The Orientation of Steam-Bearing Fractures at The Geysers Geothermal Field

Abstract

sThe Geysers steam reservoir exists primarily within Mesozoic Franciscan graywacke and an underlying 2.4 - 0.9 Ma silicic bathollith. All steam from both reservoir rock types is produced from a highly permeable and interconnected set of fractures. The top of the reservoir has a roughly antiformal shape, with a northwest trending axis that coincides with the axis of the elongate batholiths. Different fracture patterns exist in the graywacke and intrusive rocks. The orientation of steam bearing fractures within graywacke is generally random, but includes extensive low angel fracture zones. We infer from this that many of the fractures represent reopened Franciscan age structures. The distribution of these steam bearing fractures in graywacke leads to laterally extensive zones of high productivity. In contrast, high productivity in the intrusive rocks is found in a arrow, steeply dipping zones. This pattern reflects the predominately high angle fractures in the intrusive rocks that are related to recent strike slip tectonics.