Geology And A Working Conceptual Model Of The Obsidian Butte (Unit 6) Sector Of The Salton Sea Geothermal Field, California

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Conference Paper: Geology And A Working Conceptual Model Of The Obsidian Butte (Unit 6) Sector Of The Salton Sea Geothermal Field, California

Abstract
A working conceptual model has been developed for the southwestern portion of the Salton Sea geothermal system, the region encompasing CalEnergy Operating Company's imnent 'Unit 6' field expansion (185 megawatts). The model is based on examination and analysis of several thousand borehole rock samples combined with a wealth of subsurface information made available for the first time from the databases of present and prior field operators. The Unit 6 sector of the system is hosted by fluvial and lacustrine, si~ciclastisce diments and sediment^ rocks of qua tern^ age. These strata are gently folded and cut by high angle fault zones with a component of strike-slip displacement. The thicker of these zones (1) are mineralized and enriched in gouge and crush breccia (both commonly slickensided) as well as dilational microbreccia with a 'jigsaw-puzzle' texture; and (2) are hosts for the most productive thermal-fluid conduits yet encountered in this part of the field. Much of this production is derived from major faults apparently forming the upper portion of a 'negative flower s ~ c ~ r ea 'c ,o ~ ofneat ure of ~anstensionawl rench-fault regimes like the one in which the field is situated. A unique, - 100-200 m-thick, evaporitic anhyd~te-rich layer in the mudstone capping the sedimentary sequence is continuous except above the faults most productive at depth. We believe that only these faults penetrate significantly upward into the cap, providing ingress for cooler, sulfate dissolving waters from above. Unit 6 as drilled to date shares numerous attributes with the broader Salton Sea geothermal resource. The production fluids are hypersaline brines (total dissolved solids content 2025%) circulating at temperatures generally in excess of 290°C. Porosity and Permeability for fluid flow and storage are provided primarily by fractures, breccias, and veinlets, but also, in the upper part of the reservoir (and in a supra-reservoir outflow plume), by porous sandstones in which calcite has been hydrothermally dissolved. Overlying strata have not only retained their calcite, but have been mineralized locally with intergranular anhydrite, therefore providing an effective reservoir cap. In addition to a paucity or absence of calcite, the following hydrothermal features are closely correlated with the Unit 6 geothermal reservoir: (1) pervasive veinlets of various compositions; and (2) widesp~adan d c o ~ o n l ayb undant epidote, a ~ o m p ~ e d locally at deeper levels by actinolite and clinopyroxene. The most prolific therm~-fluidch annels coincide with fault con~o~ed concentrations of veinlets and dilational breccias mineralized with pos t-calc-silicate specular hematite & anhydrite. The foregoing observations and deductions are consistent with a conceptual geothermal reservoir centered above a still-cooling granitic pluton at least 2 km in diameter and -3.5 km below the modern ground level. Major zones of buoyantly upwelling hot brine above the intrusion are focused along faults. More diffuse upflow occurs in a stockwork of interconnected, mineralized fractures (veinlets). This stockwork probably formed by hydraulic rock rupture induced by explosion of isolated, fluid-filled pores heated and consequently overpressured at an expanding (prograde) thermal front emanating from the magmatic heat source. Subhorizontal stratigraphic permeability in this model is concentrated in the upper portion of the reservoir, where the balance between carbonate dissolution and calc-silicate mineralization has favored formation of sandstone aquifers. Local down~owa nd w a ~ i n g of initially cooler brine from below the cap along major faults leads to slight (5-10°C) cooling of the upflow concomitant with open-space hematite f: anhydrite mineralization. The Unit 6 geothermal reservoir is clearly open at depth and for at least a kilometer to the northwest. It extends to the southwest and (especially) to the northeast for considerably greater distances. The reservoir plunges abruptly to the southeast, but even here, by analogy with the northern part of the geothermal field, there is a high probability for encountering productive reservoir rock at depths below 2 km. Our conceptual model, coupled with documented reservoir behavior here and elsewhere in the field, strongly suggests that the immediate resource is more than sufficient to support the planned expansion.

Authors 
Jeff Hulen, Denis Norton, Dennis Kaspereit, Larry Murray, Todd van de Putte and Melinda Wright








Published 
GRC, 2003





DOI 
Not Provided
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Citation

Jeff Hulen,Denis Norton,Dennis Kaspereit,Larry Murray,Todd van de Putte,Melinda Wright. 2003. Geology And A Working Conceptual Model Of The Obsidian Butte (Unit 6) Sector Of The Salton Sea Geothermal Field, California. In: (!) ; (!) ; (!) . (!) : GRC; p. (!)