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

RESERVOIR PRESSURE AND TEMPERATURE

One of the more important defining characteristics of continuous-type (basin-centered) gas accumulations is their pressure history. Continuous-type (basin-centered) gas accumulations are always abnormally pressured. They are either overpressured or underpressured with respect to normal hydrostatic pressure gradients which range from 0.43 to 0.46 pounds per square inch per ft (psi/ft), depending on the salinity of the formation water. In the Appalachian basin, Russell (1972) first recognized abnormally low reservoir pressures in the "Clinton" sands. Later, Davis (1984), Zagorski (1988; 1991), and Law and Spencer (1993) suggested that the abnormally low pressures in "Clinton" sands and Medina Group sandstone reservoirs were indicative of a regional, continuous-type (basin-centered) accumulation. Law and Dickinson (1985) suggested that the abnormally low pressures in low-permeability reservoirs, like those in "Clinton" sands and Medina Group sandstone reservoirs, had evolved from an earlier abnormally high pressure history caused by hydrocarbon generation.

Based on analogs of continuous-type (basin-centered) gas accumulations in the Rocky Mountain region, we have developed a working hypothesis that the Clinton/Medina sandstones and the equivalent Tuscarora Sandstone in the Appalachian basin should have three pressure domains; a normally pressured domain, a low-pressured domain, and a high-pressured domain. The normal and underpressured domains have been fairly well defined by industry operators in the region. The normally pressured domain is located in east-central Ohio (Davis, 1984, Thomas, 1993) where "Clinton" sands produce oil and gas with water. This normally pressured domain coincides approximately with Clinton/Medina sandstone oil/gas play 6732. In eastern Ohio and western Pennsylvania and New York, downdip from the normally pressured "Clinton" sands, is the underpressured domain. Since the inception of this project, abnormally low pressures have been verified throughout most of northwestern Pennsylvania and adjoining western New York (play 6728). Pressure gradients measured here commonly range from 0.30 to 0.40 psi/ft. The location of the transition into updip, normally pressured reservoirs is uncertain, however, and remains an objective of this project.

The postulated overpressured domain, may be located east of the underpressured domain, where the "Clinton" sands, Medina Group sandstones, and the equivalent Tuscarora Sandstone are more deeply buried and present-day subsurface temperatures are higher than at shallower depths. Empirical evidence in the Rocky Mountain region has shown that in rock sequences that have adequate amounts of organic carbon to generate hydrocarbons (a condition not yet confirmed for Lower Silurian strata in the Appalachian basin), the top of the zone of active generation of hydrocarbons and overpressuring occurs at temperatures of about 200°F (90°C). In areas where the rocks have experienced a cooling event, a common situation in most basins, the top of overpressuring may be significantly less than 200°F(90°C). Our search for this overpressured component has just begun; however, there are some encouraging beginnings.

A report by Avary (in press) indicates that some of the Lower Silurian Tuscarora Sandstone reservoirs in the Runville, Black Moshannon, and Devils Elbow gas fields near the Allegheny structural front in Pennsylvania are overpressured (0.5 - 0.6 psi/ft). The depth to the Tuscarora Sandstone in these fields ranges from 8,100 - 11,100 ft and reservoir temperatures range from 138 - 167°F. However, because these gas fields produce from anticlinal traps there is some uncertainty about the pressure relationships. For example, high reservoir pressures (greater than hydrostatic pressures) are common in hydrocarbon accumulations that have long columns. Thus, the reservoir pressure in these fields may, in fact, be normal when considering their mode of accumulation. Through the next year, one of our objectives will be to determine the pressure conditions of the Tuscarora Sandstone. The recognition of a regionally overpressured domain in the Tuscarora Sandstone would have the effect of enlarging the geographic and stratigraphic distribution of the Clinton/Medina continuous-type (basin-centered) gas accumulation in the Appalachian basin.

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