Characteristics of discrete and basin-centered parts of the Lower Silurian regional oil and gas accumulation, Appalachian basin: Preliminary results from a data set of 25 oil and gas fields

U.S. Geological Survey Open-File Report 98-216

CONCLUSIONS

1. Oil and gas trapped in Lower Silurian shallow marine and tidally influenced (estuarine) sandstone reservoirs of the Appalachian basin constitutes a regional hydrocarbon accumulation with large recoverable gas resources. Based on subtle variations, the regional accumulation is tentatively subdivided into three parts: an eastern, basin-centered part, a western quasi-discrete part, and a wide central hybrid part having characteristics of both the discrete and basin-centered parts. Very likely, all three parts could be assessed with an EUR per well approach. A fourth part of the regional accumulation, not discussed in this report, is an eastern extension of the basin-centered part where gas is trapped locally in a thicker sandstone sequence that represents a more proximal depositional setting with a moderate fluvial component.
2. The regional accumulation is characterized by such a broad region of interconnected oil and gas fields that, other than for historical and location purposes, the term field is meaningless as an assessment unit. In practice, designated fields represent production sweet spots having high EURs per well that merge gradually with surrounding regions that are also productive but have lower EURs per well.
3. Most of the resources reside in "Clinton" sands and Medina Group sandstone in the basin-centered part of the regional accumulation where as much as several tens of TCF of gas are estimated to be technically recoverable. However, the Tuscarora Sandstone in the eastern extension of the basin-centered part of the accumulation underlies a large area and, although characterized by very low porosity and permeability and low Btu gas, its potential for basin-centered gas should not be ignored. Remaining gas and oil resources in the discrete and hybrid parts of the accumulation are located primarily beneath Lake Erie.
4. Stratigraphic traps are the dominant mode of entrapment in the discrete and hybrid parts of the accumulation. These traps are caused by subtle lateral changes in depositional and (or) diagenetic facies. Locally, anticlinal noses and fault blocks play a secondary role in entrapment. A broad water-block trap, where water is located updip from the gas-saturated reservoir intervals, seems to control entrapment of the basin-centered part of the accumulation.
5. The basin-centered part of the accumulation is characterized by: a) reservoir permeability equal to or less than 0.1 mD, b) low reservoir-water saturation and average water yield per well ranging from 9 to 13 BW/MMCFG, c) broadly defined water-block trap updip of the gas-saturated regional reservoir, d) underpressured reservoirs with gradients ranging from 0.25 to 0.35 psi/ft, e) reservoir temperature of 125° F (52° C) or greater, f) depth of production at 5,000 ft or greater, and g) a dominantly gas-bearing reservoir sequence.
6. The discrete part of the regional accumulation is characterized by: a) reservoir permeability greater than 0.1 mD, b) high reservoir-water saturation and average water yield per well of several hundred BW/MMCFG or more, c) facies-change and(or) diagenetic stratigraphic traps, d) normally pressured to slightly underpressured reservoirs, e) reservoir temperature lower than 125° F (52° C), f) depth of production at 2,000 to 2,500 ft or less, and g) a dominantly gas-bearing reservoir sequence.
7. The hybrid part of the accumulation is both oil and gas bearing and has characteristics of both the basin-centered and discrete parts.
8. Available data indicate that EURs per well are very similar for the basin-centered (median range of 98 to 400 MMCF of gas) and hybrid parts (median range of 84 to 450 MMCF of gas) of the accumulation. No EUR per well data are available for the discrete part of the regional accumulation but it is expected that they would exceed these values.
9. High-salinity formation water (brine) with TDS = 200,000 to 300,000 mg/l (ppm) commonly is produced from all parts of the regional hydrocarbon accumulation. Very likely these brines were derived from connate water derived from the Upper Silurian Salina Group evaporite beds. Moreover, these Salina-derived brines may have mixed with an earlier brine derived during dewatering and (or) gas generation in the deep Appalachian basin.
10. The basin-centered part of the Lower Silurian regional accumulation has been significantly modified by post-emplacement episodes of uplift and erosion. Among the modifications are a reduction in temperature, gas remigration and loss, a reduction in fluid pressure from above normal to subnormal, and mixing with circulating brine. Many of these observed modifications to the Lower Silurian basin-centered accumulation differ largely by degree from those affecting the classic western U.S. and Canadian examples because of the much greater time involved (200+ million years versus 50 to 60 million years) following hydrocarbon emplacement and entrapment. However, some of the differences between the Lower Silurian Appalachian basin and Cretaceous-Tertiary Rocky Mountain basin-centered accumulations may be related to fundamentally different geologic processes.

Created by the EERT WWW Staff.