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
Appendix I
Cooperstown gas field (C)
| Location: | Venango and Crawford Counties, Pennsylvania (Cochranton, Dempseytown, Franklin, New Lebanon, Sugar Lake, and Utica 7½ min quads.) |
| Discovery date: | 1982 |
| Depth (ft): | ~5,600 |
| Hydrocarbon type and GOR: | Nonassociated gas |
| Structural setting: | Regional southeast-dipping homocline with numerous southeast-plunging anticlinal noses; field is cut by numerous lineaments |
| Stratigraphic name of reservoir: | Medina Group (Grimsby Formation—best reservoir; Cabot Head Shale—minor reservoir; Whirlpool Sandstone—minor reservoir) |
| Trap: | Water block |
| Porosity: | Grimsby Formation, F
ave (core) = 10.35%, range 5.9 to 26%, F med
(core) = 9.9%, secondary intergranular porosity is dominant (caused by feldspar
dissolution) Cabot Head Shale, F ave (core) = 8.8%,
range 4.5 to 20%; F med (core) = 8.6%, local
intragranular porosity (caused by partial dissolution of shale clasts) and extensive
quartz cementation Whirlpool Sandstone, F ave (core) = 7.6%, range 2.0 to 13.7%; F med (core) = 7.5%, intergranular porosity Medina Group, F ave (core) = 5.7%, range <1.5 to 18% (Castle and Byrnes, in press) |
| Permeability: | K < 0.1 mD, except in several thin zones
defined by low log resistivity where drilling fluid has invaded the reservoir; these zones
of higher permeability are caused by secondary intergranular porosity (feldspar
dissolution); K(ave) = 0.08 mD, range 0.001 to 1048 mD (Castle and Brynes, in press) |
| Natural fractures: | None are evident in cores and thin sections; however, because of the close proximity of the field to regional lineaments, fractures are considered to be important for improving reservoir quality |
| Diagenetic features: | Extensive quartz and calcite cementation has greatly reduced the intergranular porosity of the reservoirs; the best porosity type is intergranular porosity caused by feldspar-grain dissolution; most dissolution of feldspar grains has occurred along northwest-trending lineaments |
| Water saturation and volume/salinity of produced water: | Reservoirs are characterized by low water saturation (resistivity-log readings of 80 to 200 ohm-meters), little to no water produced; trap is characterized by high water saturation (resistivity-log readings of <80 ohm-meters—can be as low as 10 ohm-meters), high salt water saturation and show of gas after hydrofracturing); irreducible water saturation (Swi) = 10-80% depending on F , Swi ~ 12% for F > 8%, Swi ~ 20% for F = 6%, Swi ~ 40% for F = 3%, brine production is insignificant except for a few local areas (Castle and Byrnes, in press) |
| Gas/water and oil/water contacts: | None; zone of high gas saturation is located downdip of the zone of high water saturation; a transitional zone is characterized by high water saturation in the gas phase |
| Reservoir pressure: | 1,570 to 1,960 psi (0.28 to 0.35 psi/ft) |
| Bottom-hole temperature: | Tave = 119° F, range 111 to 140° F; Tave ~ 140° F also reported |
| Well spacing: | |
| Ultimate production (EUR per well): | EURave = 400 MMCF of gas per well, range 170 to 430 MMCF of gas per well; about 1 TCF of recoverable gas in the field |
| References: | Zagorski (1991, 1996); Castle and Byrnes (in press) |
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