Stratigraphic framework and depositional sequences in the lower Silurian regional oil and gas accumulation, Appalachian Basin: From Jackson County, Ohio, through northwestern Pennsylvania, to Orleans County, New York

Lower Silurian sandstone units constitute the reservoir rock for a regionally extensive oil and gas accumulation in the central Appalachian basin (fig. 1A). The accumulation, referred to here as the Lower Silurian regional oil and gas accumulation, has been drilled and produced since the early 1880’s. To date, approximately 300 million barrels of oil and six to eight trillion cubic feet of gas have been produced from it in the United States and Ontario, Canada (McCormac and others, 1996; Miller, 1975; State oil and gas reports such as New York State Department of Environmental Conservation, 1998). The domi­ nant reservoirs are the “Clinton” and Medina sandstones in Ohio and westernmost West Virginia and the Medina Group (Grimsby Sandstone/Grimsby Formation and Whirlpool Sandstone) in northwestern Pennsylvania and western New York. A secondary reservoir in the Lower Silurian regional oil and gas accumulation is the Upper Ordovician(?) and Lower Silurian Tuscarora Sandstone (fig. 1A), a more proximal eastern facies of the “Clinton” sandstone and Medina Group (Yeakel, 1962; Cotter, 1982; Castle, 1998). On the basis of subtle variations, the regional accumulation is tentatively subdivided by Ryder (1998) into three parts: (1) an eastern gas-bearing part having many characteristics of basincentered accumulation (Davis, 1984; Zagorski, 1988, 1991; Law and Spencer, 1993); (2) a western gas-bearing part having characteristics of discrete fields such as a gas-water contact; and (3) a central oiland gas-bearing hybrid part having characteris­ tics of both discrete and basin-centered accumulation (Zagorski, 1996) (fig. 1A). Whereas the oil and (or) gas in the hybrid and discrete parts of the regional accumulation in Ohio are largely depleted except in the Lake Erie offshore (de Witt, 1993), gas continues to be discovered in the deeper basin-centered part of the Appalachian basin (Zagorski, 1991; Pees, 1994; Petroleum Information Corporation, 1994). The Tuscarora Sandstone is tentatively identified here with the basin-centered part of the regional accumulation (fig. 1A). However, only small quantities of gas have been produced from the Tuscarora Sandstone because of its generally poor reservoir quality and because of the low energy (Btu) content of the gas (Avary, 1996). To better understand the character and origin of the regional oil and gas accumulation and its component parts, six cross sec­ tions were drawn through the Lower Silurian sequence in parts of New York, Ohio, Pennsylvania, and West Virginia. The loca­ tions of the cross sections are shown on figure 1A and B, and preliminary results are reported in Ryder and others (1996), Keighin and Hettinger (1997), and Keighin (1998). Each cross section shows the stratigraphic framework, depositional setting, sequence stratigraphy, and hydrocarbon-producing intervals of the Lower Silurian sandstone reservoirs and adjoining strata. Cross section A–A’ presented here is about 450 mi long and trends northeastward, approximately subparallel to the deposi­ tional strike of the Lower Silurian sandstone system. Moreover, section A–A’ extends through large stretches of the basin-cen­ tered and hybrid parts of the regional accumulation. The remaining five cross sections are oriented approximately normal to and in part oblique to the depositional strike of the Lower Silurian sandstone system and they connect with section A–A’ (fig. 1A and B). Two of these cross sections (E–E’ and F–F’) tra­ verse the entire Lower Silurian regional oil and gas accumulation and its discrete, hybrid, and basin-centered parts.

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