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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 12. (December)

First Page: 1927

Last Page: 1927

Title: Geologic Framework and Hydrocarbon Evaluation of Devonian and Mississippian Black Shales in Appalachian Basin: ABSTRACT

Author(s): John B. Roen


The Paleozoic wedge of sedimentary rocks in the Appalachian basin is a repetitive cyclic rock sequence consisting mainly of three alternating rock types--siliciclastics, carbonates, and black shales rich in organic matter. Of the two black-shale sequences in the basin, the upper is the Devonian and Mississippian shales, which have attracted interest as a source of uranium, oil and gas, and pyrolytic oil. Recent geologic studies have defined their regional stratigraphy, distribution, and thickness. These studies have provided the necessary framework for resource evaluations and for geochemical characterizations, including analysis of the chemical structure of the kerogen by 13C nuclear magnetic resonance (NMR). An eastward-thickening sequence of predominantly ve y fine to fine-grained rocks contains an aggregate thickness of dark shale (rich in organic matter) that ranges from about 15 ft (4.6 m) or less in the south-central part of the basin to about 500 ft (150 m) in the northeastern part of the basin. The average organic-carbon content for the black shales ranges from about 3% in the eastern part of the basin to about 10% in the western part. Regionally, the maturation of the shales increases eastward. Various studies suggest that the Devonian and Mississippian shales west of the Allegheny front have not exceeded the catagenic stage. The 13C NMR determinations of aromaticity of kerogen from selected stratigraphic intervals suggest that a significant amount of the parent organic material was of terrestrial origin. This NMR work corr borates visual evidence such as the presence of coalified plant material and the known initial flourishing of vascular plants in Devonian time. Maturation of shale containing aromatic kerogen derived primarily from vascular plants generated mostly nonassociated "dry" natural gas. Therefore, the composition of the gas in the black shales, whether "wet" or "dry," appears to be more closely related to the kerogen type or degree of aromaticity than to the degree of thermal maturation. The Devonian and Mississippian black shales in the basin are estimated to contain 200-300 mmcf of gas. Recoverable-reserve estimates are highly variable because the source and reservoir rock are one and the same and because the exploitation conditions and techniques vary across the basin.

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