TRACING FLUID SOURCES IN THE COSO GEOTHERMAL SYSTEM USING FLUID-INCLUSION GAS CHEMISTRY
Conference Proceedings: TRACING FLUID SOURCES IN THE COSO GEOTHERMAL SYSTEM USING FLUID-INCLUSION GAS CHEMISTRY
AbstractVein and alteration assemblages from eight Coso wells have been collected and their fluid-inclusion gases analyzed by quadrupole mass spectrometry. Four major types of alteration were sampled: 1) young calcite-hematite-pyrite veins; 2) wairakite or epidote veins and alteration that are spatially associated with deep reservoirs in the main field and eastern wells; 3) older sericite and pyrite wallrock alteration; and 4) stilbite-calcite veins that are common in cooler or marginal portions of the geothermal area. The gas compositions of the fluid inclusions display systematic differences among the secondary assemblages. The highest N2/Ar ratios are found in some calcite vein samples (N2/Ar up to 5173) and in epidote assemblages (N2/Ar ratios of 1000 to 1427). These high N2/Ar ratios suggest the presence of a magmatic component in the fluids that produced these assemblages. Sericite-pyrite altered samples, and stilbite and calcite-hematite veins have fluid-inclusion gas compositions with N2/Ar ratios less than 100, indicating that these assemblages formed from meteoric fluids. High CO2 and low H2O contents (less than 98 mol %) of inclusions in wairakite and sericite-bearing assemblages suggest deposition from boiling fluids. Low CO2/CH4 ratios of inclusions in stilbite veins indicate formation from crustal fluids. The gas chemistry reflects mixing between two endmember fluids; one enriched in magmatic volatiles, and a second represented by meteoric fluids. With the exception of the epidote assemblage, the fluid-inclusion gases plot in the basaltic field on a He-N2-Ar diagram. This signature is similar to the present-day gas analyses from steam samples taken from both the Devil’s Kitchen fumarole area and from Coso production wells.
- Lutz, S.J.; Moore, J.N.; Adams, M.C.; Norman and D.I.
- PROCEEDINGS, Twenty-fourth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 25-27, 1999, 1/1/1999
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Lutz, S.J.; Moore, J.N.; Adams, M.C.; Norman, D.I.. 1/1/1999. TRACING FLUID SOURCES IN THE COSO GEOTHERMAL SYSTEM USING FLUID-INCLUSION GAS CHEMISTRY. Proceedings of (!) ; (!) : PROCEEDINGS, Twenty-fourth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 25-27, 1999.