Recent contributions to geothermal research

Several recent papers describe research relevant to characterization of geothermal targets.

To produce hot water or create an efficient circulation system, low-enthalpy geothermal reservoirs need effective heat exchange within the host rock and sustained high flow rates with no fast-paths for water breakthrough. Geothermal appraisals can effectively account for ambient temperature and, in sandstone, host-rock porosity and permeability. However, optimal stimulation of enhanced geothermal (EGS) reservoirs may depend on the size, openness, stiffness, strength, abundance, surface area, arrangement, and connectivity of naturally occurring fractures that are hard to sample or accurately predict.

Our recent papers describe how site-specific fracture information can be augmented with drilled sidewall cores (Fulcher et al. 2026), how outcrop analogs can be used to find specific, actionable information about some subsurface targets (Elliott et al. 2025; Forstner et al. 2026), and how outcrops can yield new kinds of information about fracture connectivity (Forstner and Laubach 2022) and 2D spatial arrangement (Corrêa et al. 2022)..

Geothermal research in New York

Fulcher et al. describe how in New York State and Pennsylvania, USA, Precambrian metamorphic and intrusive rocks and Cambrian to Lower Ordovician sedimentary rocks are reservoir targets for deep direct-use geothermal development. Evaluation of natural fractures and structures in the potential reservoir units at the Cornell University Borehole Observatory site was conducted through cross-scale evaluation of oriented sidewall cores, borehole image (BHI), and far-field acoustic survey data. Oriented sidewall cores in the basement complex (Cayuta Formation) reveal metasediments containing foliations, lineations, mineral-filled fractures, and breccia intervals. Basement sidewall core fracture data aid identification of fractures in BHI surveys riddled with borehole breakouts.

In contrast, sidewall and image log data for the Cambrian-Ordovician sedimentary section show that open fractures are present and allow orientation and abundance to be estimated. At various depths sandstone and dolostone sidewall cores contain quartz-filled or carbonate-filled bed-normal and -parallel microfractures. Four subvertical microfracture sets, formed sequentially, strike NW-SE (F1), NE-SW (F2), N-S (F3), and WSW-ENE (F4).

Microfracture set orientations F1, F2, and F4 match interpretations of acoustic fracture anomalies (open fractures) located tens of meters from the wellbore. In the uppermost Galway Formation sandstone, common microfracture apertures are 0.001 to 0.01 mm. The widest microfractures transition to quartz-lined and bridged open macrofractures. An open vertical macrofracture in Galway sandstone is observed in BHI surveys and a sidewall core, effectively ground-truthing the F4 fracture set. Based on comparison of core fractures with borehole image survey features, differentiation of natural from drilling-induced fractures reveals three sedimentary rock zones of elevated natural fracture frequency.

These results were compared with the outcrop studies of Elliott et al. 2025.

Elliott et al. show that exceptionally large, well-exposed sandstone outcrops in New York provide insights into folds, deformation bands, and fractures that could influence permeability, heat exchange, and stimulation outcomes of geothermal reservoir targets. Cambrian Potsdam Sandstone with <5% porosity contains decimeter-scale open, angular-limbed monoclines <0.5 km apart with associated low-porosity mm-wide cataclastic deformation bands. Crossing and abutting relationships among sub-vertical opening-mode fractures show four chronological Sets A–D, striking NNW, NE, NW, and ENE, respectively. Fracture lengths and heights range from millimeters to tens of meters. Sets A and C macro-fractures, and possibly B and D, contain quartz deposits.

