Structural Framework of the Soda Lake Geothermal Field: An Optimized Composite Model Based on Well Lithology, Gravity, and Seismic Data

H. S. McLachlan, P. C. Schwering, and J. E. Faulds
University of Nevada, Reno, NV, USA

Abstract

The Soda Lake geothermal field is a blind, fault-controlled system located ~80 km east of Reno, NV in the west-central Great Basin. It was first drilled to assess its potential for geothermal electricity production ~40 years ago, and it has been producing electricity for over 20 years. More than 28 deep wells are at and near the production site. Numerous geophysical surveys have been carried out in the area, including gravity, ground magnetics, MT, CSAMT, and 3D-3C seismic.

The Soda Lake field lies within the Basin & Range structural province, a few tens of kilometers east of the Walker Lane. This region is rich in geothermal sites, and regional-scale constraints on their occurrence are reasonably well understood. Most geothermal systems in the west-central Great Basin are associated with north- to northeast-striking, moderately to steeply dipping normal faults that are roughly orthogonal to the present-day regional extension direction (Faulds et al., 2004). The most productive fields are commonly found near the tips of these primary structures, not at their culminations. Intense geothermal fluid flow – hot and strong enough to make an individual system prospective for electricity production – is commonly localized by smaller structures that accommodate or transfer strain between the primary structures. Accommodation zones generate regions of enhanced local fracture density and tend to have subdued surface expression. Understanding them is critical for modeling subsurface fluid flow, but they are commonly poorly exposed.

Substantial evidence indicates the primary faults at the Soda Lake geothermal field strike north to north-northeast, roughly perpendicular to the contemporary extension direction. Preliminary work with cuttings and seismic data supports this model; all evidence indicates the field lies within a graben, bounded on the west by a N-S striking, east-dipping normal fault that dies off to the south, and on the east, by a NNE-striking, west-dipping normal fault that terminates to the north. Preliminary interpretation of well log lithology, gravity and seismic data indicate that the well field lies within an accommodation zone between these overlapping, antithetic faults.

While the regional-scale (1-5 km) structural setting at Soda Lake is understood, production-scale controls on fluid flow, such as the geometry of the lesser faults and the local stress regime, are still poorly characterized. A key challenge in assessment of the Soda Lake field is that it lies under hundreds of meters of unconsolidated basin fill, with few surface expressions of the fluids or the structures that guide their circulation. The Soda Lake geothermal system must be characterized remotely via information from wells and geophysical surveys. The purpose of this study is to integrate well lithology, gravity, and seismic reflection data in order to accurately model the subsurface structural framework of the hydrothermal system.

AAPG Search and Discovery Article #120140© 2014 AAPG Hedberg Conference 3D Structural Geologic Interpretation: Earth, Mind and Machine, June 23-27, 2013, Reno, Nevada