Subsurface Hydrocarbon Biodegradation in the Context of Microbial Habitability and Hydrogeologic, Thermal, and Geologic Evolution of the Colorado Plateau

Abstract Subsurface microbial ecosystems associated with hydrocarbon reservoirs are shaped by temperature, salinity, redox gradients, and fluid flow. However, the drivers of microbial activity and hydrocarbon biodegradation over geologic timescales remain poorly constrained. Here, we examine the geologic and hydrologic controls on subsurface microbial activity by comparing hydrocarbon biomarkers across several organic‐rich reservoirs in the Colorado Plateau with distinct thermal and fluid histories. Our study includes two paleo‐oil reservoirs: (a) Rainbow Rocks in the Jurassic Entrada Sandstone and (b) the Permian White Rim Sandstone; and three actively producing hydrocarbon fields: (a) Cisco Springs (Jurassic Entrada and Cretaceous Dakota), (b) Middle Mesa in Lisbon Valley (Pennsylvanian Honaker Trail), and (c) Wolf Point (Jurassic Morrison and Cretaceous Dakota). Biomarker distributions reveal spatial variability in hydrocarbon alteration. Rainbow Rocks display moderate biodegradation with loss of n ‐alkanes and depletion of isoprenoids and low‐ring aromatics, whereas the White Rim Sandstone retains higher abundances of labile compounds. Within both paleo‐reservoirs, biodegradation varies with depth, consistent with permeability‐controlled differences in fluid circulation. In the modern reservoirs, biodegradation is limited and proceeds primarily via bacterial sulfate reduction under conditions of low salinity and elevated sulfate concentrations. Thermal histories indicate that conditions favorable for biodegradation likely developed after post‐burial cooling of reservoirs to temperatures