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

Abstract:

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/R_a values of Antrim Shale gases (where R and R_a 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.