Assessing Compositional Variability and Migration of Natural Gas in the Antrim Shale in the Michigan Basin Using Noble Gas Geochemistry

The Antrim Shale was one of the first economic shale gas plays in the U.S. and has been actively produced since the 1980's. While previous studies suggest co-produced water in the Antrim is a mixture of brine from deeper formations and freshwater recharge, the extent of water-gas interactions has yet to be determined. The extent and source of thermogenic methane in the Antrim Shale are also under debate. This study uses stable noble gases' (He, Ne, Ar, Kr, Xe) isotopic ratios and their volume fractions from the Antrim Shale gases to assess compositional variability and vertical fluid migration, in addition to distinguishing between the presence of thermogenic versus biogenic methane. R/Ra values of Antrim Shale gases (where R and Ra are the measured and atmospheric 3He/4He ratios, respectively) vary from 0.01 to 0.34 suggesting dominant crustal 4He in addition to minor mantle and atmospheric He. Elevated 20Ne/22Ne ratios (up to 10.4) point to mantle Ne. Similarly crustal 21Ne*, 40Ar* and 136Xe* are also suggested. High variability of noble gas signatures in the Antrim Shale are observed, which are mainly due to variable noble gas input from deep brines and, to a smaller extent, variable in-situ production in the Antrim Shale. Estimated 4He ages considering external 4He flux for Antrim water match well with timings of three major glaciation periods (Wisconsin, Illinoian and Kansan glaciations) in the Michigan Basin, suggesting that all our Antrim samples were more or less influenced by glaciation recharge. Consistency in measured and predicted 40Ar/36Ar assuming Ar release temperatures ≥ 250°C supports a thermogenic origin for the majority of the methane in our Antrim Shale gas samples. Thermogenic methane is likely to originate at greater depth, either from deeper portions of the Antrim Shale in the central Michigan Basin or from deeper formations underlying the Antrim Shale, as the thermal maturity of the Antrim Shale in our study area is low.