Oxygen Isotope Geochemistry of The Geysers Reservoir Rocks, California

Whole-rock oxygen isotopic compositions of Late Mesozoic graywacke, the dominant host rock at The Geysers, record evidence of a large liquid-dominat-ed hydrothermal system that extended beyond the limits of the present steam reservoir. The graywackes show vertical and lateral isotopic variations that resulted from gradients in temperature, permeability, and fluid compo-sition during this early liquid-dominated system. All of these effects are interpreted to have resulted from the emplacement of the granitic "felsite " intrusion 1-2 mil-lion years ago. The 6l8O values of the graywacke are strongly zoned around a northwest-southeast trending low located near the center of and similar in shape to the present steam system