FRACTURE ANALYSIS AND RISK ASSESSMENT FOR A “LEAKY” CO2 SYSTEM IN THE ROCKY MOUNTAIN FORELAND

In an effort to mitigate increasing anthropogenic CO₂ in the atmosphere, many possible carbon sequestration solutions may be envisioned. One viable solution is CO₂ sequestration in subsurface geologic formations. Because supercritical CO₂ behaves in much the same way as hydrocarbons in the subsurface, researching hydrocarbon reservoir-trap-seal systems is important for understanding deep subsurface sequestration. Also, hydrocarbon systems can themselves provide excellent reservoir-trap-seal systems for storing CO₂. Finally, hydrothermal (brine) fluid flow in the subsurface is often associated with CO₂ migration and investigating the pathways will help to more accurately understand potential CO₂ leakage problems.

Many Laramide uplifts in the Rocky Mountain foreland are hydrocarbon traps as well as being possible sites for carbon sequestration. These uplifts are most often basement-involved, tri-shear anticlines. The Thermopolis anticline in Thermopolis, WY is an excellent example of a fault-controlled Laramide anticline with an active source of thermal water and CO₂ and it is structurally and stratigraphically similar to other hydrocarbon-producing Laramide structures in the region. At least four domes are present within the greater structure (west-to-east: King, Rose, Cedar Ridge and Warm Springs). Between the two easternmost domes, there is a high-angle transverse fault zone or relay ramp; this transverse zone locates the position of the Big Horn River as well as a large mineral hot spring. Hot spring travertine deposits occur as terraces at the river level and along the crest of the anticline on the Cedar Ridge and Warm Springs domes. Detailed fracture analyses were conducted on the limbs and crest of the anticline at these domes. Both A-C (hinge perpendicular) and B-C (hinge parallel) fracture sets were found, as well as two sets of conjugate fractures (an earlier set and a later set). Hydrothermal fluid flow to the surface is inferred to have occurred along the basement-rooted transverse fault zone and then upwards along B-C fractures in the stratigraphic succession. Understanding fractures in structural domes such as Thermopolis anticline greatly aids research in carbon sequestration by more accurately assessing the integrity of possible carbon storage sites and their potential for leakage.