The Upper Cretaceous Shallow Microbial Gas-Water System in Montana, Saskatchewan, and Alberta—Regional Variations in Stable Isotopes, Noble Gases, and ¹²⁹I/I Ratios

Ridgley, Jennie L.¹, Glen T Snyder^2
1 U.S. Geological Survey, Denver, CO
² Rice University, Houston, TX

Regional trends in the chemical and isotopic composition of microbial gas and co-produced water from shallow gas reservoirs in Montana, Saskatchewan, and Alberta reflect multiple periods of structural and hydrodynamic influences on the spatial distribution of gas and water chemistry. Samples from the Upper Cretaceous Eagle Sandstone, Milk River Formation, Medicine Hat Formation and its equivalents, Bowdoin Sandstone Member of the Carlile Shale, and upper and lower members of the Belle Fourche Shale (2WS sandstones of industry usage) show differences in the chemistry and stable isotopic composition in the gas and co-produced water, and in the noble gas composition of the gas.

⁴⁰Ar/³⁶Ar ratios (316-518) for gas samples are more than the atmospheric value (295.5), indicating the presence of excess ⁴⁰Ar (⁴⁰Ar*) from the decay of K within the crust. ⁴He/⁴⁰Ar* ratios for all gas samples are from 2 to 7 times the crustal radiogenic production ratio (4.9), indicating an excess of ⁴He relative to ⁴⁰Ar*. A crossplot of deuterium isotopes in methane and co-produced water shows disequilibria relations for some samples, indicating these gases did not form in the present aqueous environment. Mixing relations between ¹²⁹I in connate and surface waters also imply compartmentalization. Uncorrected ¹²⁹I ages of the co-produced waters range from 23.5 to 101.4 Ma (n=26), although most ages are within the range 29.6 to 65.5 Ma (n=24). The chemical and isotopic chemistry of gas and water will be used to model the influences of fluid flow and compartmentalization on formation of the microbial gas accumulations.