Org. Geochem. Vol. 5, No. 4, pp. 241-254, 1984 0146-6380/84 $3.00 + 0.00 Printed in Great Britain Pergamon Press Ltd Geology of the Devonian black shales of the Appalachian Basin JOaN B. ROEN U.S. Geological Survey, Reston, VA 22092, U.S.A. (Received 8 August 1983; accepted 28 September 1983) Abstract--Black shales of Devonian age in the Appalachian Basin are a unique rock sequence. The high content of organic matter, which imparts the characteristic lithology, has for years attracted considerable interest in the shales as a possible source of energy. The recent energy shortage prompted the U.S. Department of Energy through the Eastern Gas Shales Project of the Morgantown Energy Technology Center to underwrite a research program to determine the geologic, geochemical, and structural characteristics of the Devonian black shales in order to enhance the recovery of gas from the shales. Geologic studies by Federal and State agencies and academic institutions produced a regional stratigraphic network that correlates the 15 ft black shale sequence in Tennessee with 3000 ft of intcrbedded black and gray shales in central New York. These studies correlate the classic Devonian black shale sequence in New York with the Ohio Shale of Ohio and Kentucky and the Chattanooga Shale of Tennessee and southwestern Virginia. Biostratigraphic and lithostratigraphic markers in conjunction with gamma-ray logs facilitated long-range correlations within the Appalachian Basin. Basinwide correlations, including the subsurface rocks, provided a basis for determining the areal distribution and thickness of the important black shale units. The organic carbon content of the dark shales generally increases from east to west across the basin and is sufficient to qualify as a hydrocarbon source rock. Significant structural features that involve the black shale and their hydrocarbon potential are the Rome trough, Kentucky River and Irvine-Paint Creek fault zone, and regional dccollements and ramp zones. INTRODUCTION clature and the ~areai extent of the stratigraphic units as used here does not necessarily conform to the The U.S. Government, response to the energy short- usage of the U.S. Geological Survey. age of the 1970s, sponsored research programs to develop unconventional hydrocarbon resources. In 1976, under the auspices of the Energy Research and STRATIGRAPHY Development Administration, now the U.S. De- partment of Energy (DOE), the Eastern Gas Shales Regional setting Project was organized to study the Devonian black The wedge of Paleozoic sedimentary rocks in the shales in order to enhance their gas production. The Appalachian Basin is the result of a general cyclic U.S. Geological Survey, under interagency agreement repetitive deposition of certain rock types in an No. EX-76-C-01-2287 with DOE's Morgantown asymmetric, eastward-deepening downwarp in the Energy Technology Center was responsible for the earth's crust. Modification of Colton's (1970) cross- compilation of the regional geology of the Devonian section showing the stratigraphic and lithologic re- black shales. In addition to the financial support of lations to the sedimentary wedge of the Appalachian the Morgantown Energy Technology Center, the Basin indicates a cyclic repetition of three noticeable writer wishes to acknowledge the State geological rock types: organic-rich rocks, predominantly black surveys and universities who, in co-operation with the shale; elastic rocks, mostly silty shale, siltstones and Eastern Gas Shales Project, supplied much of the sandstones; and carbonate rocks (Fig. 2). The obvi- basic data for this regional study. ous cycle, not always complete, is a basal carbon- Interest in the economic development of the De- accous shale overlain by elastics which in turn are vonian shales has prompted a variety of geochemical overlain by carbonates. Three, and possibly four, studies to help define areas for hydrocarbon ex- cycles may be identified in the basin fill (Fig. 2). ploration. As an aid to those studies, this paper The third cycle contains at its base the Devonian presents a summary of the basinwide stratigraphic black shales. These shales are a facies within a large framework of the principal Devonian black shale westward- and southward-thinning sequence of sedi- units in the Appalachian Basin and specific regional mentary rocks. In eastern Pennsylvania the sequence tectonics of the Appalachian Basin that may affect is over 5000 ft and thins westward across the basin hydrocarbon production from the black shales. The where in central Ohio the thickness is about 600 ft. area of study is defined by the outline of the regional Southward, in central Tennessee, the thicknesses stratigraphic network prepared for the Devonian range from 0 to about 50 ft. Throughout the length shales of the Appalachian Basin (Fig. l). The nomen- of the basin, the sequence thickens from the central 241 242 JOHN B. ROEN limestones. The nonmarine facies consists of reddish colored shale, siltstone, sandstone and conglomerate. This facies, which was not studied for this report, is the