The percolation of fluids is of utmost relevance for the utilization of underground resources; however, the location and occurrence of fractures are not always known, and important characteristics of faults, such as stress state and permeability, are commonly uncertain. Using a case study at the Brady’s geothermal field in Nevada (USA), we demonstrate how permeable fractures can be identified and assessed by combining fault stress models with measurements of diffuse degassing and emanations at Earth’s surface. Areas of maximum gas emissions and emanations correspond to fault segments with increased slip and dilation tendency, and represent a fingerprint of the geothermal system at depth. Thus, linking gas fluxes with fault stress models serves as a measure of the connectivity between surface and subsurface.

INTRODUCTION

Degassing and emanation processes at Earth’s surface occur as a result of fluids ascending along fault zones, for example from magmatic intrusions, metamorphic or plutonic basement rocks, and deep-seated mantle sources. Tracing such gases and emanations into the subsurface provides significant information about major fault structures and subterranean dynamics. Analyses of diffuse degassing and emanation processes have previously been applied for various purposes, including surveillance of volcanic activities, geohazard risk assessment, environmental monitoring, and exploration for mineral resources (Barretto, 1981; Hernández et al., 2001; Cardellini et al., 2003; Pérez et al., 2004), but only to a limited degree in fault zone analysis. We have developed a novel approach for fault zone analysis by correlating fault stress models with degassing and emanations at Earth’s surface. Our results enable a three-dimensional (3-D) reproduction of potential permeability structures along fault zones that facilitate fluid circulation. These structures correspond to fault segments with an increased tendency to slip and dilate as well as to increased gas emissions and emanations at the surface. While detailed fault structures in the subsurface can be detected by extensive seismic reflection surveys, the analysis of degassing and emanation processes can build on preexisting structural models and define highly permeable sections of the identified fractures. The non-invasive technique of in-situ diffuse degassing and emanation measurements has the potential to indicate hidden fault zones where gaps in surface outcrops preclude conventional structural-geological mapping.

STUDY AREA AND METHODS

We use the example of the fault-controlled Brady’s geothermal field in the western part of the Basin and Range province (Nevada, USA) as a case study to demonstrate the applicability of the method for resource mapping and assessment (Fig. 1). The Basin and Range province is an excellent study area due to its active extension associated with normal faulting. Our focus is on favorable structural settings that promote subsurface fluid flow. Previous studies have shown that structural step-over regions of active faults (a set of left or right lateral steps) commonly represent important permeability structures for fluid circulation (Faulds and Hinz, 2015; Micklethwaite et al., 2015). For example, the left-lateral structural step-over (Fig. 1) of the NNE-oriented Brady’s normal fault zone is a major conduit for hot fluids extracted for geothermal power generation at the Brady’s power plant (Davatzes et al., 2013). Along the Brady’s fault zone, abundant geothermal surface activity is observed, such as fumaroles, alteration zones, and warm ground.

The area was explored by measuring diffuse degassing rates and emanations at the surface to assess relative fracture permeability (Table 1; Jolie et al., 2015b). The measurements along a predefined grid (50 × 50 m for CO2 and H2S; 50 × 100 m for radon) revealed the internal setup (e.g., structural segmentation) of the investigated area at high resolution, and added decisive information about the orientation of open fracture segments, complementary to existing information (Faulds et al., 2012). In particular, the anomaly pattern of statistically analyzed CO2 and H2S emissions as well as radon (222Rn) activity concentrations indicate zones of increased structural permeability and provide important information for precise mapping and delineation of permeable fault lineaments. In addition, a new measurement approach of the local dose rate (LDR; gamma radiation) was applied, which can be correlated with the geothermal system.

The subsurface structure of the study area has been visualized by a 3-D fault model, which provides the basis for fault stress analyses (Jolie et al., 2015a). The ability of faults to act as conduits or barriers for subsurface fluid flow is attributed to—among other factors—the stress conditions in the reservoir, and hence the state of stress on faults. The ambient stress field has been determined from field-based fault slip analyses and verified by borehole breakout data for the in-situ stress field. The tendency of a fracture to slip (TS) or dilate (TD) can be calculated from its orientation with respect to the ambient stress field. Fractures in high-slip-tendency orientations (Barton et al., 1995; Morris et al., 1996) as well as high-dilation-tendency orientations (Gudmundsson et al., 2002; Ferrill and Morris, 2003) are preferential flow pathways. For the study at Brady’s, we analyze summarized values of the slip and dilation tendency TSD for each fracture segment of the fault model to characterize the structural permeability in the reservoir (Fig. 1). Summarized slip and dilation tendency is: 
graphic
where τ is the shear stress, σn is the normal stress, σ1 is the maximum principal stress, and σ3 is the minimum principal stress.

RESULTS

Here, we show how the measurements of surface gases and emanations can be linked with the fault stress model to develop a parameterized conceptual 3-D model of potential fluid migration pathways, using the data collected at the Brady’s fault zone and its step-over.

