TOPIC: Stratigraphy, structure, and fluid flow at the Soda Lake geothermal field, western Nevada, USA
SPEAKER: Holly S. McLachlan
ABSTRACT: The Soda Lake geothermal field lies in the south central Carson Sink basin in northwestern Nevada. It is within the Basin and Range province, a world-class geothermal region with significant untapped potential. Basin and Range geothermal systems can often be ‘blind’, and have little or no surface expression. The Soda Lake well field taps one of these blind systems, and as a result, the controls on geothermal fluid flow are not discernable at the surface. However, Soda Lake has produced electricity for over 30 years, and a great wealth of subsurface data has been acquired since the area was first explored in the early 1970s. Both legacy and new (c. 2010) data were used in this study to model the geology of the Soda Lake field, and to estimate the most likely fluid upflow paths.
The stratigraphic framework of the field was re-modeled from drill cuttings and borehole logs at the outset of this study. A 3D geologic model of the field area was then constructed based on 1) the re-interpreted well data, 2) bedding attitude estimates from seismic surveys and borehole logs, and 3) a fault pick set obtained from a previous study of the Soda Lake 3D seismic volume. Prior to building the geologic model, the preexisting seismic fault pick set was enhanced along strike and extended to >2 km depth using well intercepts and 2D gravity profiles. The resulting fault framework served as the initial input in the modeling workflow. A ‘horizon’ model was then built around this fault framework to generate a 3D block model of geology at the 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 block model. Equilibrated well temperature data were interpreted in context with the geology, PTS data, and well schematics in order to identify likely upwelling and outflow conduits.
There are three major stratigraphic packages at the Soda Lake field: 1) unconsolidated basin fill, 2) Miocene bedrock, and 3) Mesozoic basement. Basin-fill sediment is ~1 km thick at Soda Lake, and can be divided into two units based on grain size and provenance. The basin fill section is underlain by a ~1 km thick Miocene bedrock section comprised of mafic lavas (75%), mafic to felsic tuffs, and sedimentary rocks. This late to middle Miocene package lies unconformably on Mesozoic metasediments and granite. The structural fabric of the field area is defined by a series of east-dipping, northerly-striking normal faults that bound a series of west-tilted half-grabens. Five of these east-dipping faults define the west side of a deep graben in the center of the Soda Lake field, and they step to the left in tandem a few hundred meters west of the most densely drilled part of the well field. The easternmost of these, the Blue3 fault, accommodated the most slip. A northeast-striking splay, the SWX fault, intersects the Blue3 fault slightly southeast of the left step. The hottest, highest flow rate wells are in the dilatational southeast quadrant of this fault intersection, and are interpreted to intersect upflow zones that converge on the Blue3 fault at ~1.2-1.5 km depth.
BIO: Holly S. McLachlan
M.S. Stanford University, 1993, Brecciation and its relationship to gold mineralization and wall-rock alteration in the Round Mountain epithermal gold/silver deposit, Round Mountain, Nevada, 139 p Advisor: Dr. Marco T. Einaudi.
~15 years as a geologist in the Nevada mining industry with Santa Fe Pacific Gold, Hecla, Newmont, Placer Dome, Harvest Gold and Rye Patch Gold.
PhD. UNR, 2018, Stratigraphy, Structure, and fluid flow at the Soda Lake geothermal field, western Nevada, USA; 258 p. Advisor: Dr. Jim Faulds.
The event details are as follows:
Date: Jan 10, 2019; 6:30 PM to 9:30 PM
Location: Tamarack Junction
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