A multi-isotope geochemical approach to baseline monitoring at the Carbon Management Canada’s CCS Field Research Station, prior to CO2 injection

Abstract:

Geochemical monitoring tools are useful to verify secure CO₂ storage and detect unplanned CO₂ migration. However, a robust geochemical baseline is needed prior to subsurface CO₂ injection for the effective use of these tools. Here we present the first multi-well gas and groundwater geochemical baseline characterisation of the CMC Research Institutes’ Field Research Station (FRS) located near Brooks, Alberta.

We confirm that CH₄ occurs pervasively in gas and water samples obtained from the shallow (<550 m) Upper Cretaceous bedrock succession. Using C₁/C₂+C₃ ratios, δ¹³C_CH4 , δD_CH4 we determine that the CH₄ is primarily of biogenic origin. However, we also identify the presence of a thermogenic CH₄ component that increases with depth, which is correlated with increasing radiogenic-sourced ⁴He.

Measured ⁴He concentrations exceed those that could be generated by in-situ radioactive decay of U and Th in the host Upper Cretaceous stratigraphy. ⁴He concentrations lie on a mixing line between the atmosphere and a nearby petroleum well that produces natural gas from Lower Cretaceous Viking Fm. This excess ⁴He could indicate mixing with a radiogenic component that is consistent with observed elevated nucleogenic and radiogenic-derived ²¹Ne* and ⁴⁰Ar* in several samples.

In contrast to previous work, this indicates a resolvable crustal contribution to fluids at the FRS site, showing that a fluid connection from the petroleum producing Viking Fm. is present in this portion of the Western Canada Sedimentary Basin. This highlights the additional sensitivity that noble gases can provide in resolving subsurface fluid sources.