Session 2E: Geology

Colorado has a long history of mining and hydrocarbon production as well as recent developments of wind and solar energy. Additionally, hot springs and thermal wells exist across the state, suggesting potential for geothermal resources. Previously, limited studies aimed to characterize these individual geothermal sites, with plans for local development. The statewide geothermal resource has been generally known and mapped in the state in past years, but there is significantly more data now available, which means resource understanding can be improved through a new resource assessment. Additionally, no recent public studies have examined the best pathway for economic development of these resources in Colorado that could substantially assist the energy needs of the state.

As part of a larger geothermal resource study for the state of Colorado, an initiative by the Colorado Energy and Carbon Management Commission (ECMC), Teverra completed a thermal resource evaluation with new data and produced new geothermal resource maps. In addition to evaluating the geothermal resource in Colorado, the study also examined utilization opportunities given the determined resource potential.

The geothermal resource across the state was examined for electricity production, direct use, and geothermal heat pump applications. There are several prominent sedimentary basins with significant development that may be an opportunity for well repurposing, which is also examined. There is resource with temperatures 100-175 °C throughout the state at depths that are currently drilled. In these areas, power generation could be a possibility, and technologies to utilize lower temperature resources are likely to have the greatest economic impact and benefit for the majority of the state.

This study demonstrates a new approach to understanding the geothermal resource for the state, and compares this work to what work had been previously completed on geothermal potential in Colorado.

Accurate formation temperature is a pivotal part of geothermal resource assessments in sedimentary basins. Bottom-hole temperature (BHT) measurements from well logs are an abundant data set in these basins but are also notoriously unreliable because of several reasons such as the vintage of the measurement, type of tool used, ambient surface temperature at the time of logging, thermal properties of the drilling mud, time since circulation (TSC), rate of penetration (ROP) while drilling, human error, and the thermal conductivity of the formation and formation fluids (oil, natural gas, or brine) where the temperature is measured. Consequently, BHT correction methods vary depending on the basin and available data. In south Texas, where oil and gas wells dominate the landscape, the Waples correction method is commonly used because it incorporates TSC data and ambient surface temperature to correct for the cooling effects of drilling mud on the formation while the well was being drilled.

A 12,500 square mile area in south Texas was designated for BHT correction and temperature-depth mapping because it contains most of the key sedimentary geothermal elements of the Texas Gulf Coast. These elements include: 1) the location of geopressured reservoirs in Paleogene formations, 2) the location of the Aptian and Albian shelf margins, 3) the type of transitional crust beneath the sedimentary section, 4) the location of salt diapirs, and 5) the shallowest depth to 250⁰ F in Texas. To help identify geothermal play types in the Cretaceous strata of south Texas, BHT measurements from 826 wells north of the Aptian and Albian shelf margins were corrected for temperature-depth mapping. Of these 826 wells, 272 had TSC data and were subsequently corrected using the Waples BHT correction methodology. This method resulted in an average temperature increase of 19.63%, which aligned with temperature measurements from cement bond logs, drill stem tests (DSTs), and regional BHT correction studies completed by other researchers. This 19.63% increase was then applied to the remaining 554 wells that lacked the TSC data needed to properly correct BHTs for those wells.

Corrected BHT measurements for all 826 wells were then used to calculate a temperature log from the surface to the total depth of each well. Using Petra cross section and mapping software, temperatures were correlated from well to well across the research area at 25⁰ F increments from 250⁰ F to 400⁰ F. This methodology resulted in two important products for geothermal play type identification: 1) a high-resolution series of temperature-depth maps for play fairway identification and 2) a set of temperature logs that can be incorporated into petrophysical analysis for temperature-based reservoir characterization calculations.

The Hawai‘i Play Fairway project produced the State of Hawai‘i’s first statewide geothermal resource assessment since the 1980s and Hawai‘i’s first quantitative geothermal resource probability model. Funded $2.3 million by the U.S. Department of Energy, the project operated in three phases from 2014 to 2021. During the first two phases, the project developed a statewide geothermal resource probability map and a Play Fairway methodology and identified 10 locations for geothermal prospecting. During the third phase, the project tested the methodology, drilled a temperature observation well on Lāna‘i Island to a depth of ~1km, and developed final statewide probability maps. At 1-km depth, the temperature within Lāna‘i Well 10 exceeds those of two wells in Kīlauea’s East Rift Zone and a third well in the Saddle region of Hawai‘i Island that exhibited an elevated temperature gradient from 1-2km depth. The Phase 3 work produced an updated statewide geothermal resource assessment via a newly established methodology to integrate multiple datasets into a resource probability calculation, or map. This paper presents final statewide probability maps independently for heat (PrH) and permeability (PrP); we now take fluid (PrF) as equal to one. This differs from our Phase 1 presentation of a combined resource probability achieved by multiplying the three together. Our updated approach is to now stress Heat as fundamental to current geothermal prospecting in Hawai‘i. We now recognize that a) the probability of Fluid at resource depths equals one at the 2+ km depth of Hawai‘i’s geothermal resource given Hawai‘i’s ocean environment, and b) we have little-to-no field data on permeability at depth (outside of Kīlauea’s East Rift Zone) with which to calibrate a model of permeability. We also present a map of confidence in our Probability of Heat.

