APPLICATIONS OF 3D MODELING TO GEOTHERMAL RESOURCE EVALUATION, GREAT BASIN REGION, WESTERN USA: LESSONS LEARNED IN STRUCTURALLY COMPLEX VOLCANIC TERRANES

The Great Basin region of the western U.S. is one of the largest geothermal provinces on Earth, thanks to crustal thinning, high geothermal gradients, and abundant active faults in this extensional to transtensional region. This region has been subjected to multiple orogenic events, which have included large magnitude folding and thrusting in late Paleozoic-Mesozoic time followed by regional extension-transtension in the Cenozoic. Voluminous volcanism associated with arcs and the Ignimbrite flare up have also swept across the region. Although beset with challenges due to complex structures and stratigraphy, with multiple overprinting events, detailed 3D modeling is possible given availability of well and geophysical data to provide subsurface control.

Detailed 3D models have been completed for several geothermal areas in the region, including Bradys, Soda Lake, Granite Springs Valley, Fallon FORGE, SE Gabbs Valley, Astor Pass, Neal Hot Springs, Steptoe Valley, and Tuscarora. Most 3D modeling was carried out by Dr. Drew Siler while with Nevada Bureau of Mines and Geology and USGS. Datasets incorporated in the models included geological mapping, well logs generally after review of cuttings and/or core, fault kinematic data, slip rates and age of ruptures on Quaternary faults, regional stress field, gravity, magnetics, MT, and seismic reflection profiles (if available). The 3D models provided insight into fault geometries, structural controls on fluid flow, geothermal reservoirs, and conceptual models. In some cases, it resulted in the first firm estimate of which units contained geothermal reservoirs for operating plants. The models also elucidated subsurface structure by modeling density of fault intersections and slip/dilation tendency, both crucial in controlling fluid flow. 3D geophysical inversions were employed at some sites to test results. The size of the 3D models varies, but generally do not exceed ~ 200 km³, with depths from ~2-3 km. Considering that such modeling is relatively low cost, it is worthy of investment, as the process of making the model refines the 3D perspective, which leads to more informed selection of drill sites and reduces risks. These efforts show that 3D modeling is alive and well in a complex terrane like Nevada, but by necessity is generally finer in scale and focused more locally, as compared to 3D modeling in some other parts of the country.