Geologic and hydrologic characteristics of North American coal-bearing basins with biogenic gas accumulations
|Location||International Geological Congress,oslo 2008|
|Author||Warwick, Peter D.; Flores, Romeo M.|
|Holding Date||28 September 2008|
Several basins in North America (NA) contain significant economic accumulations of coalbed gas of biogenic origin. The rank of the coal beds in the basins that contain primary biogenic gas ranges from lignite to bituminous and their ages range from Cretaceous to Paleogene. The NA basins that currently produce coalbed gas primarily sourced from microbial activity include the Gulf of Mexico (GOM) Basin in the southeastern U.S., the Powder River Basin (PRB) in Wyoming and Montana, the Western Canadian Sedimentary (WCS) Basin in Alberta, and the Cook Inlet—Matanuska Basin in southern Alaska. Biogenic gas within and below permafrost zones also has been discovered, although no production has occurred in the Coleville Basin, North Slope, Alaska. Low-rank coal in the Williston Basin in North and South Dakota, and Saskatchewan, has minor, sub-economic amounts of biogenic gas. Coal-bearing basins in NA that contain primarily coalbed gas of thermogenic origin commonly have some amount of secondary biogenic gas. Examples of basins with accumulations of mixed thermogenic and biogenic coalbed gas include the Illinois, Black Warrior (both Carboniferous), and San Juan (Cretaceous) basins in north-central, southeastern, and southwestern U.S., respectively.
There is a broad range of geologic and hydrologic characteristics associated with biogenic gas accumulations in these basins. For example, in the GOM Basin, biogenic gas occurs in subbituminous coal beds, as much as 7 m in thickness that range in depth from 500 to 1500 m. Associated coal formation water quality is high in total dissolved solids (TDS, ranging from 7,000 to 16,000 mg/L), primarily as a result of salt (Jurassic age) dissolution in the groundwater. In the PRB, biogenic gas occurs in coal beds, as much as 60 m in thickness that range in depth from 50 to 1000 m. Associated formation water quality in the PRB is relatively fresh (TDS from 270 to 2,010 mg/L). Isotopic data from coal-derived biogenic gas in both basins indicate that the primary methanogenic pathway for microbial gas generation is through the reduction of CO2; however, significant mixing of biogenic gas formed by acetate fermentation does occur in some parts of these basins. Published data from the WCS, the Cook Inlet—Matanuska, and the Coleville basins indicate that these areas have diverse geologic settings (e.g. coal thickness, rank, depths, structural setting). In contrast, low-rank coal basins with relatively minor biogenic gas generation and accumulation (Williston Basin and parts of the GOM Basin) may be limited by chemical and hydrological factors such as elevated amounts of sulfate in the groundwater or excessive groundwater flow and removal of produced gas. An improved understanding of the geologic and hydrologic conditions that control biogenic gas generation and accumulation is needed to help design exploration programs and to evaluate the potential for underground microbial stimulation and gas regeneration.