All sets have abundant associated quartz cemented microfractures that also record set orientations and crosscutting relations. Quartz cement deposits—evidence of diagenesis—are the key to identifying attributes of outcrop fractures suitable for extrapolation to geothermal targets in sandstones because they show which fractures formed in the subsurface. Set A fluid inclusion homogenization temperatures (120°C–129°C) are compatible with fracture at >3 km depth. Fractures are stiff and those ≥0.05 mm (Set C) and ≥0.1 mm (Set A) are open and potentially conducive to flow. Sets A and D are abundant in outcrops with close fracture spacing—0.18 m and 0.68 m, respectively—and define a rectangular connectivity network dominated by crossing and abutting X and Y nodes. Set A aperture distributions follow a power law with slope −0.8 up to 0.15 mm; other sets have lognormal distributions. Set A and D microfractures are weakly clustered, while macro-fractures commonly have 1D anticlustered (regular or periodic) arrangements at shorter length scales (<0.2 m). Sub-horizontal fractures are barren and may have formed near the surface. Fracture heights, lengths, and spatial arrangements show good trace connectivity but low open connectivity.

For geothermal applications, outcrop results predict low initial well-test permeabilities owing to quartz disconnecting open fractures, but stimulation of closely spaced microfractures and partly open macro-fractures could yield high surface area for heat exchange. Quantitative extrapolation of key fracture attributes like abundance, orientation, spatial arrangement, length, and open fracture connectivity is possible from outcrops to fractured reservoirs if differing thermal histories and diagenesis are accounted for.

Using these outcrops, Forstner et al. provide new insights into measuring fracture lengths for geothermal applications. Fracture lengths govern permeability and are unknowns in geothermal assessment. Along their lengths, fracture widths vary due to growth by linkage. Under the influence of diagenesis, narrow widths seal, breaking porosity continuity and reducing open length. The largest range of widths and thus susceptibility to fill occurs where fractures are linked by narrow segments.

Outcrops of a geothermal target, Cambrian Potsdam quartz arenite, contain opening-mode fractures having lengths spanning five orders of magnitude from 0.082 mm to 17.9 m. Combined lengths measured at a range of scales can be described by power laws, but at a given image resolution, lengths are best fit by exponential functions. Owing to preferential sealing of small fractures, open fractures follow exponential functions, but values depend on rules for designating fractures as continuous. En échelon segments, offset 10 mm, are connected by narrow fractures or microfractures (hard linked) not evident on outcrop 1 m-elevation LiDAR or 30 m-height drone images.

A rule that identifies where narrow and likely connected segments are located can yield lengths meaningful for flow simulation. Depending on diagenesis, continuity rules can halve or double average and maximum length values. Length values from outcrop for geothermal applications should be adjusted based on wellsite-specific diagenesis information.

Forstner and Laubach (2022) describe new approached to characterizing fracture connectivity and Corrêa et al. (2022) describe breakthroughs in describing 2D fracture spatial arrangement.

These paper are published open access.

Fulcher, S.A., Jordan, T.E., Laubach, S.E., Elliott, S.J., Nogales Herrera, V., 2026. Improving borehole fracture characterization using oriented sidewall cores for geothermal assessment – an example in central New York State USA. Geothermics 136, 103589 | Published open access | view at publisher

Elliott, S.J., Forstner, S.R., Wang, Q., Corrêa, R., Shakiba, M., Fulcher, S.A., Hebel, N.J., Lee, B.T., Tirmizi, S.T., Hooker, J.N., Fall, A., Olson, J.E., Laubach, S.E., 2025. Diagenesis is key to unlocking outcrop fracture data suitable for quantitative extrapolation to geothermal targets. Frontiers in Earth Science 13, 1545052. Published online open access 2025 | view at publisher

Forstner, S.R., Corrêa, R., Wang, Q., Laubach, S.E., 2026. Fracture length data for geothermal applications. In Gill, C.E., Goffey, G., Underhill, J.R., eds., Powering the Energy Transition through Subsurface Collaboration, Geological Society of London, Energy Geoscience Conference Series, v. 1, https://doi.org/10.1144/egc1-2024-17. Published online open access 2024 | view at publisher

Corrêa, R.S.M., Marrett, R., Laubach, S.E., 2022. Analysis of spatial arrangement of fractures in two dimensions using point process statistics. Journal of Structural Geology 163, 104726.

Forstner, S.R, Laubach, S.E., 2022. Scale-dependent fracture networks. Journal of Structural Geology 165, 104748. https://doi.org/10.1016/j.jsg.2022.104748 | view at publisher