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Coal, carbonaceous shale, and siltstone have been tentatively identified as some of the more important source rocks of gas in low-permeability gas reservoirs in the greater Green River basin of Wyoming and Colorado. An attempt to quantify the amount of gas presently contained in these source rocks was initiated during coring of the Rainbow Resources-Pacific Creek Federal 1-3 well, located north of the Rock Springs uplift in the northeastern Green River basin. The direct gas desorption method developed by the U.S. Bureau of Mines was used. Three samples of carbonaceous shale and siltstone from a depth of 13,450 to 13,500 ft (4,099 to 4,115 m) in the Upper Cretaceous Rock Springs Formation were collected and desorbed. The gas content of these samples ranges from 23.0 to 74. cu ft/ton (0.65 to 2.1 cu m/T). The weight percent of
total organic carbon ranges from 0.57 to 7.81 and the vitrinite reflectance is 1.2 Ro.
Although no coal was cored, a gas estimate ranging from 1.0 to 1.5 bcf/sq mi (0.07 to 0.11 billion cu m/sq km) was calculated using a graphic solution based on existing coal desorption data from other areas. Coal beds were identified from geophysical logs in the stratigraphic interval from the Tertiary Fort Union Formation through the Upper Cretaceous Mesaverde Group; coal rank was determined from a vitrinite reflectance profile in the Pacific Creek area. The aggregate thickness of coal in this interval is 200 ft (61 m), and the coal rank ranges from high volatile C bituminous to medium volatile bituminous.
The results obtained in this limited study indicate that the direct method of gas desorption works at great depths and provides what appears to be fairly reliable gas estimates. Calculations of the amount of gas contained in coal beds show that a very large volume of gas has been generated.
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