Southern Extent of The Geysers High Temperature Reservoir Based on Seismic and Geochemical Evidence

Abstract

In the northwest part of The Geysers steam field, the “normal”, 240 oC vapor dominated reservoir is underlain by a “high temperature” vapor dominated reservoir (HTR) at 260 to 360 oC. The southeastern extent of the HTR is unknown. A sharp, vertical boundary, the “M?4 Divide”, partitions The Geysers reservoir such that all but one of 26 historic M?4.0 and over 90% of M?3.0 earthquakes are located to its northwest. The same boundary coincides with an abrupt 2000 m (6600 ft) increase in the maximum depth of seismicity to the northwest. Steam enriched in volatile acid chloride, believed to originate in the HTR, is essentially restricted to the northwest of this line. Coincidence of this boundary for reservoir characteristics as disparate as earthquake magnitude distribution, earthquake depth and steam chloride chemistry may indicate that the M?4 Divide represents the southeastern boundary of the HTR. An alternative possibility, supported by the geometry of the “seismic floor”, is that a second, smaller HTR, as yet unidentified by drilling, exists immediately to the northwest of the M?4 Divide. Assuming that a younger intrusive heat source is required for HTR formation than for the normal reservoir, this may further imply that a separate young igneous intrusion underlies the area immediately to the northwest of the M?4 Divide.