Three-dimensional geologic mapping to assess geothermal potential: examples from Nevada and Oregon

Abstract

IntroductionIn this paper, we present a methodology for three-dimensional (3D) analysis of the structural and geologic controls on permeability development and the transmission of fluids in geothermal systems. We present these methods through examples from two geothermal fields, demonstrating the use of these methods in developing data-driven and testable conceptual models of geothermal processes.Faults and interconnected networks of faults and fractures can serve as pathways for upwelling in geothermal fields. This is particularly true in both magmatic and nonmagmatic geothermal systems occupying extensional and transtensional domains in which normal and strike-slip faults serve as fluid conduits (Moeck 2014; Moeck and Beardsmore 2014). The most conductive fault networks typically occur within complex Abstract Geologic structure plays an important role in controlling fluid flow in geothermal systems. In particular, very complex structural settings, consisting of many closely spaced and intersecting faults, host many geothermal systems. To elucidate the key geologic factors that affect fault-controlled geothermal circulation, it is critical to precisely characterize the structural and stratigraphic geometries in these complex settings. Here, we present a methodology and the results of 3D geologic analyses of two geothermal systems in the Basin and Range, USA. This methodology is a quantitative and geologically focused technique that can be used to precisely characterize geothermal areas, in a time when future geothermal growth demands increased exploration precision and efficiency. Surficial and subsurface geologic and geophysical data are synthesized in the construction of detailed 3D geologic maps of geothermal areas. Based on these 3D geologic maps, we examine several geologic attributes that control permeability development and geothermal fluid flow along faults. We use the stress state of faults and the distribution of structural discontinuities (i.e., fault intersections and fault terminations) to identify locations of upflow along faults in these geothermal systems. These results and the methodology presented herein are directly applicable to structurally controlled geothermal fields in the Basin and Range and worldwide. As development focus shifts toward blind geothermal resources, integration of precisely characterized subsurface structural information into exploration methods will be increasingly critical to continued growth in geothermal exploration and development. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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“…A concise analysis of this plot suggests an augmented relationship exists between known geothermal points and spatial data related to lithological and structural intensity. Within the Nigerian Benue trough, evidence for lithological and structural control of geothermal occurrence have been reported by Musa et al 7 , and a substantial correlation of geothermal resources with lineament intensity may point to the possible presence of numerous closely spaced intersecting faults 70 . A positive correlation with land surface temperature obtained from thermally calibrated MODIS imagery may suggest a near surface manifestation for geothermal occurrence across the study location.…”

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“…At the local or prospect scales, integration of 3D geological 191 and geophysical 192 data with the development of resource conceptual models 48 is crucial for selecting the final sites for drilling production and reinjection wells. The 3D geological models constrain multiple parameters that control subsurface permeability and heat, including fault geometries, stress state on faults, basin architecture, stratigraphic framework, rock properties and hydrogeology.…”

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“…190 Supplementary to the surface thermal information, geophysical data, geologic data, fault mapping, and other tools are used to characterize subsurface geologic structures and support drilling decisions. 194,197 An alternative possibility to cataloging geothermal energy systems is by their geologic characteristics, referred to as geothermal plays. A geologicalbased geothermal play-type catalog helps in understanding the nature of a resource and defining appropriate exploration strategies, reservoir evaluation and quantification of the geothermal potential.…”

“…191 In ref. [197], the results of 3D geologic analyses of two geothermal systems in the Basin and Range, USA were presented. The methodology is a quantitative and geologically focused technique that can be used to characterize geothermal areas.…”