GROUNDWATER CIRCULATION AND OIL BIODEGRADATION: NEW MODELING INSIGHTS INTO CARBON CYCLING IN THE WESTERN CANADA SEDIMENTARY BASIN

The Western Canada Sedimentary Basin is one of the world's most extensively studied sedimentary basins, yet the complexity of physical, biological, and geochemical processes at depths below 500 meters leaves open questions about water-rock-microbial interactions. Improving our understanding of the timescales and magnitudes of these processes is important for quantifying terrestrial carbon cycling in the subsurface and feedbacks to climate.

Oils with higher API gravity (i.e., less degraded) are found in deeper parts of the basin, while more degraded oils with lower API gravities are found in shallower regions. This study compiles geochemical and isotopic data along with indicators of hydrocarbon biodegradation for the Mannville Group and hydrologic data in Alberta, Canada. We hypothesize that meteoric water fluxes control the distribution of heavy oils within this reservoir.

Our objective is to create a biodegradation model to determine the time required for the observed amount of organic carbon and sulfate to be consumed. The model estimates the original amount of oil, aerobic conditions, and a constant supply of oxygen within a given area, porosity and zone of water-oil contact. We estimate that it would take ~70 million years to biodegrade the amount of oil based on our estimations of the original carbon in-place and required oxygen supply. Our results suggest that geologic events, such mountain building in the Late Cretaceous to Eocene, and glaciations during the Pleistocene have driven regional groundwater flow, facilitating hydrocarbon degradation.

These findings emphasize the need to understand ancient geological and hydrological conditions to fully comprehend current subsurface biogeochemical processes. In summary, our findings shed light on the timescales of natural carbon cycling processes in the deep subsurface, revealing the slow pace of these reactions and their implications for Earth's carbon dynamics and climate feedbacks.