Changes

* Source rock: Ordovician Utica Shale.<ref name=Coleetal_1987>Cole, G. A., R. J. Drozd, R. A. Sedivy and H. I. Helpern, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0007/0750/0788.htm Organic geochemistry and source-rock correlations, Paleozoic of Ohio]: AAPG Bulletin, v. 71, p. 788-809.</ref><ref name=Drozdandcole_1994>Drozd, R. J., and G. A. Cole, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0350/0387.htm Point Pleasant-Brassfield petroleum system, Appalachian basin, USA], ''in'' L. B. Magoon and W. G. Dow, eds., The petroleum system-from source to trap: AAPG Memoir 60, p. 387-398.</ref><ref name=Burrussandryder_1998>Burruss, R. C., and R. T. Ryder, 1998, Composition of crude oil and natural gas produced from 10 wells in the Lower Silurian "Clinton" sands, Trumbull County, Ohio: U.S. Geological Survey Open-file Report 98-799, 50 p.</ref><ref name=Ryderetal_1998>Ryder, R. T., R. C. Burruss, and J. R. Hatch, 1998, [http://archives.datapages.com/data/bulletns/1998/03mar/0412/0412.htm Black shale source rocks and oil generation in Cambrian and Ordovician of the central Appalachian basin, USA]: AAPG Bulletin, v. 82, p. 412-441.</ref> The Utica Shale contains type II kerogen and is thermally overmature (>1.3% R_o).

* Source rock: Ordovician Utica Shale.<ref name=Coleetal_1987>Cole, G. A., R. J. Drozd, R. A. Sedivy and H. I. Helpern, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0007/0750/0788.htm Organic geochemistry and source-rock correlations, Paleozoic of Ohio]: AAPG Bulletin, v. 71, p. 788-809.</ref><ref name=Drozdandcole_1994>Drozd, R. J., and G. A. Cole, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0350/0387.htm Point Pleasant-Brassfield petroleum system, Appalachian basin, USA], ''in'' L. B. Magoon and W. G. Dow, eds., The petroleum system-from source to trap: AAPG Memoir 60, p. 387-398.</ref><ref name=Burrussandryder_1998>Burruss, R. C., and R. T. Ryder, 1998, Composition of crude oil and natural gas produced from 10 wells in the Lower Silurian "Clinton" sands, Trumbull County, Ohio: U.S. Geological Survey Open-file Report 98-799, 50 p.</ref><ref name=Ryderetal_1998>Ryder, R. T., R. C. Burruss, and J. R. Hatch, 1998, [http://archives.datapages.com/data/bulletns/1998/03mar/0412/0412.htm Black shale source rocks and oil generation in Cambrian and Ordovician of the central Appalachian basin, USA]: AAPG Bulletin, v. 82, p. 412-441.</ref> The Utica Shale contains type II kerogen and is thermally overmature (>1.3% R_o).

* Generation-migration-accumulation: Late Devonian-Early Mississippian (370-320 Ma)<ref name=Drozdandcole_1994 /><ref name=Laughreyandharper_1996>Laughrey, C. D., and J. A. Harper, 1996, Play Obe: Upper Ordovician Bald Eagle Formation fractured play, ''in'' J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: West Virginia Geological and Economic Survey Publication V-25, p. 164-167.</ref><ref name=Nuccioetal_1997>Nuccio, V. F., C. J. Wandrey, R. T. Ryder, and A. G. Harris, 1997, Thermal maturity and petroleum generation of Middle Ordovician black shale source rocks, central Appalachian basin-controls on oil and gas in Lower Silurian low permeability sandstone reservoirs (abs.): AAPG Bulletin v. 81, p. 1560.</ref><ref name=Ryderetal_1998 /><ref name=Ryderandzagorski_2003 />  

* Generation-migration-accumulation: Late Devonian-Early Mississippian (370-320 Ma)<ref name=Drozdandcole_1994 /><ref name=Laughreyandharper_1996>Laughrey, C. D., and J. A. Harper, 1996, Play Obe: Upper Ordovician Bald Eagle Formation fractured play, ''in'' J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: West Virginia Geological and Economic Survey Publication V-25, p. 164-167.</ref><ref name=Nuccioetal_1997>Nuccio, V. F., C. J. Wandrey, R. T. Ryder, and A. G. Harris, 1997, Thermal maturity and petroleum generation of Middle Ordovician black shale source rocks, central Appalachian basin-controls on oil and gas in Lower Silurian low permeability sandstone reservoirs (abs.): AAPG Bulletin v. 81, p. 1560.</ref><ref name=Ryderetal_1998 /><ref name=Ryderandzagorski_2003 />  

