Fate and Transport of Shale-derived, Biogenic Methane

Natural gas extraction from unconventional shale gas reservoirs is the subject of considerable public debate, with a key concern being the impact of leaking fugitive natural gases on shallow potable groundwater resources. Baseline data regarding the distribution, fate, and transport of these gases and their isotopes through natural formations prior to development are lacking. Here, we define the migration and fate of CH₄ and δ¹³C-CH₄ from an early-generation bacterial gas play in the Cretaceous of the Williston Basin, Canada to the water table. Our results show the CH₄ is generated at depth and diffuses as a conservative species through the overlying shale. We also show that the diffusive fractionation of δ¹³C-CH₄ (following glaciation) can complicate fugitive gas interpretations. The sensitivity of the δ¹³C-CH₄ profile to glacial timing suggests it may be a valuable tracer for characterizing the timing of geologic changes that control transport of CH₄ (and other solutes) and distinguishing between CH₄ that rapidly migrates upward through a well annulus or other conduit and CH₄ that diffuses upwards naturally. Results of this study were used to provide recommendations for designing baseline investigations.