Date of Award:

5-2014

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

Benjamin J. Burger

Abstract

The boundary that separates reservoir rocks from caprock seals is generally considered a flow barrier for reservoir fluids. Buoyant fluids do no flow through the caprocks because they have low permeability and molecular forces at the base of the caprock resist upward flow. Deformation at the reservoir/caprock boundary may include fractures that increase permeability and lessen the effect of the molecular forces.

The injection and storage of carbon dioxide (CO2) in porous sandstone with effective top seals below earth’s surface is a possible solution for reducing the amount of human-created CO2 in the atmosphere. Uplift and erosion of the San Rafael Swell in central Utah has uncovered and exposed the Navajo Sandstone (or Page Sandstone) and the Carmel Formation, a Jurassic-aged reservoir rock/caprock pair that is very similar to rock in nearby fields where CO2 may be injected and stored. The rock exposures show that fractures may have contributed to ancient fluid leakage through the caprock.

We focus on three sites in different locations on the San Rafael Swell monocline with exposures of the reservoir/caprock pair that were analyzed for fracture characteristics. Deformation-band faults, joints, and joint clusters are present in the Navajo Sandstone and deformation-band faults are present where faulting has occurred. Structures in the reservoir rocks extend beyond the reservoir/caprock boundary and transition into open fractures in the caprock. The fractures in the caprock are bleached and mineralized, which is evidence of fluid migration through the fractures. Seal failure is most likely where fractures density is highest. Fracture density data acquired from scan-line measurements shows that fracture density is highest in fracture clusters, folded rocks, and near faults. The fracture distributions are related to the location of the sites on the monocline in which open fractures have predictable orientations.

Open fractures and faults of the study may diminish the seal integrity of the caprock and are not visible with most subsurface imaging techniques. The features in this study occur at the cm- to m-scale and may allow CO2 to leak through the caprock slowly, which could be significant over the thousands of years necessary for subsurface CO2 storage.

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