The thermal maturity of organic matter in the Green River Formation in the Piceance Creek basin was determined by vitrinite reflectance on coalified logs in the otherwise alginite-rich oil shale, marlstone, and sandstone. Only vitrinite from logs in sandstone and marlstone was used to determine thermal maturity because reflectance of vitrinite from alginite-rich oil shale generally is lower than that in associated other rock types. Mean random vitrinite reflectance (R_o) at the top of the Green River Formation ranges from about 0.30% around the perimeter of the basin, where maximum burial depth of the rocks was less than 1,000 m (3,300 ft), to 0.55% in the structurally lowest part of the basin, where maximum burial depth of the upper part of the Green River was ore than 1,500 m (4,900 ft). The Green River Formation is almost 1,200 m (3,900 ft) thick in the structurally lowest part of the basin, suggesting that the lower part of the formation in this area may have reached an R_o of 0.7%, generally accepted as the threshold for oil generation in alginitic rocks. Bitumin-filled fractures observed in core from this area of the basin support this conclusion. A lithologically similar lacustrine section of the Green River Formation in the adjacent Uinta basin, where maximum burial was as great as 5,600 m (18,400 ft), is producing large quantities of oil from overpressured, fracture-controlled reservoirs. Present-day maximum temperatures in the Green River Formation in the Piceance Creek basin are between 55 and 70°C (131 and 158°F) This temperature seems too low

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for hydrocarbon generation. However, temperatures in the past probably were high enough for hydrocarbon generation. Oil generated during this earlier, hotter period could have migrated into conventional stratigraphic and structural traps.

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