Hydrothermal factors in porosity evolution and caprock formation at the Geysers steam field, California--insight from the Geysers Coring Project

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The Department of Energy (DOE)/geothermal industry-sponsored Geysers Coring Project (GCP) has yielded 236.8 m of continuous core apparently spanning the transition between the uppermost Geysers steam reservoir and its caprock. Both zones in the corehole are developed in superficially similar, fractured, complexly veined and locally sericitized, Franciscan (late Mesozoic) graywacke-argillite sequences. However, whereas the reservoir rocks host two major fluid conduits (potential steam entries), the caprock is only sparingly permeable. This discrepancy appears to reflect principally vein texture and mineralogy. Two types of veins are common in the core--randomly-oriented, Franciscan metamorphic quartz-calcite veins; and high-angle, late Cenozoic veins deposited by ... continued below

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91-98

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Hulen, Jeffrey B. & Nielson, Dennis L. January 26, 1995.

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The Department of Energy (DOE)/geothermal industry-sponsored Geysers Coring Project (GCP) has yielded 236.8 m of continuous core apparently spanning the transition between the uppermost Geysers steam reservoir and its caprock. Both zones in the corehole are developed in superficially similar, fractured, complexly veined and locally sericitized, Franciscan (late Mesozoic) graywacke-argillite sequences. However, whereas the reservoir rocks host two major fluid conduits (potential steam entries), the caprock is only sparingly permeable. This discrepancy appears to reflect principally vein texture and mineralogy. Two types of veins are common in the core--randomly-oriented, Franciscan metamorphic quartz-calcite veins; and high-angle, late Cenozoic veins deposited by The Geysers hydrothermal system. The older veins locally contain hydrothermal carbonate-dissolution vugs, which, although concentrated at the larger fluid conduit, are scattered throughout the core. The younger veins, commonly with intercrystalline vugs, consist dominantly of euhedral quartz, calcite, K-feldspar, wairakite, and pyrite--those in the reservoir rock also contain minor epidote and illite. The corresponding caprock veins are devoid of epidote but contain abundant, late-stage, mixed-layer illite/smecite (5-18% smectite interlayers) with minor chlorite/smectite (40-45% smectite interlayers). We suggest that clots of these two expandable clays in the caprock clog otherwise permeable veins and carbonate-dissolution networks at strategic sites to produce or enhance the seal on the underlying steam reservoir. Illite/smectite geothermometry indicates that the SB-15-D caprock clays were precipitated in the approximate temperature range 180-218 C, and those in the reservoir at about 218-238 C. These temperatures, along with occurrence of the clays on commonly etched calcite, K-feldspar, or wairakite, suggest that the clays were precipitated from mildly acidic steam condensate under conditions similar to those now prevailing.

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91-98

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  • Proceedings, Twentieth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, January 24-26, 1995

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  • Report No.: SGP-TR-150-13
  • Grant Number: None
  • Office of Scientific & Technical Information Report Number: 889358
  • Archival Resource Key: ark:/67531/metadc884955

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  • January 26, 1995

Added to The UNT Digital Library

  • Sept. 21, 2016, 2:29 a.m.

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  • Nov. 29, 2016, 1:50 p.m.

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Hulen, Jeffrey B. & Nielson, Dennis L. Hydrothermal factors in porosity evolution and caprock formation at the Geysers steam field, California--insight from the Geysers Coring Project, article, January 26, 1995; United States. (digital.library.unt.edu/ark:/67531/metadc884955/: accessed July 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.