Small amounts of methane were formed in the Devonian black shales of the Appalachian basin during early, low-temperature stages of diagenesis, but most of the natural gas was generated by thermochemical conversion of solid and liquid organic matter during later, higher temperature stages. At any given locality, the amount of methane generated in the Devonian shales was determined by the amount of organic matter originally present and the extent of the transformation process; transformation was determined by the maximum depth of burial and subsurface temperature to which the rock was subjected. The transformation process was halted in its early stages in rocks of the western part of the basin, but approached completion in the east. The degree of transformation is indicated by systematic, west-to-east changes in the geochemistry of gas (^dgr¹³C of methane changes from -55 to -25^pmil), in the extractable organic matter (saturated hydrocarbons evolve from an immature to an incipiently metamorphosed assemblage), and in the solid organic matter (atomic H/C changes from 1.1 to 0.4).

The hydrocarbon geochemistry of oils derived from Devonian shales also changes systematically. On the basis of correlations with Devonian source rocks, oils on the western margin of the basin in the Mississippian Berea Sandstone must have been generated in and migrated from shales located about 100 km to the east. In contrast, the easternmost oil occurrences in lenticular sandstones were products of very local migration from adjacent shales. In the eastern part of the basin, the advanced stage of thermal maturity of both oils and extractable hydrocarbons in adjacent source rocks suggests that hydrocarbons, both in the reservoir and source rock, underwent parallel thermal maturation after migration and emplacement of the oil.

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