Session 3E: Geology

Supercritical geothermal resources have potential to decarbonise industry and power sustainable economic growth opportunities. It is inferred that fractured, low matrix-permeability metasedimentary basement rocks (greywacke) that underly much of the Taupō Volcanic Zone (TVZ), New Zealand, could host supercritical geothermal resources. These rocks have complex fracturing patterns resulting from a long history of deposition, burial, faulting, and hydrothermal activity. Supercritical fluid circulation will likely be through fractures, so it is important to understand fracture distribution and fracture permeability in greywacke. A resistivity image from a >2 km-deep well in the Kawerau Geothermal Field provide the deepest fracture dataset collected in TVZ greywacke. The borehole observations are complemented by structural information from outcrops that are not hydrothermally altered.

Observations at two outcrops of low-grade metasediments and schist show that metamorphic veins are pervasive, of varied orientation, sometimes 10s meters long, and areal density can vary a lot over 10s meters. Parameters such as faulting seem to have a larger influence on vein density than rock type or metamorphic grade.

In the borehole image, potentially permeable fractures have orientation consistent with current tectonics. Resistive (mineralised) fractures have moderate dip magnitude and no preferred strike orientation, consistent with them being mostly of metamorphic origin. Conductive fractures are interpreted as having a combination of metamorphic and tectonic origins, with some likely to have been of metamorphic origin reactivated under current tectonics. Overall, there are more fractures in sandstone than mudstone, but this is less pronounced for conductive (potentially permeable) fractures than for resistive fractures (mineralised) or resistive fractures with conductive halo (potentially permeable). Differences between lithologies are less pronounced when considering volumetric fracture densities. Lithological controls on fracturing differ amongst the three permeable zones. The two minor permeable zones are in intervals of sandstone, mudstone and interbedding mixes. The third, major permeable zone is in dominantly sandstone and an interbedded mixture of sandstone and mudstone. Potentially permeable fractures of high thickness (>20 mm) are more common within the two minor feed zones than outside feed zones. Fracture density is higher in the major feed zone than outside feed zones.

When considering both outcrop and borehole image fracture data, overall, faults and fractures consistent with current tectonics seem to matter more than lithology for permeability. The veins of varied orientations observed in outcrops seem to remain in the geothermal reservoir (i.e., the resistive fractures), and the permeable fractures in the geothermal system may be either new ones or metamorphic discontinuities (veins, layering boundaries) reactivated under current tectonic stresses.

The University of Auckland’s “Reversing Carbon Emissions in the Geothermal Industry” five-year project, aims to mitigate Green House Gas (GHG) emissions by employing innovative techniques in carbon capture and storage. A key focus of the project involves developing processes that will enable trapping of gases, especially CO2, in solid form at optimal distances from the injection point, thereby promoting negative-emissions energy generation. A high-temperature-pressure reactor is currently under construction to carry out laboratory experiments involving saturating cored samples with CO2-rich fluid. Prior to these experiments, it is critical to fully characterise the physical properties of the cored samples. Our experiments are designed to identify the favourable fluid-rock interactions and trapping mechanisms that govern CO2 entrapment. Following our experiments, we will model fluid-rock interactions and the storage capabilities of the rock. We present the mineralogical, textural, and elemental characterisation of cored samples immediately adjacent to an injection depth, prior to it being used for reinjection, from one New Zealand geothermal field. Methods for rock characterisation include; (1) scanning electron microscopy to assess microscopic void dimensions, distribution, and connectivity and to examine reactive surfaces, (2) petrography to determine mineral relationships, mineral percentages and textural components, (3) X-ray fluorescence and energy dispersive spectroscopy to identify elemental compositions, (4) computerised tomography to establish rock density, and connected flow pathways. We are working with our industry partners to undertake field trials following completion of our experiments. New Zealand is taking significant steps in addressing climate change and minimising its environmental footprint. This MBIE-funded project is a notable advancement towards achieving this goal..

The Sorik Marapi Geothermal Field is a volcano-hosted geothermal system, located in northwestern Sumatra, Indonesia. The project has been developed in three phases by KS Orka for a total of 140 MWe of power generation. Wells drilled to depths of 1500 to 2715 m have proven a commercial geothermal resource at 245 – 320°C with neutral pH in an approximately 3 km (N-S) x 2 km (E-W), irregularly shaped area along the western side of the Sumatran Fault System (SFS). Unlike other geothermal systems that have been impacted by magmatic acid vapor cores or supergene acid influx, there is no evidence that the Sorik Marapi geothermal reservoir has experienced either, rather the reservoir is geochemically similar to many other entirely neutral systems developed worldwide. It can therefore be considered a prime example of paired (but separate) acid vapor core and neutral systems. Like other geothermal resources, the base of the argillic cap provides a good marker for the top of the reservoir in some areas of the reservoir (mainly under Pad A and some Pad T wells). However, across much of the reservoir, the reservoir top does not immediately occur at the base of the smectite cap and is instead commonly capped by phyllic or overlapping phyllic and propylitic alteration. In all areas across the field the appearance of wairakite, rather than the more typical epidote, is most diagnostic of the top of the reservoir. Vein paragenesis determined from thin section petrography illustrates where wairakite is observed, it is commonly the most recent vein filling mineral. In this paper, we detail the relations between permeability, alteration mineralogy and paragenesis, and present conceptual model and alteration elements in map, 1-D, and 2-D cross-section views to illustrate these relations.

