ACTIVE SOURCE AND LABORATORY SEISMIC MEASUREMENTS ACROSS THE LITTLE GRAND WASH FAULT, UTAH: REGOLITH AND HOST ROCK INFLUENCES ON CO2 LEAKAGE:

Understanding carbon dioxide (CO2) reservoir to surface migration is crucial to successful carbon capture and sequestration approaches. Through active source p-wave and s-wave seismic imaging and laboratory measurements, we explore regolith and shallow stratigraphy across the Little Grand Wash fault, Utah. The presence of natural CO2 seeps, travertine and tufa deposits confirm modern and ancient fault-controlled CO2 leakage. We consider this fault an analogue for a long-failed sequestration site. We map stratigraphy and fault geometries through reflection imaging. Through time-lapse seismic imaging, we explore travel time and amplitude changes related to CO2 fluid migration. We estimate bulk porosity and fracture density for host rock, regolith, and fault zone from petrophysical relationships from refraction profiling and from unconfined compressive strength laboratory tests. When our seismic results are combined with existing geochemical and geological data, we characterize a 60 m wide south-dipping damage zone that represents the primary surface delivery channel for CO2 originating from reservoir depths. Where the damage zone corresponds to high CO2 flux values, low seismic velocities when compared to host rock suggest sediments have formed through host rock chemical dissolution or mechanical weathering. In contrast, high seismic velocities within other portions of the fault correlate with low CO2 flux values and suggest low porosity zones driven by enhanced cementation within the fault. While regional stress changes may account for decadal- to millennial-scale changes in CO2 pathways, we speculate that the total fluid pressure has locally reduced the fault’s minimum horizontal effective stress; thereby producing both low-and high-permeability fault segments that either block or promote fluid migration.