG wells across the Atlantic Rim. Samples were first grouped on the basis of geologic location, and then additionally defined by isotopic and water chemistry analysis into Mesaverde Group springs, Lewis Shale springs, Steele Shale springs, Sand Hill springs, enriched delta 13CDIC springs, methane springs, and subsurface samples (monitoring and CBNG wells). Two distinct water chemistry types are evident in Atlantic Rim samples, Ca-Mg-SO4-type and Na-HCO3-type waters. Atlantic Rim samples also had distinct radiogenic isotopic signatures. Stream water sourced from the Sierra Madre and waters associated with the Mesaverde Group have the highest 87Sr/86Sr ratios, whereas spring samples from the Lewis Shale have the lowest 87Sr/ 86Sr.delta13CDIC, which is enriched by bacterial methanogenesis, was used to identify coalbed waters from other natural waters. Positive delta13CDIC identified spring waters that originated from Mesaverde coalbed aquifers, including methane springs. Strongly positive delta13CDIC of Atlantic Rim CBNG samples identified those coalbed aquifers that are hydraulically isolated, whereas lower delta13CDIC identified wells within open aquifer systems and wells with inefficient casing.This study demonstrated that delta13C DIC was able to help identify hydraulic aquifer isolation, well casing integrity, springs associated with methanogenic coalbed aquifers, and areas of peak methanogenic activity, all of which can be utilized to assess gas potential and promote efficient CBNG production. This abstract supports a supplementary file.