Possible Continuous-Type (Unconventional) Gas Accumulation in Lower Silurian "Clinton" Sands, Medina Group, and the Tuscarora Sandstone in the Appalachian Basin: A Progress Report of 1995 project activities

Robert T. Ryder, Kerry L. Aggen, Robert D. Hettinger, Ben E. Law, John J. Miller, Vito F. Nuccio, William J. Perry, Jr., Stephen E. Prensky, John R. SanFilipo, and Craig J. Wandrey

Open-File Report 96-42

INTRODUCTION

In the U.S. Geological Survey's (USGS) 1995 National Assessment of United States oil and gas resources (Gautier and others, 1995), the Appalachian basin was estimated to have, at a mean value, about 61 trillion cubic feet (TCF) of recoverable gas in sandstone and shale reservoirs of Paleozoic age. Approximately one-half of this gas resource is estimated to reside in a regionally extensive, continuous-type gas accumulation whose reservoirs consist of low-permeability sandstone of the Lower Silurian "Clinton" sands and Medina Group (Gautier and others, 1995; Ryder, 1995). Recognizing the importance of this large regional gas accumulation for future energy considerations, the USGS initiated in January 1995 a multi-year study to evaluate the nature, distribution, and origin of natural gas in the "Clinton" sands, Medina Group sandstones, and equivalent Tuscarora Sandstone. The project is part of a larger natural gas project, Continuous Gas Accumulations in Sandstones and Carbonates, coordinated in FY1995 by Ben E. Law and Jennie L. Ridgley, USGS, Denver. Approximately 2.6 man years were devoted to the Clinton/Medina project in FY1995.

A continuous-type gas accumulation, referred to in the project, is a new term introduced by Schmoker (1995a) to identify those natural gas accumulations whose reservoirs are charged throughout with gas over a large area and whose entrapment does not involve a downdip gas-water contact. Gas in these accumulations is located downdip of the water column and, thus, is the reverse of conventional-type hydrocarbon accumulations. Commonly used industry terms that are more or less synonymous with continuous-type gas accumulations include basin- centered gas accumulation (Rose and others, 1984; Law and Spencer, 1993), tight (low-permeability) gas reservoir (Spencer, 1989; Law and others, 1989; Perry, 1994), and deep basin gas (Masters, 1979, 1984).

The realization that undiscovered gas in Lower Silurian sandstone reservoirs of the Appalachian basin probably occurs in a continuous accumulation rather than in conventionally trapped, discrete accumulations represents a significant departure from the 1989 National Assessment (Mast and others, 1989; deWitt, 1993). In 1989, a direct assessment (field-size distributions required for play analysis were unavailable) of the Lower Silurian sandstone play gave, at a mean value, about 1.7 TCF of gas. The 1995 estimate (~30 TCF of gas) is so much greater than the 1989 estimate (~1.7 TCF of gas) because of the interpreted continuous nature of the accumulation and the assessment methodology applied. The methodology for continuous hydrocarbon accumulations assumes that the reservoirs in the accumulation are gas-saturated and takes into account: 1) estimated ultimate recovery (EUR) per well probability distributions, 2) optimum area that a well can drain (spacing), 3) number of untested drill sites having the appropriate spacing area, 4) success ratio of previously drilled holes, and 5) risk (Schmoker, 1995b).

Davis (1984), Zagorski (1988, 1991), and Law and Spencer (1993) were among the first petroleum geologists to suggest that gas in the "Clinton" sands and Medina Group sandstones was trapped in a basin-centered/deep basin accumulation. They recognized many of the earmarks of a basin-centered/deep basin accumulation such as low-permeability reservoirs, abnormally low formation pressure, coalesced gas fields, gas shows or production in most holes drilled, low water yields, and a general lack of structural control on entrapment. Ryder (1995) adopted this interpretation by defining four continuous-type gas plays (6728-6731) in the "Clinton" sands-Medina Group interval (fig.1).

Play 6728 (Clinton/Medina sandstone gas high potential) covers a 17,000 sq mi region of western New York, northwestern Pennsylvania, eastern Ohio, and a small part of westernmost West Virginia that is very favorable for future gas resources (fig.1). Also, this play includes a large part of eastern Lake Erie. From the late 1970's to the present, this area has produced gas from drilling depths of about 2,500 to 4,500 ft in New York, 3,500 to 6,500 ft in Pennsylvania, and 5,000 to 6,500 in Ohio. A large part of play 6728 remains undrilled along its southeastern margin and in Lake Erie. Drilling depths are as great as 7,500 ft along the southeast margin of the play and between 1,500 and 2,500 ft in Lake Erie. Very likely, the entire undrilled part of the play contains gas-saturated "Clinton" sands and Medina Group sandstones. Most of the sandstone reservoirs are tight and require stimulation by hydrofracturing. In the 1980's and early 1990's, gas production from these tight reservoirs with permeability less than 0.1 milliDarcys (mD) was encouraged with price incentives administered by the Federal Energy and Regulatory Commission (FERC). Although plays 6729 through 6731 have potential for future gas resources they are less desirable than play 6728 because of thinner and probably lower quality reservoirs.

The Clinton/Medina sandstone gas plays are flanked by two additional plays that involve the Lower Silurian sandstone depositional system. The first play (Clinton/Medina sandstone oil/gas play--6732), marks an oil and gas producing region of east-central Ohio that is updip and transitional with the continuous gas plays (Ryder,1995)(fig.1). It closely corresponds to Area 1 (oil and gas in Clinton sands of Ohio) of the Lower Silurian sandstone play of deWitt (1993). Ryder (1995) considers play 6732 to be a conventional oil and gas play because of its tendency to have discrete accumulations with well-defined oil- and gas-water contacts. Oil and gas have been produced in this play since the early 1880's and are now largely depleted except for the Lake Erie part. The second play, (Tuscarora Sandstone gas play--6727), marks the approximate limit of undiscovered gas in basement-controlled anticlines and combination traps in the Tuscarora Sandstone, a more proximal, sandstone-dominated facies of the Lower Silurian clastic wedge (Ryder, 1995)(fig.1). This play extends eastward from a transitional boundary with the Clinton/Medina sandstone gas plays to near the Allegheny structural front (fig.1). Ryder (1995) classified the Tuscarora Sandstone gas play as a conventional gas play because its scattered gas accumulations seem to be largely controlled by anticlinal structures and have well-defined gas-water contacts.

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