Date of Award:

2016

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Advisor/Chair:

James P. Evans

Abstract

In the subsurface, faults can act as conduits for seal bypass or as lateral barriers to subsurface fluid flow. Recent reservoir modeling shows that the area where a reservoir-seal interface is in contact with a fault—the fault-interface corner—can be a site of high pore-fluid pressure that may cause seal failure. 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.

In order to better understand the fault-interface corner and improve models, we examined five mesoscale (cm- to m- scale) normal-slip faults that juxtapose medium cross-bedded sandstone (1-2 m thick beds) against red or green siltstone or mudstone (~1 m thick beds) in the San Rafael Swell, Utah. 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 in order to better define the fault-interface corner models and identify new fault elements previously not considered in the models.

Fault elements documented here include shale injection into faults, fault-bounded shale blocks, entrained sand blocks, and reactivated joints. Faults with shale injection have almost double the seal thickness and mineralization along the bottom slip surface of the injected shale bed. Step-over faults on either side of fault-bounded shale blocks replace overly simplified single plane faults in previous models. Geochemical alteration and mineralization includes calcite precipitation and clay development in several faults. These faults have varying spatial relationships with the zone of deformation bands. A fault with reactivated joints represents an endmember example of the fault-interface corner models with a large opening mode fracture allowing seal bypass.

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