Session 4E: Geology

Geologic mapping is a crucial component of geothermal exploration programs because faults penetrating formations with rock properties favorable to forming open space fracture permeability are an important conceptual element of almost all geothermal reservoirs that have sufficient permeability to be commercially viable. Depending on exposures and site access, geologic maps typically have several components that are relevant to the conceptual model, including; stratigraphy, faults, structural data, active and inactive surface manifestations, and type and extent of alteration. Each of these components contributes to the interpretation of the geophysics and geochemistry and the integration of these with thermodynamics in a resource conceptual model. In some cases, surface geology mapping is so ambiguous that it should be given less emphasis in the exploration strategy, for example, on volcanoes with geology obscured by recent ash eruptions. More commonly and more seriously, geologic mapping is completed for purposes other than geothermal exploration and/or with approaches inconsistent with geothermal needs, commonly producing seriously misleading results. Therefore, we have proposed quality assurance guidelines for non-specialists to consider when reviewing geology and structural interpretations and best practice standards for geologists.

For most geothermal exploration projects worldwide, existing mapping will be of insufficient detail and/or accomplished with a different purpose (e.g., regional geologic mapping, seismic hazard, mineral exploration) than what is needed for building a reliable structural model for formation-mediated fracture permeability in the geothermal resource conceptual model. Typically, the design of a geologic mapping study can be based on existing maps but all features should be reviewed, ideally using high resolution Lidar followed by detailed ground proofing, considering a larger area before focusing on the area of the known surface thermal manifestations The regional reconnaissance mapping should be reviewed to provide the regional geologic context for the local mapping and to determine the larger setting for the local fault mechanics. Specific applications of the geologic mapping includes: determining permeability patterns (structural settings, stress orientations, Quaternary fault activity, stratigraphic), identifying new areas for geochemistry sampling, designing geophysical surveys, identifying areas of relict versus active alteration, alteration type and insight into possible local erosion of the clay cap. The crucial quality assurance step for the geologic mapping is in its consistent integration with the geochemistry, geophysics and supporting data in the geothermal resource conceptual model.

Since August 2022, an exploration campaign has been conducted in Northern Alsace, France, to enhance the understanding of the area and build 3D models integrating geological, geophysical, and geochemical data. These 3D models aim to mitigate the risks associated with future lithium extraction by modeling the effects of extraction on the subsurface. This comprehensive integrative approach will help lithium resource assessments and targeting future projects.

Exploration drilling for geothermal resources is expensive and the risk of not discovering an economic resource with exploration drilling is high. Lower cost, shallower direct measurements of the subsurface have therefore been conducted as part of early-stage geothermal exploration since at least the late 1970s. These surveys prioritize low cost and rapid data gathering over a wide field area. Since the mid-2000s the technique has been further developed and data corrections have become more sophisticated. This paper compares three shallow temperature (2 m probe) systems used by public and private groups. Additionally, findings are described from a shallow temperature survey conducted in 2010 at Puchuldiza, Chile. The results indicate that shallow temperature surveys are an effective method for discovering and characterizing geothermal resources.

The northeastern part of the Reese River basin situated ~15 km southeast of Battle Mountain, Nevada, scored highly in the Nevada geothermal play fairway analysis (PFA) for hosting potential hidden geothermal systems. This site (also referred to as Argenta Rise) was therefore chosen for detailed study in the INGENIOUS project (INnovative Geothermal Exploration through Novel Investigations Of Undiscovered Systems). The high PFA scores resulted primarily from favorable structural settings (e.g., fault intersections and pull aparts) with relatively high slip rates on Quaternary faults. The INGENIOUS project is utilizing additional parameters and more rigorous analytical techniques to further advance exploration at this site. This includes integration of geological (e.g., Quaternary fault mapping) and new geophysical datasets (e.g., gravity, magnetics, MT data, and five reprocessed seismic reflection profiles) to build a structural model and to identify specific favorable sites for potential geothermal upwellings. Two-meter temperature surveys were also conducted in the area (139 measurements).

This part of north-central Nevada is characterized by systems of intersecting northerly and ENE-striking faults within the broader Humboldt structural zone, a poorly understood belt of ENE-striking faults and relatively high heat flow extending across northern Nevada. Kinematic analysis of exposed fault surfaces shows that ENE-striking faults have accommodated sinistral-normal slip, and normal slip characterizes N- to NNE-striking faults. Northeastern Reese River Valley lies within a broad left step between major ENE-striking fault zones on the northern flanks of the Argenta Rim and Shoshone Range and thus corresponds to a broad pull-apart in the ENE-striking sinistral-normal fault system. Notably, the nearby Beowawe geothermal system in Whirlwind Valley (with abundant sinter, hot springs, and a geothermal power plant) occupies a fault intersection in a relatively small left step in a major ENE-striking sinistral-normal fault and may serve as an analogue for a potential hidden system in northeastern Reese River Valley. Existing geological maps, high-resolution lidar, and seismic reflection data demonstrate that northeastern Reese River Valley is structurally complex with multiple intersections between the ENE- and N- to NNE-striking fault systems. Some of these fault intersections correspond to low resistivity anomalies, magnetic lows, and/or very subtle 2-m temperature anomalies, which may indicate hidden geothermal upwellings. 3D modeling and temperature-gradient drilling are planned to further evaluate these sites for geothermal activity.

