Geysers Geothermal Area

At the latitude of the Clear Lake volcanic field, the San Andreas transform itself is aseismic, with strain instead distributed diffusely across an adjacent, subparallel belt up to 75 km wide along the North American plate edge. Although dominantly a compressional regime, the belt is locally characterized by extensional tectonism. Such an extensional zone is believed to have focused Clear Lake magmatism and accompanying high heat flow. Only a fraction of the magma — about 100 km3 — has been erupted, with most crystallizing as large plutons, including The Geysers felsite.

Host rocks for the felsite and much of The Geysers steam reservoir are tectonically interleaved metaturbidites, metabasalts, metacherts, and serpentinites of the subduction-related, late Mesozoic Franciscan Assemblage. The bulk of the steam reservoir in the lithocap (the entire volume of rock above a pluton, not to be confused with the caprock) of the felsite occurs in brittle metagraywacke and argillite with subordinate metabasalt, variously metamorphosed to greenschist and blueschist grades. The metagraywackes have negligible unfractured permeability, on the order of a few nanodarcies (10−22 m2).

The vapor-dominated system is a two-tiered feature. The so-called “normal” reservoir constituting most of the system had pre-exploitation temperature and pressure of about 240°C and 35 bars. In the northern part of the field, a deep, “high-temperature” (commonly up to 300°C, and reaching 342°C) reservoir underlies and is in apparent pressure communication with the normal reservoir regime. At least the normal reservoir can be represented as a “heat-pipe”, wherein (1) steam is the pressure-controlling phase in intercommunicating fractures; (2) liquid water is tightly held, in part by adsorption and capillary forces, in microfractures of the intervening matrix blocks; and (3) heat is transferred upward by steam while down-flowing steam condensate returns fluid mass to deeper levels of the reservoir."^[6]