Date of Award:

12-2017

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

Kelly K. Bradbury

Committee

Peter S. Mozley

Abstract

The midcontinent United States, a region which typically does not experience many earthquakes, has experienced a significant increase in the number of earthquakes over the last decade. This increase in earthquake activity has been linked to wastewater injection, a process in which large volumes of wastewater from oil and gas extraction are injected into deep (2-3 km), high-permeability sedimentary rocks, near low-permeability Precambrian (>540-million-year-old) crystalline ‘basement’ rocks. The contact between these two rock types is referred to as the Precambrian nonconformity. Injection-induced earthquakes occur on or near basement-hosted faults due to an increase in pore fluid pressures, which implies that there may be a hydrological connection between the basement-hosted faults and the injection point. We hypothesize that the Precambrian nonconformity greatly influences this hydrological connection.

We investigate the geologic properties of Precambrian nonconformity zone outcrop and core analogs to examine how the geology of the nonconformity zone controls fluid flow. Methods include mapping of geological materials and deformation structures (faults and fractures), mineralogical analysis, and geochemical analysis. These data sets allow us to infer the nature of fluid flow in the past, and make predictions about fluid flow in the future. In addition, this information is used to inform hydrological models, improving the ability to predict earthquakes due to wastewater injection.

We identify three main geological scenarios that are likely to be encountered at the nonconformity. These are: 1) basal conglomerate, 2) weathered/altered horizon, and 3) mineralized contacts. These scenarios, or combinations of these scenarios, may be fractured or faulted, resulting in a variety of hydrological implications. The permeability of basal conglomerates and weathered horizons at the contact depends on the textures and minerals that are present. Regolith, clast-supported granitic wash, or poorly cemented conglomeratic horizons, may act as high permeability conduits, whereas a clay-rich grus or granitic wash, or tightly cemented conglomerate, may act as low permeability barriers. Mineralized contacts may act as low permeability barriers due to a reduction of pore space. The mineralized contact shows that the introduction of warm brines by modern injection may result in mineralization or chemical weathering, dynamically affecting permeability over time depending on the mineralogy of the host rock and chemical composition of the injected brine.

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