| Title | Comparison of Reservoir and Caprock Core from The Northwest Geysers Steam Field, California--Implications for Development of Reservoir Porosity |
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| Authors | Hulen, Jeffrey B.; Walters, Mark A.; Nielson, Dennis L. |
| Year | 1991 |
| Conference | Geothermal Resources Council Transactions |
| Keywords | Drilling; Reservoir Engineering; USA; California; Sonoma; Geysers; Hydrothermal Alteration; Lithology; Mineralization; Logs; Prati 5; Prati 12; Photomicrographs; Steam Entry |
| Abstract | Representative drill cores from the Northwest Geysers steam reservoir and its caprock provide clues to the evolution of reservoir porosity as well as fluid storage mechanism. In the reservoir core, Franciscan (Late Mesozoic) vein calcite has been hydrothermally dissolved or replaced with epidote, actinolite, and other calc silicates deposited in a high temperature, liquid dominated hydrothermal system antedating the modern steam field. This mineralization produced texturally intricate calc silicate networks and vug lining with intrinsically high surface areas favorable for absorption and effective storage of the current steam field's liquid water reserves. In the tight caprock core, Franciscan calcite has remained stable, calc silicates are sparse, and potential porosity in young bladed calcite veins has been sealed extensively with hydrothermal sericite and wairakite. |