Abstract

Coalbed natural gas (CBNG) production typically requires the extraction of large volumes of water from target formations, thereby influencing any associated reservoir systems. We describe isotopic tracers that provide immediate data on the presence or absence of biogenic natural gas and the identify methane-containing reservoirs are hydrologically confined. Isotopes of dissolved inorganic carbon and strontium, along with water quality data, were used to characterize the CBNG reservoirs and hydrogeologic systems of Wyoming's Atlantic Rim. Water was analyzed from a stream, springs, and CBNG wells.

Strontium isotopic composition and major ion geochemistry identify two groups of surface water samples. Muddy Creek and Mesaverde Group spring samples are Ca-Mg-SO 4–type water with higher 87Sr/ 86Sr, reflecting relatively young groundwater recharged from precipitation in the Sierra Madre. Groundwaters emitted from the Lewis Shale springs are Na-HCO 3–type waters with lower 87Sr/ 86Sr, reflecting sulfate reduction and more extensive water-rock interaction.

To distinguish coalbed waters, methanogenically enriched graphic was used from other natural waters. Enriched graphic, between −3.6 and +13.3‰, identified spring water that likely originates from Mesaverde coalbed reservoirs. Strongly positive graphic, between +12.6 and +22.8‰, identified those coalbed reservoirs that are confined, whereas lower graphic, between +0.0 and +9.9‰, identified wells within unconfined reservoir systems.

These results demonstrate that graphic analysis provides immediate data to help identify Atlantic Rim groundwater sources, hydraulic reservoir confinement, springs associated with methanogenic coalbed reservoirs, areas of peak methanogenic activity, and to help assess gas potential and promote efficient CBNG production.

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