Secondary mineralogy and oxygen-isotope geochemistry of two peripheral steam-exploration boreholes at the Geysers Geothermal field, California

Conference · Sun Dec 31 04:00:00 EST 1995

OSTI ID:175663

Hulen, J B; Moore, J N ^[1]

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1. Univ. of Utah, Salt Lake City, UT (United States)

Two deep, non-commercial, steam-exploration bore-holes completed in similar Franciscan-Assemblage (late Mesozoic) metaclastic sequences peripheral to The Geysers steam field show dramatic differences in: (1) the intensity of hydrothermal alteration and vein mineralization: and (2) the corresponding extent of fluid-rock interaction revealed by whole rock oxygen-isotopic ratios. In borehole Bud Taylor No. 3, completed at a depth of 3237 m just northwest of the field, hydrothermal alteration and veining are minimal, metamorphic calcite and pumpellyite persist to the bottom of the well, and {delta}{sup 18}O values range between about +12 and +14{per_thousand} - values which are also regionally prevalent in the Franciscan Assemblage. In the 3717 m-deep borehole Tellyer 1-24, just east of the field: (1) hydrothermal alteration and veining (including K-feldspar, epidote, ferroaxinite, and tour-maline) are relatively intense below about 800 m; (2) most Franciscan calcite has apparently been removed by dissolution; and (3) 8180 values show a progressive downward decrease from about +17{per_thousand} in near-surface samples to about +5{per_thousand} at total depth. These secondary mineral and oxygen-isotopic signatures are virtually identical to those prevailing in the steam reservoir, within which downward-increasing temperature was largely responsible for the observed whole-rock {delta}{sup 18}O values. However, by contrast with rocks hosting the steam reservoir, those penetrated by Tellyer 1-24 contain hydrothermal vein calcite to nearly the deepest levels penetrated. This vein calcite may block what would otherwise be productive steam channels.

• DOE Contract Number: AC07-95ID13274
• OSTI ID: 175663
• Report Number(s): CONF-951037--
• Country of Publication: United States
• Language: English

Related Subjects

15 GEOTHERMAL ENERGY
CALIFORNIA
EXPLORATION
GEOCHEMISTRY
GEOTHERMAL FIELDS
GEOTHERMAL FLUIDS
GEOTHERMAL WELLS
HYDROTHERMAL ALTERATION
VAPOR-DOMINATED SYSTEMS