* Reservoir rocks: Lower Silurian Clinton-Medina in eastern Ohio and western Pennsylvania and Tuscarora Sandstone in central Pennsylvania. The reservoir interval ranges in thickness from 100 to 600 ft (30-183 m).<ref name=Ryderandzagorski_2003 /> The thermal maturity of the reservoir ranges from 1.1 to 2.0% R_o.<ref name=Wandreyetal_1997 />

* Reservoir rocks: Lower Silurian Clinton-Medina in eastern Ohio and western Pennsylvania and Tuscarora Sandstone in central Pennsylvania. The reservoir interval ranges in thickness from 100 to 600 ft (30-183 m).<ref name=Ryderandzagorski_2003 /> The thermal maturity of the reservoir ranges from 1.1 to 2.0% R_o.<ref name=Wandreyetal_1997>Wandrey, C. J., R. T. Ryder, V. F. Nuccio, and K. L. Aggen, 1997, The areal extent of continuous type gas accumulations in Lower Silurian Clinton sands and Medina Group sandstones of the Appalachian basin and environments affeted by their developments: U.S. Geological Survey Open-File Report 97-272, 12 p.</ref>

* Porosity: 5-10%<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />

* Porosity: 5-10%<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />

* Permeability: <0.1 md<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />

* Permeability: <0.1 md<ref name=Ryder_1998 /><ref name=Ryderandzagorski_2003 />

* Environments of deposition: Fluvial, estuarine, and inner marine shelf in the eastern part to outer marine shelf and tidal in the western part<ref name=Cotter_1983 /><ref name=Brettetal_1990 /><ref name=Castle_1998 />

* Environments of deposition: Fluvial, estuarine, and inner marine shelf in the eastern part to outer marine shelf and tidal in the western part<ref name=Cotter_1983>Cotter, E., 1983, Shelf, paralic, and fluvial environments and eustatic sea-level fluctuations in the origin of the Tuscarora Formation (Lower Silurian) of central Pennsylvania: Journal of Sedimentary Petrology, v. 53, p. 25-49.</ref><ref name=Brettetal_1990>Brett, C. E., W. M. Goodman, and S. T. LoDuca, 1990, Sequence, cycles, and basin dynamics in the Silurian of the Appalachian foreland basin: Sedimentary Geology, v. 69, p. 191-244.</ref><ref name=Castle_1998>Castle, J. W., 1998, Regional sedimentology and stratal surfaces of a Lower Silurian clastic wedge in the Appalachian foreland basin: Journal of Sedimentary Research, v. 68, p. 1201-1211.</ref>

* Reservoir pressure: Reservoirs are normally pressured in the updip part of the Clinton-Medina in eastern Ohio, producing oil, gas, and water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). In western Pennsylvania reservoirs are underpressured and produce mainly gas with very small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients of 0.39-0.25 psi/ft in the underpressured part of the system. In central Pennsylvania, the Tuscarora Sandstone, equivalent to the Clinton-Medina, is overpressured and produces gas with small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients ranging from 0.50 to 0.60 psi/ft in the overpressured Tuscarora Sandstone in central Pennsylvania. The variable pressure gradients within the stratigraphic interval are shown in [[:file:BasinCenteredGasFig12.jpg|Figure 10]].

* Reservoir pressure: Reservoirs are normally pressured in the updip part of the Clinton-Medina in eastern Ohio, producing oil, gas, and water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). In western Pennsylvania reservoirs are underpressured and produce mainly gas with very small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients of 0.39-0.25 psi/ft in the underpressured part of the system. In central Pennsylvania, the Tuscarora Sandstone, equivalent to the Clinton-Medina, is overpressured and produces gas with small amounts of water ([[:file:BasinCenteredGasFig9.jpg|Figure 7]], [[:file:BasinCenteredGasFig11.jpg|Figure 9]]). Ryder and Zagorski<ref name=Ryderandzagorski_2003 /> reported pressure gradients ranging from 0.50 to 0.60 psi/ft in the overpressured Tuscarora Sandstone in central Pennsylvania. The variable pressure gradients within the stratigraphic interval are shown in [[:file:BasinCenteredGasFig12.jpg|Figure 10]].

* Seals: The top seal is interpreted to be the shales, carbonates, and evaporites in the overlying Upper Silurian.<ref name=Drozdandcole_1994 /> The updip seal has been identified as a water block.<ref name=Zagorski_1988 /><ref name=Zagorski_1991 /><ref name=Ryderandzagorski_2003 />

* Seals: The top seal is interpreted to be the shales, carbonates, and evaporites in the overlying Upper Silurian.<ref name=Drozdandcole_1994 /> The updip seal has been identified as a water block.<ref name=Zagorski_1988 /><ref name=Zagorski_1991 /><ref name=Ryderandzagorski_2003 />

* Gas accumulations: Downdip from normally pressured, water-bearing reservoirs; lacks downdip water contact ([[:file:BasinCenteredGasFig11.jpg|Figure 9]])

