ISOTOPIC ANALYSIS OF AQUIFER SYSTEMS, ATLANTIC RIM, CARBON COUNTY, WYOMING: A NEW TOOL FOR CHARACTERIZING COALBED NATURAL GAS SYSTEMS

Coalbed natural gas (CBNG) production requires the extraction of water from target formations. This process can alter local aquifers and larger hydrologic systems in a producing area. We describe isotopic tracers that provide immediate quantitative information on the presence or absence of natural gas and the degree to which the methane-containing strata are hydrologically isolated. Isotopic analysis, specifically ⁸⁷Sr/⁸⁶Sr and the carbon isotope signature of dissolved inorganic carbon (δ¹³C_DIC), supplemented by field measurements and water chemistry analysis, were used 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.

Sr isotopic composition and major ion compositions identify two groups of water samples from the Atlantic Rim. Stream water sourced from the Sierra Madre and waters associated with the Mesaverde Group are Ca-Mg-SO₄-type and have higher ⁸⁷Sr/⁸⁶Sr ratios reflecting relatively young ground waters recharged from precipitation in the Sierra Madre. Lewis and Steele shale samples are Na-HCO₃-type waters with lower ⁸⁷Sr/⁸⁶Sr resulting from longer residence time during which sulfate reduction and water-rock interaction produced the observed chemical and Sr isotopic compositions.

δ¹³C_DIC, which is enriched by bacterial methanogenesis, was used to distinguish coalbed waters from other natural waters. Enriched δ¹³C_DIC, between -3.6 to 13.3‰, identified spring waters that originated from Mesaverde coalbed aquifers. Strongly positive δ¹³C_DIC, between 12.6 to 22.8‰, of Atlantic Rim CBNG produced water samples identified those coalbed aquifers that are hydraulically isolated, whereas lower δ¹³C_DIC, between 0.6 to 9.9‰, identified wells within open aquifer systems and wells with poorly cemented casing.

These results demonstrate that ⁸⁷Sr/⁸⁶Sr ratios and δ¹³C_DIC provide immediate quantitative data to help identify ground water sources, hydraulic aquifer isolation, well completion 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.