Stratigraphic and Structural Framework of the Soda Lake Geothermal Field, Western Nevada - Implications for Understanding Geothermal Systems in the Great Basin Region

Abstract

This report provides a detailed stratigraphic analysis, conceptual structural model, and three-dimensional (3D) geological model of the Soda Lake geothermal field, a geothermal system in the Carson Sink basin in northwestern Nevada. We first present an updated stratigraphy of the field area based on re-analysis of drill cuttings, geophysical logs, and radiometric dates from key igneous units. The stratigraphy was validated against bedrock sections in surrounding ranges and basin-fill sections derived from wells in the Carson Sink. In the second section, we present a detailed 3D geological model of the field based on three inputs: 1) well intercepts interpreted in context with the new stratigraphy, 2) block tilt estimates from borehole bedding attitude logs, and 3) a fault framework derived from drill intercepts and a 3D seismic reflection survey. The stratigraphic framework at the Soda Lake geothermal field is dominated by 1) ~1 km of late Miocene to Quaternary basin-fill sediments and volcanic units, 2) an underlying ~1-km-thick section of middle to late Miocene volcanic and sedimentary rock, and 3) Mesozoic metamorphic and granitic basement. Unconsolidated late Miocene to Quaternary basin fill can be divided into distinctive upper and lower units. The upper unit is highly variable but dominated by pebble to silt-size quartzo-feldspathic detritus. It contains 2–3 marker beds, the most significant of which is a 30- to 60-m-thick mudstone layer at ~245–275 m depth. Basin-fill sediments below this mudstone horizon are moderately coarse grained but become increasingly lithic-rich and epiclastic down section. The lower basin-fill unit includes a lithic lapilli crystal ash tuff and an underlying ~300 to 400-m-thick section of lacustrine silt and mud. The basin fill is interrupted by a small, 5.1 Ma trachyandesite volcanic center near the heart of the Soda Lake well field. The basin-fill sediments are underlain by a ~1-km-thick section of middle to late Miocene volcanic and sedimentary strata. It is dominated by mafic flows that are similar to Miocene lavas in the surrounding highlands. The Tertiary bedrock section also includes minor intervals of intermediate to felsic tuff, clastic sediments, and limestone. Two of the thicker and more continuous non-volcanic intervals serve as stratigraphic marker units. Granitic and metasedimentary Mesozoic basement units intercepted in wells at the Soda Lake geothermal field are correlated with recognized assemblages in the surrounding region. The structural framework at the Soda Lake geothermal field is dominated by a series of east-dipping, N5˚E-striking normal faults that are ~10 km in length. This set of faults bound a series of west-tilted fault blocks and half-grabens. The most closely spaced of these east-dipping faults are near the center of the Soda Lake well field and define the west side of a deep central graben. They step slightly to the left in tandem a few hundred meters west of one of the current production zones. Maximum throws on the largest of these faults range from 200 to 450 m at the Mesozoic basement contact, and dips range from 65˚to 75˚ east. The maximum fault block tilt in the central graben is ~35˚ west, based on well intercepts and dipmeter data. A subordinate set of north-striking, west-dipping normal faults is also present, and limited data suggest they are similar in strike length to the more dominant east-dipping fault set. A ~2-km-long, nearly vertical, southeast-dipping, east-northeast-striking cross-fault is present in the production area and appears to intersect the most prominent east-dipping faults slightly south of the step-over. Offset on all fault sets appears to be roughly contemporaneous, with extension constrained from ~13 Ma to present. Tilt versus age measurements suggest graben development slowed significantly at ~5 Ma. However, the main faults continued to accommodate minor normal slip into the late Pleistocene. The east- and west-dipping normal faults are inferred to intersect beneath the central graben. Upwelling fluids at the field appear to be controlled by a hybrid setting that includes the minor step-over, fault intersection between the north- and east-northeast-striking faults on the west side of the central graben, and intersecting oppositely dipping normal faults beneath the central graben. Three or more historically or presently active wells intersect the east-dipping, northerly striking normal faults that form the west side of the central graben. Two currently active geothermal wells ≥700–800 m southeast of the step-over intersect the main west-dipping fault that bounds the east side of the central graben. Two notable stratigraphic features may influence the geothermal system. The ~5 Ma trachyandesite volcanic center that underlies the well field may provide subvertical fracture permeability, enhancing shallow production from wells that do not intersect the major normal faults. Production may also be enhanced by the low permeability mudstone horizon in the upper basin fill that may act as a clay cap for the geothermal upwellings along subjacent faults.