The Innovative Geothermal Exploration through Novel Investigations Of Undiscovered Systems (INGENIOUS) project aims to discover new, economically viable hidden geothermal systems in the Great Basin region by building on previous work in play fairway analysis and machine learning. A key objective of this project is to develop an exploration workflow to reduce geothermal exploration risks for hidden geothermal systems. A single preliminary play fairway workflow was developed from the assessment of the regional INGENIOUS geological, geophysical, and geochemical datasets. This workflow provided new preliminary predictive geothermal fairway maps for the INGENIOUS study area, which encompasses most of Nevada, western Utah, southern Idaho, southeastern Oregon, and easternmost California. However, a recent study (incorporating machine learning techniques) of a portion of Nevada identified four geologic domains and determined that the relative importance of individual datasets or features as indicators of geothermal potential may differ across these domains. The INGENIOUS study area includes a much larger and more geologically diverse region; therefore, additional geologic domains or sub-regions are expected. To assess the sub-regions in the INGENIOUS study area, principal component analysis and k-means clustering were applied. Preliminary results indicate that the INGENIOUS regional data cluster into groups that relate to different geologic domains in the Great Basin region. These include domains such as the Walker Lane, extensional western Great Basin region, broad lower strain region in the eastern Great Basin of western Utah and eastern Nevada, Quaternary volcanic fields, and the area adjacent to the Snake River Plain. These clusters are assessed to determine the key geologic drivers of the identified clusters and if they relate to key components of geothermal conceptual models. Understanding this variability can provide key insights for the exploration and characterization of hidden geothermal systems in the Great Basin region and could indicate the need to develop multiple geothermal conceptual models and play fairway workflows for the INGENIOUS study area.

At the 2023 GRC conference, we discussed the methodology for geothermal play fairway analysis (GPFA) and their associated outputs, Common Risk Segment (CRS) & Composite Common Risk Segment (CCRS) Maps. The CRS maps define areas that contain the same general Probability of Success (PoS) for each individual risk element based on the input data. Cutoff values or classes are then applied to each map with color assignments indicating high (red), medium (yellow) and low (green) risk areas for each element under consideration. Of prime importance and the first risk element to be interrogated is an adequate heat resource, without which there is no geothermal prospect to exploit.

Applying the GPFA process to the Texas/Gulf Coast region, two anonymously high geothermal gradient regions were identified. One in Northeast Texas within the Haynesville oil and gas play and the second located to the Southwest in the Eagle Ford region. Ambient background geothermal gradients for the majority of the Texas/Gulf Coast are approximately 35-40°C/km, whereas these locations contain elevated gradients (60-100°C/km), creating geothermal project conditions with potential for significantly reduced drilling costs.

In this presentation, we will investigate the geological mechanisms driving these elevated geothermal gradients; the presence of salt diapirs in the Northeast and geopressured geological formations to the Southwest.

The Great Salt Lake (GSL) basin has been identified as a potential area for sedimentary geothermal exploration, primarily based on the presence of deep legacy oil and gas wells that have high geothermal gradients and temperatures relative to many other areas in the Basin and Range province. However, previous studies have not provided detailed descriptions of sedimentary geothermal play concepts or characterized sedimentary geothermal reservoirs that might be exploited in the basin. In this study, new seismic structural cross sections, petrophysical data from legacy oil and gas wells, and outcrop analog studies are integrated to generate and characterize new concepts for potential sedimentary geothermal exploration and development in the basin.

Structural mapping from terrestrial gravity surveys and seismic reflection data defines major extensional faults and three major sub-basins where sedimentary geothermal reservoirs are buried by 2–3 km of Miocene to recent sediment. These are the Gunnison Bay basin, the Gilbert Bay basin, and the Willard Bay basin.

The most prospective stratigraphic reservoirs are fractured Paleozoic carbonates, which reach temperatures greater than 150 °C as shallow as 2.5 km depth in the basin. Units of interest are the Ordovician Fish Haven Dolomite and the Silurian Laketown Dolomite. Both of which are massively bedded, commonly vuggy, and pervasively fractured. Conservative estimates of log-derived porosity for these carbonates vary between 5%–10%, which is higher than many analogous fractured commercial petroleum reservoirs. These potential reservoir units are exposed in the Promontory Mountains providing information on natural fracture characteristics used to model ranges of production flow rates and define optimal horizontal drilling orientations for conceptual geothermal developments.

The reservoir characterization is used to generate techno-economic models of conceptual sedimentary geothermal development and highlight fairways where potential sedimentary resources exist and could be developed. While much of the GSL basin will be difficult to develop due to sensitive environmental issues, the northern extent of the Willard Bay basin is onshore, above the GSL highstand and represents a favorable greenfield sedimentary geothermal exploration target area with no deep well penetrations. Additionally, the approach detailed here is beneficial as it can be applied to other sedimentary reservoirs and plays in the Great Basin.