Surface Signal versus Fault Traces—2-D Horizontal Model

In the first step, surface emanations and diffuse degassing rates are correlated with each other and with quantitative structural-geological information along mapped fault lineaments (Fig. 2). This combination enables a two-dimensional (2-D) assessment of increased degassing rates and emanations. A moderate to very strong correlation was determined among CO2, H2S, 222Rn, and 222Rn/220Rn. The very weak correlation between LDR and the other parameters results from an anthropogenic effect on the LDR along Interstate Highway 80 in the northwestern sector (Jolie et al., 2015b). Structural permeability is not correlated to fault zones with either high slip or high dilation tendencies, but rather to a combination of both (Fig. 1). At Brady’s, the occurrence of anomalous degassing rates and emanations displays a geometrically constrained shape, indicating a fault-controlled distribution (Fig. 2). Peak CO2 and H2S emissions and peak 222Rn activity concentrations were measured in the Brady’s fault step-over along faults characterized by maximum TSD values. This area reflects the major fault-controlled permeability structure.

Surface Signal versus Fault Stress Model—3-D Model

In the second step, diffuse degassing rates and emanations are correlated with three selected faults of the 3-D fault stress model developed by Jolie et al. (2015a). Along the illustrated faults, segments of increased degassing and emanations and of abundant geothermal surface manifestations are concentrated and correlate with increased slip and dilation tendencies (Fig. 3). These results show that the Brady’s system is a multiple upflow system with a set of distinct fault-controlled upflows. Increased 222Rn/220Rn ratios point to major geothermal upflow zones where fluids migrate from depth to the surface.

Joint interpretation with the 3-D fault stress model indicates that fault lineaments with increased degassing rates and emanations are very deep-rooted fault zones, which reach into the Mesozoic metamorphic and plutonic basement below 1300 ± 200 m depth. Fault lineaments with minor gas emissions and emanations are commonly synthetic faults connected to the main faults.

Several other optimally oriented faults in the study area also reach into the deep basement but lack increased gas emissions and emanations along surface fault traces. This lack is possibly related to (1) hydraulically isolated fault blocks, (2) a minimized fault connectivity and reduced fluid flow, (3) fault sealing by mineral precipitation and/or geothermal alteration, or (4) reservoir rocks with very low porosity and permeability. Results of the quantitative structural analysis, however, indicate that such faults still hold the potential to transport fluids, which would make them suitable for the development of enhanced geothermal systems by reservoir stimulation.

DISCUSSION

This novel approach combining diffuse degassing and emanation surveys with 3-D fault stress models can be significant for a broad range of geoscientific disciplines by providing a method for an improved understanding of connectivity between surface and subsurface.

We show that anomalous gas emissions and emanations at the surface—in particular CO2, H2S, and 222Rn—are a sign of permeable fault segments and represent a fingerprint of the geothermal system at depth, indicating areas where the geothermal resource base could be exploited. Three-dimensional fault segments with increased slip and dilation tendencies are indicative of increased structural permeability and enable one to trace degassing and emanation anomalies from the surface to the reservoir, and therefore point to potential target regions for geothermal production wells. Production wells at Brady’s target these segments, but do not everywhere hit the central anomalies as shown in Figure 3. Among other factors, this could explain the declining performance of the geothermal power plant (Krieger and Sponsler, 2002).

The method developed above was used to detect permeable structures linking the Brady’s geothermal system with the surface. Not all potentially permeable structures give rise to surface emanations and degassing processes, and not all surface emanations and degassing processes are indicative of subsurface fault structures. Current efforts focus on the development of improved filter techniques to enable a distinction between different sources. The study area is an excellent test site to illustrate both surface and subsurface effects, and to distinguish between geogenic (e.g., geology, geothermal fault) and anthropogenic (e.g., infrastructure) effects based on a statistical data set separation (Jolie et al., 2015b). The approach presented here can be applied using parameters other than the ones we used; in other geological environments, gases and surface emanations not originating from magmatic processes may become more interesting (e.g., from biogenic processes).

Fault stress modeling enables the further characterization of fault segments, irrespective of the presence of fluids at depth. The reliability of the method depends strongly on the knowledge of the acting stress field and the quality of the fracture model. In this study, a constant stress field orientation was assumed over the full vertical extent of the 3-D model. Integrated stress field analyses for the whole length of existing or new boreholes and associated fault stress modeling will add further details and enable the development of an integrated fault stress model.

This method is not limited to geothermal exploration, and could be applied to fault stability analyses, earthquake monitoring, volcanic risk assessment, environmental monitoring, monitoring of seafloor emission sites (especially above known gas hydrate accumulations), and others. It could be particularly useful when applied to carbon capture and storage sites or to reservoirs enhanced by fracking, where leakage along faults might be hazardous to the overlying environment. Targeted monitoring of such areas by the presented approach could trigger the development of adequate mitigation measures in time to reduce environmental risks.

ACKNOWLEDGMENTS

We thank Ormat Technologies, Inc., and OLAM Spices and Vegetables, Inc., for permission to conduct the study. Special thanks are addressed to Matthew David Pope for his enthusiastic assistance during the fieldwork. Thanks also to James Faulds (University of Nevada) for establishing the basis of this study by detailed field mapping. We thank Alan Morris and an anonymous reviewer for helpful reviews. The 3-D model is developed with earthVision software (Dynamic Graphics, Inc.). Slip- and dilation-tendency analyses are performed with 3DStress software (Southwest Research Institute).

Gold Open Access: This paper is published under the terms of the CC-BY license.