STRUCTURAL CONTROLS ON CO2 LEAKAGE AND DIAGENESIS IN A NATURAL LONG-TERM CARBON SEQUESTRATION ANALOGUE: LITTLE GRAND WASH FAULT, UTAH

The Little Grand Wash normal fault in Green River, Utah, hosts 10-15 naturally occurring CO₂ seeps. Four of these are active, and the other inactive seeps are marked by travertine deposits. We studied the association of fault structure with CO₂-related alteration as an analogue for the long-term (10³- to 10⁴-year) effects of carbon leakage through reservoir faults. We combined 1:700-scale field mapping of fault structure and alteration with laboratory analyses of mineralogical, isotopic, and textural changes in an effort to assess controls on the migration of CO₂.

The 200-meter-wide fault zone contains 4-5 major subparallel fault segments that form multiple soft- and hard-linked relay ramps. The field area also includes an inactive travertine deposit and related sandstone alteration: outcrop-visible coloration, porosity-occluding calcite cementation (in otherwise low-calcite rocks) and abundant, occasionally fabric-obliterating veins. Detailed structural mapping shows that the travertine is located at an intersection of major fault segments that constitutes the hard link of a 400-meter-long relay ramp. Mapping the distribution of sandstone alteration reveals a decrease in intensity away from the hard link and travertine, indicating that sandstone alteration is CO₂-related. At distances greater than 10 m, intense alteration becomes rare and patchy, and is observed only at fault intersections. In contrast, along non-intersecting faults we observed very limited veins and calcite cementation. Alteration is thus limited to the major fault intersections. It does not permeate the fault zone or extend outside the zone, suggesting that these intersections are the preferred conduits for upward flow of CO₂-laden fluid.

In thin section, intergranular and fracture porosity near the travertine mound is extensively or completely occluded by calcite cement. XRD shows the presence of iron oxides like goethite in altered sandstone, and suggests a reduction in clay minerals with proximity to the travertine. Permeability of samples immediately adjacent to the travertine is reduced by up to two orders of magnitude, to about 1-10 md, compared to sandstone away from the travertine. The observed reduction in porosity and permeability would restrict the upward flow of CO₂-charged water through fault conduits over time.