eastern terrestrial component of a large delta Crq system that grades westward into progressively finer grained marine turbidities. The siltstones and sand- stones of the turbidite sequence are either thin- bedded sheet deposits, indicate of a relatively low energy, distal basin deposit or thicker-bedded sand- stones with current and channel structures, indicative of a proximal, higher energy, slope or shelf environ- ment. These turbidites are interbedded with, and grade basinward, to dark, organic rich shales. The dark colored shales represent the extreme distal por- tions of the deltaic wedge. The dark gray, brownish- black to black shales are thought to have accumu- lated in tranquil anoxic basinal waters. Their euxinic depositional site allowed the preservation of the finely macerated organic detritus which imparts the dark color to the shale. Sporadic limestone beds with pelagic and benthic faunas are suggestive of a higher energy, oxygenated water, shoal environment. The regional facies relationship of the black shales and \ contiguous rocks within the Appalachian Basin are Fig. 1. Index map showing location of stratigraphic cross- shown on the generalized cross-section (Fig. 2). sections and the Devonian outcrop belt in the Appalachian The basic repetitive depositional cycle recognized Basin. Cross-section network outlines the area of study. within the Paleozoic section of the basin is also recognizable within the Devonian shale sequence. Although much reduced in thickness and not without region eastward to the outcrop belt in the folded the expected variation due to missing units, the Appalachians. lithologic sequence of the Devonian cycle is similar to The principal lithologic components of this De- the larger Paleozoic cycle of the basin. In New York, vonian wedge are marine clastics with minor amounts de Witt and Colton (1959) recognized this cyclic of carbonate rock and nonmarine elastics. The ma- sedimentation pattern. A complete cycle consists of a rine facies includes black shale, gray shale, siltstone basal black shale overlain by lighter gray shale, and sandstone, and lesser amounts of argillaceous siltstone, and sandstone with a limestone at the top. NW SE Ohio Virginia Devonian and Mis~ssippian clasfics Pennsylvanian and Permian clastics S I Devonian bleck shale ':-' -- ,,, .;::.~:::: .:.'~:";':! Silurian and Devonian carbonates Ordovician and SJurian clastics Ordovician black shale "-I- ,~' , '1 I I ~ I Cambrian and Ordovician carbonates :~ Carbonates Feet" c=== I' -I Cambrian clastics Black shale I ,,.,;,"#~ .'i! 50 Miles i Cycle ",-G Fig. 2. Stratigraphic cross-section of the Appalachian Basin showing the cyclic relationship of the basin fill. The Devonian black shale is the basal sequence of cycle 3. Modified from Colton (1970). Devonian black shales in Appalachian Basin 243 A complete cycle is represented by the Hamilton found in the southern part of the basin. The quartz Group at the base of the Devonian black shale is most always angular to subangular having an sequence. The cycle includes the basal black Mar- equant to elongate shape. The extremely fine grained cellus and Skaneateles Shales, the overlying Moscow nature generally precludes anything other than the and Ludlowville Shales and the Tully Limestone at monocrystalline variety of quartz. Also, because of the top. A few thin limestone beds within the thick the very fine grained character of the quartz, it is Hamilton sequence suggests the presence of a few difficult to determine authigenic quartz, secondary incomplete subcycles. Above the Hamilton Group in overgrowths, and to interpret possible multicycle the northeastern part of the basin there are several sedimentation. Some of the quartz-rich silt laminae recognizable cycles; however, only the basal black contain a few overgrowths and other forms of sec- shale and the overlying gray shale and coarser clastics ondary quartz. are present. Toward the western and southern part of Feldspar is present in amounts less than that of the basin the cycles become compressed losing the quartz; probably less than 10~o. The shape and grain gray clastics, and in areas such as central Tennessee size of the feldspar is similar to that of the quartz. the cycles are represented almost entirely by black Differentiation between feldspar and quartz is shale. difficult because of the very fine grain size. The grain size also precludes determination of the type of Lithology and related properties feldspar. However, sporadic plagioclase grains are The color of the Devonian shales ranges from identified by their albite twinning. medium light gray (N 6) to black (N 1) with vari- Variable amounts of mica are found in the black ations of brownish black (5 YR 2/1) and olive black shale. Conant and Swanson (1961) report as much as (5 Y 2/1). The color value notations are from the 10~ muscovite or sericite; Stenbeck (1981) reports Rock Color Chart of Goddard et al. (1948). In about 5-30~ chlorite, muscovite, and other mi- general reference to the Devonian black shales, caceous minerals contained in the black shale.