Identification of microbial and thermogenic gas components from Upper Devonian black shale cores, Illinois and Michigan basins

Differentiation of microbial versus thermogenic methane in coalbed and black shale accumulations can affect strategies for exploration and may influence the total gas content in a given area. Early identification of these processes from crushed core materials, even before formation fluids and produced gas samples are available, could permit a more efficient and cost-effective exploration. Total gas contents and compositional and isotopic data from New Albany Shale core materials are presented, which delineate regional occurrence of microbial, thermogenic, and mixed gas generation in the Illinois Basin. These trends are consistent with those identified from detailed prior studies of produced gas and water chemistry from the same locations. The most useful markers for microbial gas in crusbed core gases are elevated CO2 contents characterized by high delta(13) C-CO2, values (> 5 parts per thousand). Core gas analyses from wells in which microbial gas is identified commonly have significantly more total gas absorbed than do core samples from wells producing gases solely of thermogenic origin. These observations are independent of variations in sample depth and organic carbon content in a given core. Thus, this integrated case study of core and produced gases in the Illinois Basin illustrates that the areas containing microbial gas, in addition to early thermogenic gas, may be more productive than pure thermogenic zones for these early to immature unconventional gas deposits.