| Keywords |
Crump geyser, Warner Valley, Oregon, hot springs, geothermal, faulting, geophysics, gravity, magnetics, 3D modeling, heat and groundwater flow simulations |
| Abstract |
Warner Valley in southern Oregon (USA) hosts a geothermal system characterized by several thermal springs and siliceous sinter mounds. Crump geyser, that issued from a well soon after being drilled in 1959 on the west side of the valley, underwent frequent eruptions of boiling water. These manifestations prompted several geological, geochemical, and geophysical investigations of the valley’s geothermal resources. The present work focuses on 3D potential field modeling, incorporating a diverse range of datasets, aimed at characterizing the structural setting of the natural hydrothermal system around Crump geyser. Warner Valley forms a narrow north-south trending extensional basin that developed as an asymmetric graben in a tectonically complex region situated in the northwest corner of the Basin and Range Province. The regional geology consists predominantly of interbedded Neogene sediments and Neogene to Paleogene volcanics that have been faulted by a series of obliquely oriented NW and NNE-trending extensional faults. The large contrast in rock properties (density, magnetic susceptibility, and magnetic remanence) between the basin sediments and volcanic rocks renders potential field methods (gravity and magnetics) particularly well-suited to mapping and modeling subsurface geologic structures such as faults that juxtapose contrasting rock types and result in distinct gravity and magnetic anomalies. We performed high-resolution gravity and ground magnetic surveys locally around Crump Geyser, as well as regionally, to constrain basin geometry, characterize intra-basin faults that are obscured by basin fill, and study fault interactions. Furthermore, this work is aimed at identifying areas favorable to hydrothermal flow that will help guide further exploration of the area’s geothermal system. We also performed rock-property measurements on samples from several stratigraphic sections in the region, as well as on two ~1000m cores recently drilled near Crump Geyser to constrain model properties. Our modeling efforts began with a series of intersecting 2D profile models across the area that extend through existing wells and integrate information from: recent local-scale geologic mapping, seismic and airborne magnetic surveys, well cuttings and core, borehole geophysical logs, and rock property measurements to correlate subsurface and outcrop stratigraphies. We built in mapped contacts and fault interpretations into the models, and incorporated surfaces from seismic interpretations, which provided useful subsurface control on some structures. Further control on these structures and constraints on additional structures were determined from the potential field data. Maximum horizontal gradients of magnetic (pseudogravity) and gravity (isostatic) grids were used to constrain the lateral extent of faults and contacts, and we used tilt derivative calculations to estimate the depth to magnetic source solutions, which provided initial estimates of depth-to-magnetic basement. We exported horizons from the 2D models to build 3D surfaces for import to the initial 3D model. We developed the 3D model through a series of forward manipulations, and structural and property inversion steps. Property inversions began with average rock properties and were permitted to vary within the range of measurements determined from rock property samples. In addition, we integrated results from 3D voxel-based magnetic vector inversions to aid in modeling the vector magnetization of strongly magnetic volcanic units whose magnetizations are not well constrained by outcrop rock property measurements. Combined, these methods provide better control on subsurface stratigraphy and structure to guide exploration drilling and hydrologic models. Mapping and modeling results reveal buried intra-basin structures that intersect the range front at Crump Geyser. We suggest these fault intersections, as well as the geometry of the basin, are important for promoting permeability and facilitating hydrothermal flow within the Crump Geyser geothermal system. To test this, we perform preliminary heat and groundwater flow simulations to evaluate our conceptual model of reservoir location, heat accumulation, and hydrothermal fluid delivery via structurally controlled outflow zones deduced from the potential field results. |