The Coso Geothermal Field is a large, high temperature system located in eastern California on the western edge of the Basin and Range province. The East Flank of this field is currently under study as a DOE-funded Enhanced Geothermal Systems (EGS) project. The work described here was undertaken to evaluate the geology and thermal history of the East Flank, in order to better understand how the rocks will behave during hydraulic and thermal stimulation. The petrology and mineralogy of East Flank injection wells 34A-9 and 34-9RD2 were studied. Well 34A-9 is the hottest well at depth in the field, reaching nearly 350oC.
Most of the wells in the East Flank are dominated lithologically by diorite and granodiorite. The two study wells contain a substantial interval of granite. The granites tend to be much less altered and veined than the diorite. The granodiorite is intermediate in veining and alteration between the two other rock types. The granite at Coso ranges in age between 89 to 53 Ma. Thus, there has been abundant time for alteration to occur. The presence of only weak alteration in the granites suggests that they are less fractured and therefore less permeable than the other rock types. Thus, the diorite is possibly a better stimulation target than the granite.

The veining is most commonly calcite, quartz, chlorite, and hematite. Euhedral quartz is especially prevalent in these two wells compared to other wells on the East Flank. In the deepest samples from well 34A-9, veins of epidote, chlorite, quartz, adularia, sericite, and late wairakite are found. A general mineral paragenetic sequence has been developed for the East Flank. The mineral assemblages suggest 34A-9 was drilled through the caprock and into the uppermost part of the reservoir rocks of a relict geothermal system.
The temperatures indicated by the clay minerals are much lower than would be predicted from the present measured downhole temperatures, but are in good agreement with fluid inclusion temperatures. These relationships imply the system is currently being reheated.

Lost circulation zones tend to correlate with zones of relatively substantial calcite veining. Fluid inclusion measurements indicate some of the calcite was deposited by old, high salinity fluids not related to the current geothermal system. Other inclusions contain low-salinity fluids that could be related to the current geothermal system. Petrographic relationships demonstrate that some of these fracture zones have been episodically reactivated and that vein porosities have been affected by younger fluids. In many samples, dissolution of early calcite is indicated by the presence of solid inclusions of early calcite contained in later, coarse-grained calcite.

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