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A thorough understanding of the interrelationship of the geologic history, thermal maturation, and petrographic characteristics of a coal bed is necessary to interpret its present methane content. Coalification of organic matter occurs contemporaneously with burial through the interaction of temperature and pressure during geologic time. In initial stages of coalification, pressure is an important factor in volume, pore, and moisture reduction. In later stages of coalification, temperature and duration of heating are more significant. During this period, methane and other gases are generated as coalification by-products. The degree of coalification, referred to as rank or thermal maturity, is commonly measured by vitrinite reflectance. This value has been used by the Bureau of Mines t determine the rank of Permian and Pennsylvanian coal beds from several boreholes in southwestern Pennsylvania. From these values, estimates of former depths of burial and coalification temperatures may be made. Owing to the discontinuous nature of the Permian coal beds, detailed lithologic correlation of noncoal marker units was necessary to ensure that the coal bed reflectance values were placed in proper stratigraphic sequence. The thermal maturity, as indicated by vitrinite reflectance, was found to relate directly to by-product gas content.
The average vitrinite reflectance gradient of all the bore holes is 0.10%/100 m (320 ft), which corresponds to gradients measured for the Rocky Mountain Foothills of Canada. An estimate of the paleogeothermal gradient for the study area, using the Karweil nomogram to appraise the paleotemperatures of the coal beds, indicates a significantly higher gradient than that proposed earlier. The former maximum depth of burial for these coal beds, based on the estimated paleogeothermal gradient, would be 1.2-1.5 km (0.7-0.9 mi).
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