| Abstract |
A digital surface geology map of The Geysers geothermal field was constructed to provide an additional surface constraint for a three-dimensional structural model being constructed by Calpine Corporation. Twenty, hand-drawn and mostly unpublished, paper maps of portions of the surface geology at The Geysers were compiled and digitized in ArcGIS. The names of the lithologic units from the individual maps, as well as the unit descriptions, were standardized across The Geysers to conform to the surface expression and near-surface well log descriptions. Prior to this compilation there was not a complete and consistent geologic map of the entire Geysers geothermal field. By assembling and unifying the various geologic maps, spatial correlations between surface geology and subsurface discontinuities became more apparent. The compilation shows the northwest Geysers area is lithologically distinct from the southeast Geysers, with the geothermal field divided by the previously described, northeast-oriented Cobb Creek fault zone. This compilation also confirmed the dominance of northwest-striking lithologic units and faults, and revealed numerous younger (Quaternary) steeply dipping faults and discontinuous shear zones striking north-northeast and northeast throughout The Geysers geothermal field. Subtle geomorphologic features, such as faceted spurs – combined with the distribution of hydrothermal alteration, and outcrop characteristics – identify the surface expression of these northeast-striking faults and shear zones. Some of these northeast-striking, steeply-dipping, trans-tensional structures are shown by others to extend into the subsurface and hydraulically compartmentalize The Geysers steam reservoir between the dominant northwest-striking faults that limit the extent of the Geysers steam reservoir. The surface expressions of two additional shear zones in the southeast Geysers, herein called the Felsite fault and the Magnitude 4 Divide shear zone, were previously identified in the subsurface by utilizing lithology logs and induced seismicity patterns. The Geysers digital compilation map with updates from additional field mapping (2014-2015) further constrains the surface boundary of Calpine Corporation’s development of a three-dimensional, structural model using Paradigm Geophysical SKUA GOCAD software (see companion paper, this workshop, Hartline et al., 2016), and allows increased confidence in the interpretation of structural features that appear to compartmentalize The Geysers steam reservoir. |