Play Fairway Analysis of the Eastern Great Basin Extensional Regime, Utah: Preliminary Indications

摘要

We are assessing the geothermal potential including possible blind systems of the Eastern Great Basin extensional regime of western Utah through a Play Fairway Analysis (PFA) of existing geoscientific data. A PFA working model is adopted where magnetotelluric (MT) low resistivity upwellings suggesting geothermal fluids may coincide with dilatent geological structural settings and observed thermal fluids with deep high-temperature contributions. Unique to this setting is the superposition of active rifting with a N-S strike over E-W oriented belts of mid-Cenozoic plutonic rocks and possible NE-SW Precambrian shear trends. Data from ～470 high-quality MT sites have been undergoing 3D inversion using a new edge finite element formulation. Inversions show that several low resistivity upwellings project in the vicinity of Crater Bench, central Sevier Desert, Cove Fort, and east flank of the northern Mineral Mountains. The former three have known thermal expressions but the latter has no such association although it is along strike to the north of the Crater Knoll-Red Knoll Quaternary eruptions. There is a strong suggestion of control of resistivity structure by ENE-oriented structural trends possibly reactivated in E-W rifting. Reanalysis of seismicity shows that both swarm and non-swarm earthquake clusters locate in close proximity to the low resistivity upwellings near Cove Fort and the northern Mineral Mountains, while there is no seismic signature near the upwelling in the central Sevier Desert. Structural geology analysis based upon compilation of mapping, gravity and high-resolution (5 m often) DEM data has identified thirty-five areas of interest within and near the project fairway representing geometries that can produce critical stresses for dilatency, some of which overlap with MT structures. Major element geochemistry of thermal waters is dominated by chloride and sulphate compositions. Silica geothermometry values correspond well with direct reservoir temperatures, but the somewhat higher Na-K inferred temperatures may reflect deeper basement equilibration. Produced fluid compositions often are not compatible solely with the reservoir rocks of production, and imply substantially larger rock volumes are contributing. Also correlated with reservoir temperature are ~3He (R/Ra) values, indicating that magmatic input is associated with convective flow. To date, results appear consistent with our PFA working model described above.