Abstract

Coal rank is used to measure the degree of organic metamorphism, which is of great importance in the evaluation of the hydrocarbon potential of a region.The rank-is determined on true coal seams and coaly inclusions in sandstones by measuring the reflectance of vitrinite, which is a major constituent of coal. A large number of samples, encompassing nearly the entire area of Upper Paleozoic deposition in the Atlantic region, was examined. From these an isoreflectance map has been constructed. The map shows a distinct pattern of regional metamorphism that resembles the tectonic development of the region. The known oil and gas occurrences coincide with the areas of lowest rank.In this region the coalification is predominantly postdeformational, being caused by the maximum depth of burial that existed after folding. In the higher-rank coals the increase in rank with depth can be measured accurately by the reflectance (R0). In these coals different R0-depth factors were obtained in different areas, which can be related to different geothermal gradients.In the lower-rank coals (below 62 % fixed carbon) the reflectance parameter is useful only for broad rank assignments, and precise rank predictions at depth cannot be made from surface observations. However, on suitable borehole samples the rank can be measured with the reflectance. This has been done on eight exploration wells and has resulted in finding the approximate vertical position of the oil "deadline" with regards to the degree of organic metamorphism.Factors limiting the selection of promising areas for future exploration are discussed, with reference to the vertical and regional changes in rank that have been observed.

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