USING NOBLE GASES TO CONSTRAIN THE TIMING OF INITIATION OF MICROBIAL METHANOGENESIS IN THE ILLINOIS BASIN, USA

Deep subsurface methanogens produce approximately 20% of the world’s methane from organic-rich coals and shales, however the rate at which this major energy resource is generated is unknown. In the Illinois Basin economic reservoirs of microbial methane occur in the Mississippian-Pennsylvanian coals and the Upper Devonian New Albany Shale in association with anomalously low salinity formation water. Knowing the age of the groundwater will enable the estimation of the maximum age of the onset of microbial methanogenesis, and in-situ metabolic rates. The Mississippian-Pennsylvanian aquifers show freshwater recharge throughout the basin margins that penetrates up to 300 m depth. In contrast, the New Albany Shale exhibits a plume of dilute water that extends from the basin margin to over 750 m depth. The axis of this plume is oriented along the axis of the Laurentide Ice Sheet, suggesting glacially controlled freshwater recharge. However for both aquifers, low δ¹⁸O values expected from glacial recharge (<-15‰) are absent, and mixing trends indicate a relatively high δ¹⁸O endmember value (~-7‰) for the freshwater recharge. This relatively high δ¹⁸O value may result from: 1) recharge of modern precipitation (-11.0 to -4.5‰ δ¹⁸O), 2) a mixture of isotopically depleted Pleistocene-aged recharge and formational brines, 3) an isotopically enriched Pleistocene aged source, or 4) an older isotopically enriched source. These hypotheses are investigated by coupling published paleoclimate data for the Illinois Basin area with recent groundwater sampling for carbon-14 and noble gases. Carbon-14 concentrations are low (<2 percent modern carbon), however the presence of any active carbon indicates either the average age of the water is less than 50,000 years old, or that the samples have minor amounts of modern water contamination. In comparison, preliminary helium-4 ages indicate that the average residence time of the water may be as long as 25,000 years in the coal, and range from 8 million to 50,000 years in the New Albany Shale. However, when ages are corrected for an external flux of helium-4, average groundwater ages decrease to 60 years in the coal, and range from 23,000 to 200 years in the New Albany Shale. These age estimates constrain the onset of methanogenesis to at least the past 8 million years, and possibly the Pleistocene.