Noble Gas Signatures in Antrim Shale Gas in the Michigan Basin - Assessing Compositional Variability and Transport Processes

Recent studies in the Michigan Basin looked at the atmospheric and terrigenic noble gas signatures of deep brines to place constraints on the past thermal history of the basin and to assess the extent of vertical transport processes within this sedimentary system. In this contribution, we present noble gas data of shale gas samples from the Antrim shale formation in the Michigan Basin. The Antrim shale was one of the first economic shale-gas plays in the U.S. and has been actively developed since the 1980’s. This study pioneers the use of noble gases in subsurface shale gas in the Michigan Basin to clarify the nature of vertical transport processes within the sedimentary sequence and to assess potential variability of noble gas signatures in shales.

Antrim Shale gas samples were analyzed for all stable noble gases (He, Ne, Ar, Kr, Xe) from samples collected at depths between 300 and 500m. Preliminary results show R/R_a values (where R and R_a are the measured and atmospheric ³He/⁴He ratios, respectively) varying from 0.022 to 0.21. Although most samples fall within typical crustal R/R_a range values (~0.02-0.05), a few samples point to the presence of a mantle He component with higher R/R_a ratios. Samples with higher R/Ra values also display higher ²⁰Ne/²²Ne ratios, up to 10.4, and further point to the presence of mantle ²⁰Ne. The presence of crustally produced nucleogenic ²¹Ne and radiogenic ⁴⁰Ar is also apparent with ²¹Ne/²²Ne ratios up to 0.033 and ⁴⁰Ar/³⁶Ar ratios up to 312.

The presence of crustally produced ⁴He, ²¹Ne and ⁴⁰Ar is not spatially homogeneous within the Antrim shale. Areas of higher crustal ⁴He production appear distinct to those of crustally produced ²¹Ne and ⁴⁰Ar and are possibly related the presence of different production levels within the shale with varying concentrations of parent elements.