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AAPG Bulletin


AAPG Bulletin, V. 87, No. 5 (May 2003),

P. 847-872.

Nature, origin, and production characteristics of the Lower Silurian regional oil and gas accumulation, central Appalachian basin, United States

Robert T. Ryder,1 William A. Zagorski2

1U.S. Geological Survey, 12201 Sunrise Valley Dr., MS 956, Reston, Virginia 20192; email: [email protected]
2Great Lakes Energy Partners, P.O. Box 550, Hartville, Ohio 44632; email: [email protected]


Robert T. Ryder is a petroleum geologist with the Eastern Energy Resources Team of the U.S. Geological Survey. He has 29 years of experience with the Federal Government (Denver and Reston) and 5 years with the petroleum industry (Shell Oil). Most of his research is concentrated on the regional framework, origin, and resource assessment of petroleum-bearing sedimentary basins. His most recent research topics include the petroleum geology of the Appalachian basin and basins of China. Ryder has a B.S. degree in geology from Michigan State University and a Ph.D. in geology from Pennsylvania State University.

William A. Zagorski is vice president of exploration for Great Lakes Energy Partners. In 23 years with the petroleum industry, he has worked in the Appalachian, Black Warrior, Illinois, Michigan, and Uinta basins. The stratigraphic and structural habitats of Appalachian natural gas accumulations are topics of major interest to him. He has generated and evaluated numerous Appalachian gas plays in such strata as the Cambrian Rose Run sandstone, Ordovician Black River-Trenton limestone, Silurian Medina Group sandstone, Devonian shale, and Upper Devonian sandstone. Zagorski has B.S. and M.S. degrees in geology from the University of Pittsburgh.


The senior author thanks Ben E. Law, former U.S. Geological Survey colleague, for sharing his knowledge of basin-center gas accumulations and for his encouragement and technical discussions throughout this investigation. Other individuals who kindly contributed their time, thoughts, discussions, and data to the investigation are Robert C. Burruss (U.S. Geological Survey); James W. Castle (Clemson University); Doug Core (consulting geologist); Robert D. Hettinger (U.S. Geological Survey); Christopher D. Laughrey (Pennsylvania Topographic and Geological Survey); Vito F. Nuccio (U.S. Geological Survey); Ronald A. Riley (Ohio Division of Geological Survey); Craig J. Wandrey (U.S. Geological Survey). Nancy R. Stamm (U.S. Geological Survey) spent numerous hours preparing and artistically modifying the illustrations. Critical comments and suggestions by U.S. Geological Survey reviewers Robert C. Burruss, Thaddeus S. Dyman, and Ione Taylor and AAPG reviewers Katharine Lee Avary, Steve Laubach, and Sam Root improved the presentation and technical quality of the manuscript.


Low-permeability sandstones of the Lower Silurian regional oil and gas accumulation cover about 45,000 mi2 (117,000 km2) of the Appalachian basin and may contain as much as 30 tcf of recoverable gas resources. Major reservoirs consist of the "Clinton" sandstone and Medina Group sandstones. The stratigraphically equivalent Tuscarora Sandstone increases the area of the Lower Silurian regional accumulation (LSRA) by another 30,000 mi2 (78,000 km2). Approximately 8.7 tcf of gas and 400 million bbl of oil have been produced from the Clinton/Medina reservoirs since 1880.

The eastern predominantly gas-bearing part of the LSRA is a basin-center gas accumulation, whereas the western part is a conventional oil and gas accumulation with hybrid features of a basin-center accumulation. The basin-center accumulations have pervasive gas saturation, water near irreducible saturation, and generally low fluid pressures. In contrast, the hybrid-conventional accumulations have less-pervasive oil and gas saturation, higher mobile-water saturation, and both normal and abnormally low fluid pressures.

High mobile-water saturation in the hybrid-conventional reservoirs form the updip trap for the basin-center gas creating a broad transition zone, tens of miles wide, that has characteristics of both end-member accumulation types. Although the Tuscarora Sandstone part of the basin-center gas accumulation is pervasively saturated with gas, most of its constituent sandstone beds have low porosity and permeability. Commercial gas fields in the Tuscarora Sandstone are trapped in naturally fractured, faulted anticlines.

The origin of the LSRA includes (1) generation of oil and gas from Ordovician black shales, (2) vertical migration through an overlying 1000-ft (305-m)-thick Ordovician shale; (3) abnormally high fluid pressure created by oil-to-gas transformation; (4) updip displacement of mobile pore water by overpressured gas; (5) entrapment of pervasive gas in the basin center; (6) postorogenic uplift and erosion, causing gas leakage and a marked reduction in fluid pressure.

Most future natural-gas production in the Clinton/Medina sandstones is anticipated to come from the basin-center accumulation. The Tuscarora Sandstone has additional gas resources but typically low reservoir porosity and permeability, and the likelihood of low-energy (in British thermal units) gas reduce the incentive to explore for it.

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