Assessing compositional variability and migration of natural gas in the Antrim Shale in the Michigan Basin using noble gas geochemistry

This study uses stable noble gases' (He, Ne, Ar, Kr, Xe) volume fractions and isotopic ratios from Antrim Shale natural gas to assess compositional variability and vertical fluid migration within this reservoir, in addition to distinguishing between the presence of thermogenic versus biogenic methane. R/Ra values, where R is the measured ³He/⁴He ratio and Ra is the atmospheric value of 1.384±0.013×10^-6, vary from 0.01 to 0.34 suggesting a largely dominant crustal ⁴He component with minor atmospheric and mantle contributions. Crustal ²¹Ne, ⁴⁰Ar and ¹³⁶Xe contributions are also present but the atmospheric component is largely dominant for these gases. Crustal contributions for ²¹Ne, ⁴⁰Ar and ¹³⁶Xe vary between 1.1% and 12.5%, between 0.7% and 19% and between 0.1% and 2.7%, respectively. A few samples present higher than atmospheric ²⁰Ne/²²Ne ratios pointing to the presence of a small mantle Ne component. High horizontal and vertical variability of noble gas signatures in the Antrim Shale are observed. These are mainly due to variable noble gas input from deep brines and, to a smaller extent, variable in-situ production within different layers of the Antrim Shale, in particular, the Lachine and Norwood members. Estimated ⁴He ages, considering external ⁴He input for Antrim Shale water, vary between 0.9ka and 238.2ka and match well for most samples with the timing of the major Wisconsin glaciation, suggesting that Antrim Shale water was influenced by glaciation-induced recharge. Consistency between measured and predicted ⁴⁰Ar/³⁶Ar ratios assuming Ar release temperatures ≥250°C supports a thermogenic origin for most of the methane in these samples. This thermogenic methane is likely to originate at greater depths, either from the deeper portion of the Antrim Shale in the central portion of the Michigan Basin or from deeper formations given that the thermal maturity of the Antrim Shale in the study area is rather low.