Low-maturity oil is present in the Kyle Hot Springs and Dixie Valley geothermal systems in the Basin and Range of western Nevada. The Kyle oil is exclusive-ly free-flowing (500 barrels produced accidentally
with warm water at 80?C). The oil at Dixie occurs with produced geothermal water (initially 250?C) in a wellhead "bleed", and in calcite-aragonite-saponite scales deposited on hangdown strings in the upper 1500 m of at least four production wells. Organic
geochemical analysis of the oils from both sites reveals that they were derived from Tertiary lacustrine hydrocarbon source rocks which are regionally sub-mature -- incapable of generating liquid hydrocarbon
unless hydrothermally heated. Biomarker transfor-mation ratios of the oils indicate that they were (1) generated early in the "oil window" at temperatures near 100?C; and (2) not appreciably affected there-after
even by the elevated temperatures prevalent in the Dixie Valley system. The Dixie oils had to have entered the wellbores below casing at about 2600 m
depth, so their preservation at reservoir temperature can only be explained by brief residence time in the hydrothermal system. Preliminary thermal modeling with optimally favorable geologic and kinetic para-meters
indicates an absolute maximum of 5000 yr. A more realistic appraisal suggests that the oils may have been drawn in by a production-induced "pres-sure sink" in a matter of months to years. In support of this contention is the magnesium-rich saponite associated with the scale-hosted oils. Magnesium is nearly absent from the natural geothermal fluid, but
abundant in surrounding cooler basinal fluids, with which the oil may have gained access to the heart of the geothermal system. These findings are prelimi-nary. Should they withstand further scrutiny, howev-er,
they could provide powerful new contraints on the Dixie Valley systemís hydrothermal history as well as its production-influenced reservoir performance.

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