THERMAL CONSTRAINTS ON THE LATERAL EXTENT OF THE GEYSERS VAPOR-DOMINATED RESERVOIR

Recent well abandonment activities in the Unit 15 area of The Geysers geothermal field have provided a unique opportunity to investigate equilibrium thermal conditions to depths as great as 1.7 km near the southwestern boundary of the system. The southwestem boundary of The Geysers is characterized by a decrease in surface heat flow over a distance of less than 1 km from more than 400 mW/m2 to less than 250 mW/m2 outside the field. This boundary runs parallel to a series of northwest-trending faults which juxtapose various lithologic elements of the Franciscan complex. The new thermal data reveal a complex relationship between variations in near-surface ( < 200 m) heat flow, deep (> I km) heat flow, and the top of the vapor-dominated reservoir (> 1.5 km). The southwestern boundary of the reservoir at depth is characterized by an abrupt change in heat flow, and this change appears to correlate with the subsurface projection of mapped fault zones. Preliminary analyses suggest that advective heat transport along the fault zones may be significant, with heat flow in the units below these faults approximately equal to the surface heat flow found outside the reservoir. This lower value of heat flow presumably reflects the deep conductive heat source underlying The Geysers, whereas the higher value of heat flow above the reservoir reflects convective heat transfer within the vapor-dominated system. Modeling of the variation of heat flow with depth should yield detailed constraints on the geometry of this boundary