CARBON AND HYDROGEN ISOTOPIC SIGNATURES OF COALBED GASES FROM INDIANA: METHODOLOGICAL AND GENETIC IMPLICATIONS

Coalbed methane (CBM) samples from the Springfield Coal Member of the Petersburg Formation and the Seelyville Coal Member of the Linton Formation (Pennsylvanian) in Indiana have been analyzed for isotopic composition of carbon and hydrogen to: 1) assess the origin of gas, biogenic versus thermogenic, and 2) investigate the isotopic fractionation during canister desorption experiments.

Carbon isotopic values of the analyzed coals average at -55.1‰ for the Seelyville coal and at -58.5‰ for the Springfield coal, whereas hydrogen isotopic values average at -196‰ and -191‰, respectively. Such a range of values suggest that the coalbed gas studied is of mixed thermogenic/biogenic origin, and that the biogenic component is most likely formed by microbial CO2 reduction pathway. Generally, the gas from the Seelyville coal is slightly more thermogenic than the gas from the more shallow Springfield coal.

To investigate if isotopic fractionation occurs during the canister desorption process, coalbed gas samples of the Springfield and Seelyville coals from Gibson and Sullivan Counties were collected systematically from the desorption canisters, starting the first day of desorption and continuing for a period of 3 to 4 months. The preliminary results suggest that there is no isotopic fractionation occurring during the desorption process. For one of the canisters from which the entire volume of desorbed gas was collected and sampled, carbon isotopic values showed no variability (with a standard deviation of 0.55‰) during the entire desorption process (~3 months). We would like to address an additional question: Are there isotopic differences between the coalbed gas and the gas desorbed from the clastic sediments directly overlying the coal? To answer this question we are investigating a set of coal and roof rock samples from Sullivan County.

Further research will incorporate compositional analyses of coalbed gases and their variations during desorption. This will include the isotopic composition of hydrocarbons heavier than methane and the isotopic composition of CO2 present in coalbed gases. These analyses will help to better define the origin of gases, especially the possibility of bacteriogenic methane generation of the CO2 reduction pathway.