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abstract

Clay-mineralogy, mean random vitrinite reflectance (Rm), and fluid inclusion homogenization temperature (Th) from host-rock and vein samples in cored intervals from 5,000–18,000 ft (1500–5500 m) indicate that paleotemperature was higher than the temperature currently measured in siliciclastic rocks of the Pinedale anticline, Green River basin, Wyoming. The cored intervals are from lower Tertiary and Upper Cretaceous rocks in the El Paso Natural Gas Wagon Wheel no. 1 well.

Compositional analyses of mixed-layer illite/smectite (I/S) clay from sandstone and shale in the Wagon Wheel core show a progressive increase in ordering and the number of illite layers with depth (and temperature). Temperatures determined from changes in the composition and ordering of I/S, and Rm data, imply that the rocks of Wagon Wheel reached about 200°C at 18,000 ft (5500 m). Uncorrected log temperature at this depth is about 135°–150°C. The thermal gradient calculated at maximum burial temperature (25°C/ km), however, is similar to the thermal gradient established by present-day borehole temperature (24°C/km). These data suggest that maximum temperature was 30°–50°C higher than present-day uncorrected borehole temperature. The apparent temperature decrease can be explained by erosion of about 5,600 ft (1700 m) of section, as calculated from a surface Rm intercept of 0.33%, with a geothermal gradient that has remained constant since maximum burial. Major uplift of the Pinedale anticline occurred after apparent maximum burial and temperature were established in the Neogene.

Th of 130°–150°C from aqueous fluid inclusions in quartz within calcite and quartz veins at 17,000 ft (5200 m), however, conform to the present temperature regime. Fractures probably formed during uplift of the Pinedale anticline and were later mineralized in a temperature regime much like that of the present.

Primary, hydrocarbon-bearing fluid inclusions in veins from 8,000–17,000 ft (2,400–5200 m) are evidence for petroleum migration occurring during the filling of these late fractures. Secondary oil fluid inclusions, trapped in healed mierofractures that crosscut authigenic quartz in these veins at a depth of 17,000 ft (5200 m), indicate that petroleum migration continued sometime after these deep fractures were mineralized.

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