FLUID INCLUSION EVIDENCE OF THE TRANSITION FROM LIQUID- TO VAPOR-DOMINATED RESERVOIR CONDITIONS IN THE NORTHWEST GEYSERS GEOTHERMAL FIELD, CALIFORNIA

The Geysers geothermal field is located 150 km north of San Francisco, California. It is the largest geothermal field in the world, generating 720 MWe of electricity, and one of the few that produce only dry steam. The geothermal system is centered on a large (>300 km³), young (1.1 to 1.8 Ma), shallow (≥700 m) intrusive complex, with concentric zones of contact metamorphism and hydrothermal alteration in the adjacent wall rocks. Two vapor-dominated reservoirs have been recognized in the northwest portion of the field; an upper reservoir with temperatures of ~240^oC, and a deeper, hotter reservoir with measured temperatures as high as 400^oC. The high-temperature reservoir is developed in Franciscan greywacke contact metamorphosed to biotite hornfels.

Veins cutting the biotite hornfels record high-temperature, low-pressure contact metamorphism, an earlier liquid-dominated hydrothermal system and the transition from a liquid-dominated to the current vapor-dominated regime. The veins contain early biotite (± clinopyroxene, amphibole, tourmaline and apatite); this assemblage is overprinted by quartz (± actinolite and plagioclase) and late stage Fe>Zn-Cu sulfide minerals (± chlorite and quartz). Fluid inclusions have been studied in core samples from two wells in the northwest field located ~3.5 km apart, Prati-5 St-1 and L’Esperance-2. Both saline liquid-rich fluid inclusions containing up to 14 solid phases at room temperature, and vapor-rich inclusions are common in vein quartz. These fluid inclusion assemblages are interpreted to record the boiling off of the early liquid-dominated system. Energy dispersive spectroscopy analyses of the residues from decrepitated inclusions indicate the liquids were enriched in Cl, Na, Ca, Fe, K, F and potentially Mn, C, B, S and P. Modern measured temperatures fall within the range of total fluid inclusion homogenization temperatures (T_h(tot)). In Prati-5, the maximum measured temperature is 314^oC and T_h(tot) range from 298 to 386^oC. In L’Esperance-2, the maximum measured temperature is 296^oC and T_h(tot) range from 255 to 353^oC. These temperature relationships may indicate that the transition from liquid-dominated to vapor-dominated conditions is relatively recent, and/or that temperatures in the Northwest Geysers have been relatively stable since quartz deposition.