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AAPG Bulletin, V.
Geochemical signature of formation waters associated with coalbed methane Wayne A. Van Voast1
Wayne A. Van Voast has worked as a well-site geologist in the United States and Canada, as a hydrologist with the U.S. Geological Survey in Minnesota, and as a mineral fuels geologist and a hydrogeologist with the Montana Bureau of Mines and Geology. He has published numerous technical reports on coal and ground water and is currently researching the aspects of the occurrence of methane with coal.
The Montana Bureau of Mines and Geology is gratefully acknowledged for providing time and administrative support for this article. Special thanks go to James Castro, chief chemist for the Bureau, for technical editing of the chemical presentation. Numerous agencies provided data on production-water quality; specific gratitude is owed the Alberta Geological Survey, the Colorado Department of Public Health and Environment, the Colorado Oil and Gas Conservation Commission, the Montana Bureau of Mines and Geology, and the Utah Division of Oil, Gas, and Mining.
Formation waters associated with coalbed methane have a common chemical character that can be an exploration tool, regardless of formation lithology or age. Effectively devoid of sulfate, calcium, and magnesium, the waters contain primarily sodium and bicarbonate and, where influenced by water of marine association, also contain chloride. The distinct geochemical signature evolves through the processes of biochemical reduction of sulfate, enrichment of bicarbonate, and precipitation of calcium and magnesium. Cation exchange with clays may also deplete the dissolved calcium and magnesium, but is not prerequisite. Low sulfate/bicarbonate ratios characterize these waters and are also common but less pronounced with occurrences of conventional oil and gas. Waters rich in sulfate, calcium, and magnesium occur in many coalbed aquifers but are not found in association with methane.
Users of total dissolved solids data should ensure that the values reflect adjustments of bicarbonate concentrations to simulate evaporation residues. Results that erroneously sum the entire bicarbonate content can be far too high in these bicarbonate-rich waters, thereby exacerbating the issues of disposal.
Evaluations of prospects and choices of exploration targets can be enhanced by an added focus on the geochemical signature that should be expected in association with methane. Knowledge of the geochemical signature may also be useful in the commonly protracted testing of wells. The appearance of high sulfate concentrations in water analyses can justify early curtailment of test pumping and can prompt the siting of subsequent drill holes farther from areas of recharge.
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