* Gas accumulations: Downdip from normally pressured, water-bearing reservoirs; lacks downdip water contact ([[:file:BasinCenteredGasFig11.jpg|Figure 9]])

* Depth to accumulation: 6500 ft (1981 m) in western Pennsylvania to 12,000 ft (3658 m) in central Pennsylvania<ref name=Ryderandzagorski_2003 />

* Depth to accumulation: 6500 ft (1981 m) in western Pennsylvania to 12,000 ft (3658 m) in central Pennsylvania<ref name=Ryderandzagorski_2003 />

* Gas quality: Gas is interpreted to be a product of thermally cracked oil.<ref name=Lawanddickinson_1985 /><ref name=Lawandspencer_1993 /><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> Gas in the Clinton-Medina sandstone is generally composed of 79-94% methane; 3-12% ethane, propane, and C₄₊ hydrocarbon; and 3-9% nitrogen and carbon dioxide.<ref name=Burrussandryder_1998 /><ref name=Ryderandzagorski_2003 /> In the Tuscarora Sandstone, gas is commonly dry (C1/C1-5 = 0.98-0.99), with nitrogen and carbon dioxide contents of 4-22% and <1-83%, respectively.<ref name=Ryderandzagorski_2003 />

* Gas quality: Gas is interpreted to be a product of thermally cracked oil.<ref name=Lawanddickinson_1985 /><ref name=Lawandspencer_1993>Law, B. E., and C. W. Spencer, 1993, Gas in tight reservoirs-an emerging source of energy, ''in'' D. G. Howell, ed., The future of energy gases: U.S. Geological Survey Professional Paper 1570, p. 233-252.</ref><ref name=Lawetal_1998a /><ref name=Ryderandzagorski_2003 /> Gas in the Clinton-Medina sandstone is generally composed of 79-94% methane; 3-12% ethane, propane, and C₄₊ hydrocarbon; and 3-9% nitrogen and carbon dioxide.<ref name=Burrussandryder_1998 /><ref name=Ryderandzagorski_2003 /> In the Tuscarora Sandstone, gas is commonly dry (C1/C1-5 = 0.98-0.99), with nitrogen and carbon dioxide contents of 4-22% and <1-83%, respectively.<ref name=Ryderandzagorski_2003 />

* Sweet spots: structural and stratigraphic

* Sweet spots: structural and stratigraphic

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<gallery mode=packed heights=200px widths=200px>

file:BasinCenteredGasFig10.jpg|{{figure number|8}}Geologic column of OrdovicianñDevonian rocks in eastern Ohio and western Pennsylvania, Appalachian basin (modified from Law and Spencer<ref name=Lawandspencer_1993 />).

file:BasinCenteredGasFig10.jpg|{{figure number|8}}Geologic column of Ordovician-Devonian rocks in eastern Ohio and western Pennsylvania, Appalachian basin (modified from Law and Spencer<ref name=Lawandspencer_1993 />).

file:BasinCenteredGasFig11.jpg|{{figure number|9}}Generalized cross section CC' showing normally pressured, underpressured, and overpressured parts of the Clinton-Medina-Tuscarora interval (modified from Ryder and Zagorski<ref name=Ryderandzagorski_2003 />). The underpressured and overpressured areas of the Clinton-Medina-Tuscarora represent the indirect BCGA part of the interval where gas and minor amounts of water are produced, and the normally pressured area represents the conventional part of the interval where oil, gas, and water are produced. The hybrid, underpressured area represents transition from oil, gas, and water production in eastern Ohio to gas and minor water production in the BCGA.

file:BasinCenteredGasFig11.jpg|{{figure number|9}}Generalized cross section CC' showing normally pressured, underpressured, and overpressured parts of the Clinton-Medina-Tuscarora interval (modified from Ryder and Zagorski<ref name=Ryderandzagorski_2003 />). The underpressured and overpressured areas of the Clinton-Medina-Tuscarora represent the indirect BCGA part of the interval where gas and minor amounts of water are produced, and the normally pressured area represents the conventional part of the interval where oil, gas, and water are produced. The hybrid, underpressured area represents transition from oil, gas, and water production in eastern Ohio to gas and minor water production in the BCGA.

file:BasinCenteredGasFig12.jpg|{{figure number|10}}Composite pressure gradient showing pressure end members (normal, underpressuring, and overpressuring relationships) within the Clinton-Medina-Tuscarora interval (modified from Law et al.<ref name=Lawetal_1998a />).

file:BasinCenteredGasFig12.jpg|{{figure number|10}}Composite pressure gradient showing pressure end members (normal, underpressuring, and overpressuring relationships) within the Clinton-Medina-Tuscarora interval (modified from Law et al.<ref name=Lawetal_1998a />).