FROM 3D GEOLOGY TO 3D GEOTHERMAL RESOURCE MAPS: PROGRESS AND REMAINING CHALLENGES (Invited Presentation)

Assessment of geothermal resources on regional scales requires the development of 3D temperature models to identify accessible heat at economically viable depths (currently shallower than 7 km). Uncertainty in 3D temperature maps is reduced by integrating a broad collection of surface and subsurface data into regional geologic models, which can constrain thermal properties based on geology and can be calibrated against available temperature data. Recent studies at the U.S. Geological Survey have focused on the Great Basin in Nevada, the onshore Gulf Coast, and the Williston Basin, while ongoing work includes the Anadarko Basin, the Denver-Julesburg Basin, the Greater Green River Basin, and the Permian Basin. Collectively, these studies have predicted drilling depths to reach temperatures sufficient to produce electricity (moderate (>90°C) and high (>150°C)). This presentation will draw on the Great Basin and Williston Basin studies to illustrate various ways 3D geologic models have been constructed, and how they have resulted in 3D temperature maps that support new geothermal assessments.

Highlighting the importance of disentangling conductive versus convective heat flow, the Great Basin geothermal study first developed an estimated conductive heat flow map of the region. This was then utilized to predict temperature at depth using the steady-state heat equation which compared favorably with borehole temperature data throughout the region.

The regional Williston Basin study investigated the potential for deep, hot sedimentary geothermal resources. The study first focused on constructing a 3D geologic model of the basin stratigraphy based on dense borehole data. The 3D model incorporated compaction of sedimentary rocks and estimated thermal properties such as conductivity and radiogenic heat. These varying rock properties can also inform the existence of permeability in regions of sufficiently high temperature, which is required for heat extraction. The model is calibrated to tens of thousands of wells with downhole temperature data to generate maps for regional heat flow and subsurface temperature. Finally, the thermal model is calibrated through geologic time and has been or will be applied to other subsurface energy resources beyond geothermal (i.e., petroleum, natural hydrogen).