Targeting structurally controlled permeability in geothermal fields remains a challenge because of the difficulties in characterizing subsurface structures in terms of their hydraulic behavior within the reservoir. Measurement sensitivity in combination with approximately 80% circumferential coverage (in an 8-1/2-inch wellbore) makes the Fullbore Formation MicroImager (FMI) useful for identifying subsurface structures and providing direct data on in-situ stress orientations in the formation intersected in the logged interval. Accurate, high-resolution wellbore images paired with skilled image interpretation enables fractures and faults to be characterized as open (hydraulically conductive) or closed (non-conductive, typically due to mineral deposition). Knowledge of the depths and orientations of permeable structures is beneficial at the local scale (within the logged wells) and at the reservoir scale. The improved understanding of permeability distribution is used to optimize operations and improve decisions about future well targets in conventional hydrothermal projects.

Two exploration wells were logged on behalf of CPC Corporation Taiwan in 2020, using SLB’s FMI, HGNS, TLD, HRLA, and ITRI’s PT logging tools. The production wells are located in the Tuchang Geothermal Prospect Area, Yilan County, Northeastern Taiwan. These logs were run mainly in the 8-1/2-inch diameter production interval of the wells, and the results have been analyzed and interpreted. This has enabled improved classification of fractures and faults, including basic properties (direction), and determination of whether the fractures and faults are open or healed.

Conductive fractures and faults are observed in much of the logged interval in each exploration well; together, they show a dominant strike orientation of NE-SW, with dip magnitudes in the range of 5°-90°. Drilling-induced fractures and borehole breakouts are also observed, with dip statistics showing a dominant strike orientation of NNE-SSW to NE-SW for drilling induced fractures and NW-SE for borehole breakouts. The interpretation of the logging results is consistent with the predominant structural / tectonic trend in the area (most mapped faults are oriented NE-SW). conductive fractures and faults are important contributors to permeability in both exploration wells. Large conductive fractures and faults with NE-SW, NW-SE, and N-S strike orientations have been correlated to permeability indicators such as the presence of thermal plumes and sudden increases in temperature.

Puna Geothermal Venture (PGV) injection well KS-21 was spudded in November 2022 and drilled to 8050 ft total depth in January 2023. The subsequent injection test demonstrated that KS-21 has an injectivity index of ±2.7 gpm/psi, marking the highest permeability observed in the field since Kilauea’s 2018 eruption. Targeting deep permeability associated with NE-trending fractures aligned parallel to the trend of the Lower East Rift Zone, KS-21 drilled through a sequence of subaerial and submarine basalts intruded by diabase dikes, encountering high permeability through a multi-cross of key target fractures. Analysis of drill cuttings and an acoustic borehole image log, the first successful image log at PGV, provide new insight into subsurface conditions and the permeability structure of the field. In this study, we (1) detail the conceptual framework of the PGV resource, (2) characterize the geologic results of KS-21, and (3) conduct a comparative analysis of permeability in new production and injection wells, providing new constraints on the conceptual model to guide future well targeting and resource management.

“Blind” geothermal systems, lacking active geothermal surface manifestations such as hot springs or fumaroles, are difficult to discover. However, there are often surface or near-surface materials indicative of subsurface geothermal activity above a blind system. These are typically products from springs/fumaroles related to the system that are no longer active or exposure of hydrothermal alteration minerals due to faulting and/or erosion. Examples of shallow or surface materials indicative of possible geothermal activity include silica polymorphs (e.g., opal/chalcedony, found in silica sinter, silicification or veining), carbonates (e.g., tufa, travertine, calcite veining), advanced argillic alteration (e.g., kaolinite, alunite), intermediate argillic alteration (smectite, illite) and borates. We utilize publicly available airborne hyperspectral data in the visible and short-wave infrared (SWIR) wavelength range over western Nevada to create thematic maps of geothermal indicator minerals over known geothermal systems and prospects for use in exploration. We map several benchmark areas with known geothermal surface materials: Salt Wells, and Lee Allen. We then apply these techniques to produce mineral maps of prospective resource areas in Gabbs Valley, Hawthorne, and Bell Flat, Nevada. We synthesize these maps with existing data to explore how they may inform exploration activities and potential conceptual models.