Application of Geophysical data for resolving Geothermal Stratigraphic Reservoirs: A target beneath the Black Rock Desert, Utah, USA

Abstract:

Magnetotelluric (MT), gravity, and thermal data are an integral part of geothermal resource exploration throughout the world. The Black Rock Desert, Utah, may be unique, with large datasets of MT soundings and gravity measurements in combination with oil exploration wells extending up to 5 km depth possessing a variety of geophysical logs, and proven high heat flow in the central part of an underlying basin (temperatures exceeding 240°C at 3 m depth). Throughout the BRD, there are large variations in lithology and, consequently, resistivity and density. Wireline geophysical data indicate basin fill signatures of 1 to 10 ohm-m and 1.6 to 2.6 g/cm³; bedrock signatures are 10 to over 1000 ohm-m and 2.5 to 2.8 g/cm³. Gravity data indicate an anomaly amplitude on the order of -30 mGal as the basin signature. Massive salt sections, when emplaced in clay-rich basin fill, show resistivities on the order of 100 ohm-m. The upper portions of the 1D, 2D and 3D resistivity models have reasonable agreement with the wireline data whereas in the central part of the basin, the deeper portions of the wells and the models have disparities that are an order of magnitude different. Preliminary 2D gravity models show correlation with the inferred resistivity models in regards to structure on the basin margins, however the relationship diminishes toward the center of the basin. The most striking difference is the bottom of Pavant Butte well where temperatures reach 240°C and in-situ resistivities are 100 ohm-m, but the modeled resistivities are an order of magnitude lower (<10 ohm-m). Possible explanations for this difference are the existence of aligned conductive fracture networks deep within the bedrock with a small fraction of crustal fluids in the pore space or differences in the averaging scale of MT data versus downhole wireline data. Signatures of a deeply rooted system are more than likely detected with MT soundings and gravity modeling provides inferred basin structures as well as geometries. The signature of our specific target (stratigraphic reservoir at 3 to 5 km depth) requires further investigation leading to reasonable delineation using a multiple-technique approach, possibly a joint interpretation of 3D resistivity and gravity models with local controls (i.e., well data).