Two adjacent DOE multiwell experiment (MWX) boreholes were drilled to a depth of 2,520 m (8,268 ft) below the valley of the Colorado River near Rifle, Colorado. High quality vitrinite reflectance data ranging from 0.88 to 2.07% were obtained from cores from the Upper Cretaceous coaly interval (1,340 to 2,423 m; 4,396 to 7,949 ft). Remnants of basalt flows that cap highlands above the valley are 9 m.y. old, and 1,500 m (4,921 ft) of Eocene and Pliocene sediments have apparently been eroded from above the site by the Colorado River since the basalt was extruded. The excellent reflectance data and the geologic setting allow a test of three ways to evaluate maturation of vitrinite.

Figure

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Logarithms of the reflectance values form a linear trend with depth. Extrapolation of this trend to 0.20% R_o, a value thought by some to mark the normal surface intercept, reaches only 460 m (1,509 ft) above present ground, instead of the 1,500 m (4,921 ft) predicted on the basis of postulated erosion. Such a deep intercept would indicate that maximum temperatures were attained much more recently than the volcanism (and subsequent to removal of 1,040 m [3,412 ft] of sediment). If the plot is extended up to the +1,500 m (+4,921 ft) pre-erosion level, the intercept is only 0.08% R_o, leading to an unlikely paleotemperature situation.

Time-independent models of the general dependence of reflectance on temperature yield a paleotemperature gradient of about 115°C/km and surface intercepts (20°C) of -350 to +250 m (-1,148 to +820 ft) relative to the present ground level. These models require that a strong heating occurred at the site at nearly the present time, and are not in accord with available facts.

Time-dependent models give a paleotemperature gradient of 50° to 65°C/km in the sampled interval (41 to 54°C/km in the overlying non-coaly section). Intercepts of 20°C are +1,300 to +1,700 m (+4,265 to +5,577 ft) above the present ground. These models agree logically with the present 46°C/km gradient across the sampled interval and the removal of 1,500 m (4,921 ft) of overburden, with maximum temperature just a bit after maximum burial. Of the three approaches tried, the time-dependent model is the only one which works in this situation.

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