STRATIGRAPHIC RELATIONSHIPS IN MESOZOIC BASEMENT ROCKS AT THE DESERT PEAK EAST EGS AREA, NEVADA

摘要

Several plutonic units within Mesozoic basement rocks of the east Desert Peak area are potential EGS reservoirs. Sill-like Jurassic diorites and Cretaceous granodiorite intrusives occur at temperatures exceeding 400℉ (204℃) within the basement. Geologic studies are being conducted to determine the lateral and vertical distribution of the basement lithologies, the nature of contacts (depositional, fault, or intrusive) with surrounding schistose metasedimentary rocks, and the character and mineralogy of natural fractures and veins in the rocks to identify suitable targets for hydrofracturing and stimulation in subsequent phases of this enhanced geothermal system (EGS) project. Dioritic intrusives in the Jurassic section are tentatively correlated to the Humboldt mafic complex; a sequence of gabbroic to intermediate composition intrusives and comagmatic basaltic volcanics that are closely associated with shallow marine metasediments (quartz arenite of the Boyer Ranch Formation; gypsum and carbonate of the Lovelock Formation), and which host the high-temperature Dixie Valley geothermal system 75 km to the east. In the Desert Peak geothermal area, a thick (100 m) quartz monzodiorite sill within the Jurassic section can be correlated across the EGS project site (drillholes DP 23-1 and 35-13 TCH) and to outcrops near the Desert Queen mine (about 6 km NE of the EGS area). However, complicated structural and stratigraphic relationships with surrounding Jurassic intrusive and metasedimentary units and the presence of both Cretaceous and Miocene dikes in the monzodiorite suggest variable physical and mechanical properties and possible unpredictable behavior as a result of well stimulation procedures. A better lithologic target for future wellbore stimulation may be the Cretaceous-aged granodiorite that intrudes the Jurassic section. The homogeneous Cretaceous granodiorite is a more massive intrusive body (at least 750 m thick) and below a sericitized roof, is generally weakly altered with only a few discrete fracture/fault zones.