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

BURIAL, THERMAL, AND PETROLEUM GENERATION HISTORY MODELS

Except for local CAI values for conodonts recovered from the Packer Shell and "Clinton" sands (Nancy R. Stamm, written commun. 11/18/94), thermal maturation indices are unavailable for most of the Lower Silurian sequence in the study area. In the absence of thermal maturation indices, paleotemperatures for the Clinton/Medina sandstones were estimated from burial and thermal history models calibrated with data from other stratigraphic units.

Burial, thermal, and petroleum generation history models were generated from data in six deep drill holes in Ohio and West Virginia (fig. 14). From northwest to southeast these drill holes are: 1. Pan American No. 1 Windbigler, Morrow County, Ohio; 2. Redstone No. 3 Barth, Coshocton County, Ohio; 3. Lakeshore No. 1 Marshall, Guernsey County, Ohio; 4. Amerada No. 1 Ullman, Noble County, Ohio; 5. Exxon No. 1 Deem, Wood County, West Virginia; and 6. Exxon No. 1 Gainer-Lee, Jackson County, West Virginia (fig. 14). These models provide an estimate of the thermal history in three Clinton/Medina plays (6728, 6730, 6732) and the adjoining Tuscarora Sandstone play (6727). All models assume that 5,000 ft (1.5 km) of overburden have been removed from the drill site during post-Alleghanian uplift and erosion. Thermal gradients used in the models were constrained by Rock-Eval data from the Middle Ordovician Utica Shale (Ryder and others, 1991), vitrinite analyses from Pennsylvanian strata (Chyi and others, 1987) and CAI data from selected strata (Harris and others, 1978). Because overburden removal estimates remained constant, geothermal gradients were adjusted until the calculated thermal maturity based on vitrinite kinetic reactions (BasinMod Program, Platte River Assoc., Inc.) matched the measured maturity.

The burial, thermal, and petroleum generation history model of the Amerada No. 1 Ullman drill hole is shown in figure 15. Table 4 lists the stratigraphic sequence, geochemical data, and measured thermal maturation indices used to constrain the model. Geothermal gradients required to match the modeled (calculated) maturity with the selected measured thermal maturation indices are: 1.60°F/100 ft (2.90°C/100m) for the beginning (523 Ma) and end (0 Ma) of the model and 1.80°F/100 ft (3.27°C/100m) for an Alleghanian-age heat pulse at 230 Ma. This model suggests that at maximum burial, in Early Triassic time, the "Clinton" sands in the No. 1 Ullman drill hole had reached a maximum temperature of about 250°F (120°C). The model further suggests that the main phase of oil generation in the Middle Ordovician Utica Shale, a probable source of hydrocarbons in the "Clinton" sands and Medina Group sandstones, occurred in Permian time (fig 15). Abundant natural gas was probably generated from the Utica Shale in the vicinity of the No. 1 Ullman drill hole in early Mesozoic time as a result of the thermal cracking of oil and availability of types II and III kerogen. Fracturing by several mechanisms--basement tectonics, gas generation in the Utica Shale during maximum burial ~230 Ma, and thermoelastic expansion during post-Alleghanian uplift--may have permitted much of this natural gas to migrate vertically between 1,000 and 1,400 ft into "Clinton" sands and Medina Group sandstone reservoirs. Stable isotopic compositions of Medina Group gas in New York and Pennsylvania support the vertical migration theory (Jenden and others, 1993; Laughrey and Baldassare, 1995). Thus, the proximity of source and reservoir rocks that characterize Rocky Mountain continuous-type (basin-centered) gas accumulations may not apply to the proposed "Clinton"/Medina/Tuscarora continuous-type gas accumulation.

The required 1,000 to 1,400 ft of vertical migration, through fractured Upper Ordovician shale and siltstone, may be a deterrent to gas moving into the "Clinton"/Medina sandstones from the underlying Utica Shale. Particularly troublesome are the high formation pressures needed to displace water from the "Clinton"/Medina continuous-type accumulation. These high formation pressures may be difficult to achieve with the proposed source-to-reservoir distance.

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