The Geysers Coring Project, Sonoma County, California, USA - Summary and Initial Results

Abstract

The Geysers Coring Project (GCP), a US. Department of Energy (DOE) - geothermal industry joint venture, has yielded the first substantial length (236.8 m) of continuous core from The Geysers vapor-dominated geothermal reservoir. The new corehole, a sidetrack from an existing steam-production well, penetrated a fractured and veined, weakly metamorphosed, Franciscan (late Mesozoic) sequence of turbiditic origin. This sequence is disrupted in the corehole by two throughgoing fluid conduits which likely would have been steam entries rather than fluid-loss zones had the corehole been air-drilled. The conduits coincide with superimposed concentrations of two types of veinlets common throughout the core: (1) randomly-riented, Franciscan metamorphic quartz-calcite veinlets; and (2) high-angle, late Cenozoic, Geysers hydrothermal veinlets consisting principally of euhedral quartz, bladed calcite, pyrite, and (below a depth of 435.8 m) epidote. This mineral assemblage and its texture indicate precipitation at a minimum temperature of 220?C from at least intermittently boiling water. Locally corroded calcite blades coated with sericite (illite or mixed-layer tite) may record invasion of the veins by late-stage, acidic steam-condensate. vugs in Franciscan veins and intercrystalline vugs in hydrothermal veins are locally prominent, particularly in and around the larger of the two fluid-loss zones. The hydrothermal veins are fragile and commonly refractured along strike. However, many of these fractures and others which are unmineralized appear to be artifacts related to mechanical or thermal stresses induced or relieved by the drilling process. Detailed studies underway on the core, including two intervals retrieved and frozen under pressure to preserve contained fluids, will provide Geysers steam-field operators with new information for refined reservoir modeling. The GCP has demonstrated the value of wireline coring as an exploration tool in high-temperature vapor-dominated geothermal regimes.