Temperature-depth measurements from shallow water wells and deep monitoring wells are used to investigate the thermal interplay between fluid flow in faults and in aquifers in the southern San Luis basin. Two areas with the highest geothermal gradients are located in the Mirada graben on southern margin and in the No Agua-Timber Mountain area on the western margin of the basin. The ENE-striking, north down, left-lateral Embudo fault system is an accommodation structure connecting two Rio-Grande-rift half-graben basins, the west-dipping Española Basin to the south and the east-dipping San Luis basin to the north. The Embudo fault cuts the older, north-striking, 8-km-wide Picuris-Pecos fault zone, a Laramide structure that generally is an E-down, strike-slip fault. Toward the east, the Embudo fault transitions to the N-striking W-dipping 2-km-wide, Sangre de Cristo fault zone by progressively changing to a dip-slip fault. The intense fracturing of basement rocks caused by the complex interactions among these faults provides pathways to connect deep aquifers with shallow aquifers. Within the Picuris-Pecos fault zone, a series of horsts and grabens, referred to collectively as the Miranda graben, controls the location of Ponce de Leon warm springs. Three water wells (128 to 273 m deep) were logged in Miranda Canyon. Geothermal gradients (32–58°C/km) in all three wells are modified by flowing water. The highest elevation well records downflow and the two lower elevation wells record upflow. Discharge temperatures at the low-elevation end of this system are as high as 35°C.

A second area with elevated geothermal gradients extends from the No Agua rhyolite dome along the western margin of the basin eastward to Timber Mountain near the center of the basin. Three thermal profiles collected near No Agua record lateral fluid flow in a thin zone (1–5 m) in a basalt interval. The well at No Agua displays upflow of 33°C water from the screened interval (276–330 m) within a sandstone just above granite basement. In contrast, the three wells at Timber Mountain, which were measured in air, have linear gradients of 50 to 70°C/km in wells 120 to 170 m deep. The temperature data, in combination with published water chemistry data, indicates that the recharge area for this system lies to the west in the Tusas Mountains. The meteoric water has circulated deeply, picking up heat and chemical constituents and is discharging along buried faults.

Temperature profiles collected from 24 shallow wells scattered throughout the basin are disturbed by fluid flow through vesicular and flow-breccia intervals between the basalt flows of the Taos Plateau volcanic field, masking the deeper thermal structure of the basin. Three monitoring wells (410–900 m deep) penetrate below the basalt into the underlying rift-fill sediments. These wells have geothermal gradients of 26.4–28.3 °C/km.

South-central Alaska holds significant geothermal potential due to its location along the Pacific Ring of Fire, a region globally known for its volcanic activity and geothermal resources. The Cook Inlet Region lies above the Pacific Plate subduction zone and contains active volcanoes, including Mt. Augustine.

Alaska has some of the highest residential and commercial electricity prices in the nation, while its per capita energy consumption is the second highest, in part because of the State's small population, cold climate, and energy-intensive industries. South-central Alaska houses approximately 550,000 people (75% of the entire Alaskan population). Power sales in 2020 exceeded 4,408 GWh, of which 82% came from fossil-fuel produced electricity (natural gas and coal) and 18% from renewables (primarily hydroelectric). In December 2021, the Governor of Alaska requested a study on the potential impacts of achieving an 80% renewable energy portfolio for South-central Alaska.

Mt. Augustine is an andesitic stratovolcano typical of subduction zones and located in the Kamishak Bay in the southern part of the Cook Inlet, approximately 60 miles west of Anchor Point and 175 miles SW of Anchorage. Numerous previous researchers have concluded that it contains a shallow magma chamber making it attractive for geothermal exploitation. In September 2022, GeoAlaska was awarded a permit to explore for geothermal resources across 3 onshore tracts of land, totaling 3,048 acres on the southern part of Augustine Island, primarily where the Upper Jurassic Naknek Formation outcrops.

In the summer of 2023, Acoustic Magnetotelluric (AMT) data from 28 sites and gravity data from a further 215 locations were acquired, processed, and interpreted. Additionally, 20 rock samples were collected from cliff sections along the southern margin of the island, primarily from within the outcropping Naknek Formation.

The jointly inverted geophysical data reveal an area in the southern part of the island of low resistivity and high density strata lying above a zone of higher resistivity. We interpret this as the sub-crop of the Naknek Formation acting as a seal, trapping a potential hydrothermal system below, within the fractured basement. The Naknek Formation is likely to underly much of the southern part of the island and is such an effective seal, it has all but limited any above sea-level surface geothermal features that one would expect for a typical andesitic geothermal resource. As such, we describe this Mt Augustine resource as being an atypical blind geothermal system, suggesting a largely horizontal circulation pattern. An analogue for this system could be Cerro Pabellón GPP in northern Chile, which has a current installed capacity of approximately 83 MWe.

As a result of this collected data, additional exploration activity will be undertaken during 2024/25 including the drilling of an exploration well.