Dissertation/Thesis Abstract

Stratigraphy, Structure, and Fluid Flow at the Soda Lake Geothermal Field, Western Nevada, USA
by McLachlan, Holly S., Ph.D., University of Nevada, Reno, 2018, 282; 10841261
Abstract (Summary)

This study assessed the geologic setting of the Soda Lake geothermal field, which lies in the southern part of the Carson Sink basin of northwestern Nevada within the Basin and Range of the western USA. The Basin and Range is a world-class geothermal province with significant untapped potential, particularly in blind (no surface hot springs or steam vents) geothermal systems. Blind systems probably comprise the majority of geothermal resources in the region, with many lying buried under thick accumulations of sediments in the broad basins that make up >50% of the province. Locating fault-hosted blind geothermal systems in these basins is challenging, and identifying the most prospective parts of these systems is even more demanding. The Soda Lake geothermal field is one of the more deeply buried known systems in this region. This study was undertaken to elucidate the stratigraphic and structural framework of the Soda Lake field, and to determine the probable controls on fluid flow in the production areas. Due to the depth of basin-fill sediments at the Soda Lake field, the structural setting and specific controls on fluid flow are not discernable at the surface. However, the Soda Lake geothermal field has produced electricity for over 30 years, and a wealth of subsurface data has been acquired since the field was first targeted for geothermal exploration in 1972-73. The abundant well data and geophysical surveys in particular provided a foundation for investigation of the geologic setting of the field.

This study was divided into three major parts. In the initial part of the study, a stratigraphic framework was developed for the Soda Lake area from analysis of cuttings, borehole geophysical logs, and radiometric dates of key igneous units. It was validated against exposed stratigraphic sections in the surrounding ranges and interpreted basin-fill sections derived from wells across the Carson Sink basin. Pursuant to this in the second part of the study, a comprehensive 3D geologic model of the Soda Lake field was construct from three inputs: 1) the new stratigraphic framework model, 2) bedding attitude estimates from seismic reflection surveys and borehole logs, and 3) a fault framework derived from both well data and geophysical surveys. The Soda Lake fault framework had been modeled from seismic reflection and borehole data in previous studies. In this study, one of the seismic fault pick sets was enhanced along strike and extended to >2 km depth using well data and forward modeled gravity. This enhanced fault framework served as the initial input to the Soda Lake geologic model. A ‘horizon model’ based on stratigraphic well intercepts and attitude data was then built around the fault framework to generate a 3D geologic block model for the Soda Lake field. In the final phase of this study, the Soda Lake temperature anomaly was modeled in a series of cross-sections extracted from the geologic model. The temperature anomaly was interpreted in context with the geologic model and production data in order to identify the main upwelling and outflow conduits. Key controls on fluid upwelling and probable fluid flow pathways were catalogued based on the spatial relationship between the temperature anomaly and the geologic model of the field area.

There are three major stratigraphic divisions at the Soda Lake geothermal field. The field is situated in and beneath ∼900-1100 m of unconsolidated basin-fill sediments. The basin-fill section is divided into an upper 300-500 m thick, relatively coarse-grained, quartzo-feldspathic unit, and a lower ∼150-300 m thick mud-rich unit. The unconsolidated basin fill is interrupted by a 5.1 Ma trachyandesite body that is up to ∼750 m thick in the central part of the Soda Lake well field. The body consists of a buried vent edifice near one of the main production wells, 50-90 m thick outflow aprons, and a conical root on the west side of the well field that can be traced to the Miocene bedrock contact. About 1 km of Miocene bedrock underlies the basin-fill section. The Miocene bedrock section is dominated by mafic lavas, interbedded with lesser tuff, clastic sedimentary rocks, and minor limestone. No early Miocene or Oligocene strata have been found at the Soda Lake field area. The middle to late Miocene section overlies Triassic-Jurassic metamorphic basement and Jurassic-Cretaceous granite. (Abstract shortened by ProQuest.)

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Indexing (document details)
Advisor: Faulds, James E.
Commitee: Bassett, Scott, Calvin, Wendy, Faulds, James E., Henry, Christopher, Louie, John
School: University of Nevada, Reno
Department: Geology
School Location: United States -- Nevada
Source: DAI-B 80/02(E), Dissertation Abstracts International
Subjects: Geology
Keywords: Fluid flow, Geology, Geothermal field, Soda lake, Stratigraphic framework, Structural framework
Publication Number: 10841261
ISBN: 978-0-438-42771-6
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