Stratigraphy, Structure, and Fluid Flow at the Soda Lake Geothermal Field, western Nevada, USA
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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.The structural framework of the Soda Lake geothermal field is dominated by a series of east-dipping, ~N5˚-10˚E-striking normal faults. Maximum throws on the largest of these faults ranges between 200-450 m at the Mesozoic basement contact, and dips range from 65˚-75˚ east. The east-dipping faults bound a series of west-tilted half-grabens. Maximum block tilt in the central part of the well field ranges up to ~35˚ west. A subordinate set of north-striking, west-dipping normal faults are also present, but they have little apparent influence on the polarity or magnitude of block tilting. The east and west-dipping, northerly striking normal faults appear to be roughly contemporaneous, with extension constrained from ~13 Ma to present. Block tilting slowed significantly after the 5.1 Ma Soda Lake trachyandesite was emplaced. However, the main faults continued to accommodate a minor amount of extension into the late Pleistocene.A step-over in the east-dipping normal fault system and intersection between this system and a northeast-striking fault appear to control geothermal upwelling at the Soda Lake field. Three to five of the more closely spaced east-dipping, N5˚-10˚E striking faults define the west side of the central graben. These faults are less than ~0.6 km apart, and they step to the left in tandem a few hundred meters west of the central part of the Soda Lake well field. The easternmost fault in this set, the Blue3 fault, appears to have accommodated the most slip. A northeast-striking splay, the SWX fault, intersects the Blue3 fault on the west side of the central graben, slightly south of the step-over. Production wells at the Soda Lake field are distributed over ~1 km2 area in the dilational southeast quadrant of the intersection between the Blue3 and SWX faults. The innermost graben bounding fault, the Blue3 fault, is probably the main discrete conduit for upwelling fluid at the field. The trachyandesite vent may serve as a secondary conduit for upwelling fluids in the shallow subsurface and may influence the shape and elevation of the shallow thermal aquifer. Two or three stratigraphic horizons appear to control lateral fluid flow at the Soda Lake field, although stratigraphic permeability is probably secondary to fault-controlled permeability in guiding fluid flow. The most significant stratigraphic permeability is at the contact between basin-fill sediments and Miocene bedrock at ~1-1.2 km depth. The structural setting at Soda Lake appears to be less complex than some other producing geothermal fields in the region, such as Desert Peak, Bradys, and McGinness Hills, as evidenced by more widely spaced faults and fewer fault intersections at Soda Lake. This may explain the relatively modest levels of production at Soda Lake compared to many other fields in the region. Nonetheless, the stratigraphic and structural relationships defined at Soda Lake provide important insights and possible analogues for other potential blind geothermal systems in the region, especially those hidden within sedimentary basins.