Using Noble Gases to Assess the Compositional Variability and Sources of Natural Gas in the Antrim Shale, Michigan Basin, USA

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 gas (He, Ne, Ar, Kr, Xe) isotopic ratios and volume fractions from the Antrim Shale gas to assess compositional variability and vertical fluid migration, in addition to distinguishing between the presence of thermogenic versus biogenic methane.

High horizontal and vertical variability of noble gas signatures in the Antrim Shale are observed, which are due to variable noble gas input from deep brines and, to a smaller extent, variable in-situ production in different layers of the Antrim Shale. Estimated 4He ages considering external 4He input for Antrim water are consistent with glaciation-induced recharge. Consistency in measured and predicted 40Ar/36Ar assuming Ar release temperatures ≥250°C supports the presence of thermogenic methane in the Antrim for most of our gas samples. This thermogenic methane is likely to originate at greater depths as the maturity of the Antrim Shale is low.

AAPG Datapages/Search and Discovery Article #90218 © 2015 Eastern Section Meeting, Indianapolis, Indiana, September 20-22, 2015