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The Evolution of Coalbed Reservoir Fluids from Outcrop into the Basin: Applying Isotopic and Geochemical Techniques to Define Fluid Pathways and Methanogenic Processes, with Implications for Coalbed Natural Gas Production

Quillinan, Scott A.; Frost, Carol D.; McLaughlin, J. F.

This study characterizes the geochemical and isotopic processes of groundwater (GW) in coalbed reservoirs in two Wyoming coalbed natural gas (CBNG) production areas. These study sites include the Big George coalbed of the Tertiary Fort Union Formation in the Powder River Basin (PRB) CBNG field, and the Cretaceous Mesaverde coals of the Atlantic Rim CBNG field. Focusing on water quality and δ13CDIC, δ18O and δD we compare data from regional transects through these areas to determine geochemical trends that identify the influence of faults, determine methanogenic pathway, define GW flow paths, and constrain the timing of methanogenesis.

Our results demonstrate that δ13CDIC and δD vary modestly but regularly along flow path, reflecting continued methanogenesis and the influence of different methanogenic pathways near recharge and at depth. As a result of methanogenesis, δ13CDIC rapidly increases proximal to outcrop and continues to increase into the basin (3.5‰ to 26‰). Near the basin axis in the deeper coalbeds of the PRB, enriched δ13CDIC decreases (23‰ to 13‰) and δD increases (-151‰ to -135‰) with increasing residence time, effects attributed to methanogenesis dominated by CO2 reduction. δD of coalbed water is fractionated as a result of methanogenesis by this pathway, thus the δD plots above the meteoric water line and is not indicative of initial meteoric conditions. Methanogenic processes do not affect the δ18O of the formation water. Though Na+ and HCO3 increase along flow path, only the HCO3 increase is directly related to methanogenesis. Our results suggest that methanogenesis may increase HCO3 concentrations of reservoir fluids by as much as 65 meq/L.

The gradual increase in δ13CDIC, δD and HCO3 with no change recorded in δ18O is the result of uninterrupted flow from outcrop into the basin. However, we have documented that faulted areas can be associated with the disruption in the isotopic and geochemical trends described above. Abrupt changes in δ13CDIC, δD, δ18O, HCO3 among other constituents in fluids sampled proximal to faults indicate that the faults are non-sealing. Non-sealing faults may affect CBNG production, making it more difficult to lower the hydrostatic head needed to produce the gas. Detailed examination of the stable isotopic and geochemical trends of a CBNG reservoir with adequate data distribution can be instructive in defining methanogenic processes, GW flow paths, and the hydraulic confinement properties of a reservoir.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013