Sedimentary basins containing low-rank coals like those that occur within the Illinois Basin have recently attracted interest for coalbed methane (CBM) exploration because microbially-produced, biogenic coalbed gas can occur in  economically significant quantities even when coals are not sufficiently matured to have produced thermogenic gases. Our research project aims at reliable biogenic coalbed gas prediction via comprehensive geochemical and isotopic monitoring. In our first year of PRF-funding we advanced several related coalbed gas sub-projects.

We compiled gas characteristics along the southeastern margins of the Illinois Basin and scrutinized regional versus local gas variations in Seelyville and Springfield coal beds (manuscript submitted to the International Journal of Coal Geology). Our findings suggest that relatively high permeability and shallow depths of these Indiana coals (100-250 m) allowed inoculation with methanogenic microbial consortia, thus leading to microbial methane generation along the eastern marginal part of the Illinois Basin. These high volatile bituminous coals with a vitrinite reflectance R_o ~ 0.6% contain significant amounts of coal gas (~3 cm³/g) with ≥ 97 volume % microbial methane. Geographic lateral and vertical variations of coal gas parameters such as the ratio of [methane/(ethane + propane)], the ¹³C/¹²C ratio of methane, and the carbon isotopic difference between carbon dioxide and methane in Springfield and Seelyville coals in Indiana are typically small. Amounts of coal gas can vary significantly vertically within a coal seam and laterally from location to location. Therefore sampling of an entire core section is required to yield an accurate estimate of coal gas reserves across a seam. Some gas and co-produced water parameters drift over the time of production, e.g. the ratio of [methane/(ethane + propane)] increases, and the ¹³C/¹²C ratio of methane and the amount of “total dissolved solids” in co-produced waters decrease. Furthermore, prolonged CBM production may enhance access of fresh oxygenated meteoric waters into relatively shallow coal seams resulting in microbial methane oxidation

The total coalbed gas content is controlled by the amount of generated, adsorbed and preserved microbial methane, and therefore can be locally variable depending on (i) microbial accessibility (via cleats and microfractures of proper size), (ii) adsorption potential for methane molecules (micropore distribution) of a particular coal section, and (iii) availability of target organic moieties for potential biodegradation and methanogenesis.

Some of our most valuable, currently accumulating data derive from a research-based relationship with the owner/operator of a commercial CBM play in the Illinois Basin in Southern Indiana. We have access to (i) CBM production histories, (ii) collecting and desorbing coal from new exploratory boreholes, and (iii) unrestricted sampling of CBM production wells. We collected comprehensive field and laboratory data, starting at the exploration phase and continuing through the subsequent years of CBM production. We have sampled gas and production waters from several wells periodically over last 27 months to investigate production-related compositional and isotopic trends. No significant trends have been detected as yet. We will continue this periodic sampling in the next years and expect that more time is needed for consistent trends to develop.

We started collecting coalbed gases for geochemical and isotopic characterization from two monitoring and one CO₂ injection well. CO₂ will be injected into the coal beds in May 2008. In the next phases of the project we will investigate to what extent the injection of CO₂ will influence the characteristics of gases in the coalbed. We will continue measuring gases from the two monitoring wells to detect compositional and isotopic changes that will constrain/trace CO₂ migration.

Our first year of PRF funding for research on coalbed gas partially supported the dissertation projects of two graduate students Wilfrido Solano (Ph.D. in August 2007, now at Chevron/Texaco in Houston, TX) and Dariusz Strąpoć (Ph.D. planned for October 2007, then at Conoco-Phillips). It also helped us to attract two new graduate students in 2007 who will continue the geochemical monitoring as part of their Ph.D. and M.Sc. theses.