Date of Award:

5-2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Department name when degree awarded

Geology

Committee Chair(s)

James P. Evans

Committee

James P. Evans

Committee

Peter S. Mozley

Committee

Dennis Newell

Abstract

As groundwater, liquid and gas hydrocarbons, or CO2 fluids move through the subsurface, faults can act as pathways or barriers to flow. Recent studies also show that when a fault juxtaposes high permeability sandstone against a low permeability shale, the corner at the sandstone-shale interface and the fault can become a site of high pressure that may fracture the seal and allow fluids to escape. This can have negative implications for industries dependent on the quality of that seal, for example, petroleum, CO2 sequestration, waste fluid injection, and nuclear waste storage industries.

We examined five small-scale faults in the San Rafael Swell, Utah, in sandstone, siltstone, and mudstone sequences where permeable sandstones (1-2 m beds) are juxtaposed with low permeable siltstones or mudstones (~1 m thick beds). These faults give us insight into the role small-displacement faults have in reservoir-seal systems when looking at faults with similar displacements, lithologies, and tectonic settings. Outcrop observations, X-ray diffraction mineralogy, whole rock geochemistry, petrography, fractured grain density, and porosity data were used to gain insight into past fluid compositions, cross-cutting relationships, and fault seal qualities.

We described how clay, smeared into a fault, can slow or stop flowing fluids. We also found evidence that calcite in the fault had been dissolved, suggesting that these faults can be a pathway for fluids through a seal. We also found that faulted reservoir seals may be able to heal themselves and prevent fluids from escaping. We created a fault-outcrop map for each fault and simplified them to create four new models in an effort to make current modeling more robust.

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