Fault-controlled CO₂ leakage from natural reservoirs in the Colorado Plateau, East-Central Utah

The study investigated a natural analogue for soil CO₂ fluxes where CO₂ has naturally leaked on the Colorado Plateau, East-Central Utah in order to identify various factors that control CO₂ leakage and to understand regional-scale CO₂ leakage processes in fault systems. The total 332 and 140 measurements of soil CO₂ flux were made at 287 and 129 sites in the Little Grand Wash (LGW) and Salt Wash (SW) fault zones, respectively. Measurement sites for CO₂ flux involved not only conspicuous CO₂ degassing features (e.g., CO₂-driven springs/geysers) but also linear features (e.g., joints/fractures and areas of diffusive leakage around a fault damage zone). CO₂ flux anomalies were mostly observed along the fault traces. Specifically, CO₂ flux anomalies were focused in the northern footwall of the both LGW and SW faults, supporting the existence of north-plunging anticlinal CO₂ trap against south-dipping faults as well as higher probability of the north major fault traces as conduits. Anomalous CO₂ fluxes also appeared in active travertines adjacent to CO₂-driven cold springs and geysers (e.g., 36,259 gm^-2d^-1 at Crystal Geyser), ancient travertines (e.g., 5,917 gm^-2d^-1), joint zones in sandstone (e.g., 120 gm^-2d^-1), and brine discharge zones (e.g., 5,515 gm^-2d^-1). These observations indicate that CO₂ has escaped through those pathways and that CO₂ leakage from these fault zones does not correspond to point source leakage. The magnitude of CO₂ flux is progressively reduced from north (i.e. the LGW fault zone, ~36,259 gm^-2d^-1) to south (i.e. the SW fault zone, ~1,428 gm^-2d^-1) despite new inputs of CO₂ and CO₂-saturated brine to the northerly SW fault from depth. This discrepancy in CO₂ flux is most likely resulting from the differences in fault zone architecture and associated permeability structure. CO₂-rich fluids from the LGW fault zone may become depleted with respect to CO₂ during lateral transport, resulting in an additional decrease in CO₂ fluxes within the SW fault zone. In other words, CO₂ and CO₂-charged brine originating from the LGW fault zone could migrate southward over 10-20 km through a series of high-permeable aquifers (e.g., Entrada, Navajo, Kayenta, Wingate, and White Rim Sandstones). These CO₂-rich fluids could finally reach the southernmost Tumbleweed and Chaffin Ranch Geysers across the SW fault zone. The potential lateral transport of both CO₂ and CO₂-laden brine can be further supported by similar CO₂/³He and ³He/⁴He ratios of gas and a systematic chemical evolution of water emitted from the regional springs and geysers, which suggest the same crustal origins of CO₂ and CO₂-rich brine for the region.