Geologic Model of the Baca Geothermal Reservoir, Vallee Caldera, New Mexico

Abstract

Logging of drill cuttings from Union Oil Company's Baca geothermal project has resulted in a detailed geologic picture of the Baca geothermal reservoir. The reservoir is located in a complex geologic environment with both fracture and stratigraphic permeability, which have been modified (largely reduced) by hydrothermal alteration. The geothermal system is principally hosted by ash-flow tuffs and volcanic last i c rocks associated with the development of the Valles and To1 edo cal deras and precedi ng expl os i ve rhyol i t i c eruptions. The cooling history of these rocks has produced non-welded tuff s, with considerable inter granular permeabi 1 i ty and extensive hydrothermal alteration, through densely welded ash-fl ow tuffs with 1 ittl e intrinsic permeabi 1 i ty. L i t h o logic units beneath the ash-fl ow tuff sequence are commonly altered and metamorphosed but cont a in few hot-water entries, although some, such as the Madera Limestone, are regional groundwater aquifers outside the caldera. Previous workers have reported that the Baca field has a permeability-thickness product of about 6,000 md-ft, which is low compared with other producing geothermal areas. A model forth e development of the Redondo Creek resurgent dome, within which the Baca field is located, explains these low permeabilities by emphasizing the nature of strain and resulting fracture permeability in the host lithologies. The model also helps emphasize the importance of inherit e d structures in the formation of the dome and suggests that these faults may be the principal conduits for geothermal fluids from depth in the system. Another imp1 i cation of model i ng dome development is t h at the magmatic heat source for the geothermal system lies beneath present drilling depths.