This paper integrates existing and new geologic information to develop a structural model of the Desert Peak geothermal system.

The major structural features of the field include: 1) a horst block composed of pre-Tertiary basement rocks that host the geothermal fluids; 2) vertical offsets of up to 1500 ft on the northern and southern faults bounding the horst; 3) Jurassic metamorphic rocks in the horst rock directly overlain by silicified Tertiary tuffaceous rocks; 4) a thick package of pre-Tertiary weakly metamorphosed mudstones that overlies Jurassic rocks in non-productive, downdropped blocks north and east of the reservoir; 5) a Cretaceous granitic pluton northeast of the field, but not beneath the main production area; 6) a thick sequence of Tertiary basalt flows; and 7) the development of calcite veins and chlorite-bearing fault gouge along reactivated Mesozoic-age thrust faults separating major stratigraphic units in the basement rocks.

Well 27-15, the well proposed for hydraulic stimulation and creation of an enhanced geothermal system, is located north of the horst block in downthrown basement rocks. The proposed stimulation interval in this well is composed of complexly interstratified fault splices containing Jurassic-age diorites, Triassic phyllites, and hornfels that have undergone hydrothermal alteration and contact metamorphism. These rocks contain tourmaline, amphibole, and biotite that were deposited by high temperature magmatic fluids (>325oC). Overlying pre-Tertiary mudstones in the upper part of the basement sequence, in contrast, contain only low to moderate temperature clay minerals (

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