Gas Geothermometry for Drillhole Fluids From Vapor Dominated and Hot Water Geothermal Fields

Abstract

Little information has been published on the hydrothermal alteration minerals occurring at depth in the Geysers steam field , California. Steiner (1958) reported the occurrence of wairakite from a well; McNitt (1964) identified pyrite , sericite , calcite , quartz, siderite , apatite and chlorite in cores of Franciscan graywacke and greenstone. Recently, Union Oil Geothermal Division furnished a set of well cores from the cap rock overlying the steam reservoir for geophysical studies (Lockner -- e t a l . , 1980); sample localities are shown in Figure 1. Cores of metagraywacke and greenstone from 4 wells---Thermal 10 (TH10) , Lakoma Fame 6 (LF6), D & V 1 (DV1) and Horner State (HS1)---were compared to unaltered Franciscan metagraywacke (GP5) from surface exposures (Table 1). Several previously unreported alteration minerals were found in the cored rocks, including epidote, tremolite - actinolite , prehnite and tourmaline. This note describes the observed alteration minerals and some of the factors that controlled their growth.