NOBLE GAS AND HYDROCARBON GEOCHEMISTRY OF COAL BED METHANE FIELDS FROM THE ILLINOIS BASIN

Increasing global energy demands and mandates for cleaner burning, lower carbon emitting, fuels are driving an energy transformation from coal toward hydrocarbon gases produced from unconventional resources such as shales and coal bed methane (CBM). Today natural gas makes up more than 25% of the energy production in the US, while CBM constitutes 9.3% of total dry-gas production. Despite these factors, there are still many unknowns regarding the source (biogenic vs. thermogenic), residence time, and gas-water interactions that determine the occurrence of CBM and its economic viability.

Most of the previous research on CBM fields explore the molecular (C₁/C₂+) and isotopic (δ¹³-CH₄) composition of hydrocarbons in relation to water chemistry (e.g., [DIC] and δ¹³C-DIC). These studies typically attribute the very dry (C₁/C₂+= >2,000) natural gas to methanogenesis because of relatively light δ¹³-CH₄ <-50⁰/₀₀ and correlations between δ¹³-CH₄ and δ¹³-DIC, amongst other lines of evidence. Because of their inert nature and well-defined isotopic end members in crustal fluids (e.g., air-saturated water and radiogenic gases), noble gases can provide data to constrain the residence time of biogenic gas accumulations in sedimentary basins and provide a mixing component that is external to the hydrocarbon system. Thus, by combining noble gas and hydrocarbon molecular and stable isotope tracers one may provide insight on the origins and interactions associated with the generation of CBM.

Here we present noble gas (He, Ne, Ar, Kr, and Xe) and hydrocarbon data from CBM fields from western Indiana in the Illinois Basin. Our preliminary data confirms the presence of dry natural gas dominated by methanogenesis, but also suggests significant and quantifiable contributions from migrated thermogenic gas, most likely sourced from a post-mature shale in the Illinois Basin. The ingrowth of radiogenic ⁴He is used to determine groundwater and natural gas residence times in coal seams associated with microbial methane generation. The fluids produced from CBM fields in this study area have a residence time that ranges from 6,100 to 52,000 years assuming a standard crustal ⁴He release model.