Bottom-Hole Temperatures in Oil and Gas Wells: Window to the Deep Thermal Regime and Geothermal Potential of Sedimentary Basins in the Eastern Great Basin

Abstract

Oil and gas wells are relatively abundant compared to deep geothermal wells. As such, they constitute the primary window into the deep thermal regime of western Utah and eastern Nevada. Bottom hole temperature (BHT) data from the log headers of over 460 wells were examined as part of an evaluation of the geothermal potential in deep sedimentary basins in the eastern Great Basin. The wells studied constitute nearly all of the oil and gas wells in the region with reliable thermal data. The BHTs of these wells were measured shortly after drilling was completed. Consequently, the data are thermally disturbed due to circulation and represent minimum temperatures compared to the in situ formation temperatures. As a result, several correction methods were applied to these data to assess the true thermal regime. Drill stem test temperature data were also used when available, and were usually more trusted as true indicators of formation temperatures.

The basement beneath most of the basins are Paleozoic carbonates with generally high permeability that enables regional fluid flow. While permeability and sufficient quantities of hot fluids are critical components of geothermal systems, deep groundwater circulation may flush heat from a given area, resulting in anomalous areas of decreased heat flow. For example, a zone of suppressed heat flow (60 mW/m²), compared to typical Basin and Range heat flow of 85 ± 10 mW/m², largely surrounds the productive Cove Fort hydrothermal system in Utah and is thought to result from large-scale fluid movement through laterally extensive carbonate formations. A similar setting is the likely cause of the “Ely heat flow low” in Nevada. Additionally, the BHT scatter within a given basin, while partly a factor of data quality, may also reflect localized heat transport due to deep groundwater circulation.

The economically viable temperature-depth target for basin-centered stratigraphic geothermal reservoirs is 150–200°C at depths of 2–4 km, and at least eight basins may have some potential to host stratigraphic reservoirs. Of these basins, the Black Rock Desert in Utah and Steptoe Valley in Nevada are by far the hottest and have proven temperatures, from equilibrium temperature profiles and BHTs, well within the economic temperature-depth window.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018