ABSTRACT: Timing and Temperature of Petroleum Entrapment in the Grant Canyon Oilfield, Nevada

Jeffrey B. Hulen, S. Robert Bereskin, Michele Lemeiux

Vein minerals in dolomite breccia record the temperature, relative timing, and geochemical environment of petroleum entrapment in the prolific Grant Canyon oilfield. Early "saddle" dolomite and late-stage quartz host two- and three-phase (liquid water and/or oil plus vapor) fluid inclusions. Primary oil-bearing inclusions, however, are confined to the quartz, suggesting that initial petroleum entrapment followed saddle dolomite deposition but accompanied subsequent quartz veining. Primary inclusion with a wide range of oil/water ratios are commonly found in individual growth zones of single quartz crystals, indicating entrapment from a dispersion of oil droplets in aqueous solution.

All the Grant Canyon fluid inclusions observed to date homogenize to the liquid phase at relatively low temperatures: aqueous inclusions in saddle dolomite at 112.7-132.7°C; primary oil, water, and oil/water inclusions in quartz at 103.8-120.9°C; and secondary inclusions with oil and water in various proportions at 90.4-123.9°C. Ice-melting temperatures reveal that aqueous fluids associated with petroleum in both primary and secondary inclusions are relatively dilute (<2.2 wt% equiv. NaCl).

Current reservoir conditions at Grant Canyon are similar to those implied by the fluid-inclusion systematics summarized above: dilute thermal waters from reservoir depths reach temperatures of 121°C (up to 171°C in the nearby--1.4 km--Bacon Flat field). We suggest that at both fields, oil migration, entrapment, and perhaps maturation may be intimately related to development of a dilute, moderate- to high-temperature, meteoric-hydrothermal system.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990