Changes

Ettensohn<ref name=Etnsn85a>Ettensohn, F. R., 1985a, The Catskill delta complex and the Acadian orogeny: A model, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 39–49.</ref><ref name=Etnsn85b>Ettensohn, F. R., 1985b, Controls on development of Catskill delta complex basis facies, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 65–77.</ref><ref name=Etnsn1992>Ettensohn, F. R., 1992, Controls on the origin of the Devonian–Mississippian oil and gas shales, east-central United States: Fuel, v. 71, p. 1487–1492, doi:10.1016/0016-2361(92)90223-B.</ref><ref>Ettensohn, F. R., 2004, Modeling the nature and development of major Paleozoic clastic wedges in the Appalachian Basin, U.S.A.: Journal of Geodynamics, v. 37, p. 657–681, doi:10.1016/j.jog.2004.02.009.</ref> assigned the period of deposition of the Marcellus Shale to the second of four tectonically related depositional phases associated with the Devonian Acadian orogeny. Each of Ettensohn's four tectophases of the Acadian orogeny represent regressive episodes that were further subdivided into four stages: (1) the beginning of tectonism and rapid subsidence leading to the accumulation of black shales; (2) deposition of gray shales and siltstones because of impending collision and regression; (3) collision causing widespread uplift and regional disconformities; and (4) widespread accumulation of limestone during a tectonically quiet transgressive period.

Ettensohn<ref name=Etnsn85a>Ettensohn, F. R., 1985a, The Catskill delta complex and the Acadian orogeny: A model, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 39–49.</ref><ref name=Etnsn85b>Ettensohn, F. R., 1985b, Controls on development of Catskill delta complex basis facies, in D. W. Woodrow and W. D. Sevon, eds., The Catskill delta: Geological Society of America Special Paper 201, p. 65–77.</ref><ref name=Etnsn1992>Ettensohn, F. R., 1992, Controls on the origin of the Devonian–Mississippian oil and gas shales, east-central United States: Fuel, v. 71, p. 1487–1492, doi:10.1016/0016-2361(92)90223-B.</ref><ref>Ettensohn, F. R., 2004, Modeling the nature and development of major Paleozoic clastic wedges in the Appalachian Basin, U.S.A.: Journal of Geodynamics, v. 37, p. 657–681, doi:10.1016/j.jog.2004.02.009.</ref> assigned the period of deposition of the Marcellus Shale to the second of four tectonically related depositional phases associated with the Devonian Acadian orogeny. Each of Ettensohn's four tectophases of the Acadian orogeny represent regressive episodes that were further subdivided into four stages: (1) the beginning of tectonism and rapid subsidence leading to the accumulation of black shales; (2) deposition of gray shales and siltstones because of impending collision and regression; (3) collision causing widespread uplift and regional disconformities; and (4) widespread accumulation of limestone during a tectonically quiet transgressive period.

The Acadian orogeny was the result of a probable collision between a part of the North American plate and a microcontinent called the Avalonian terrain.<ref>Williams, H., and R. D. Hatcher, 1982, Suspect terranes and accretionary history of the Appalachian orogen: Geology, v. 10, p. 530–536, doi:10.1130/0091-7613(1982)102.0.CO;2.</ref> Ettensohn<ref name=Etnsn85a /><ref name=Etnsn85b /> linked basin deformation and subsidence to fold-belt orogeny, where a migrating foreland basin (i.e., proximal trough) was created cratonward (westward) of the orogen with a forebulge, the Cincinnati arch, located even farther cratonward to the west. This model suggests that major orogenic highlands (Acadian Highlands) were located to the east of the Marcellus depositional basin from which clastic sediments were derived. These highlands also contributed to deformational loading, providing the accommodation space for accumulating sediments within the subsiding basin.

The Acadian orogeny was the result of a probable collision between a part of the North American plate and a microcontinent called the Avalonian terrain.<ref>Williams, H., and R. D. Hatcher, 1982, Suspect terranes and accretionary history of the Appalachian orogen: Geology, v. 10, p. 530–536, doi:10.1130/0091-7613(1982)102.0.CO;2.</ref> Ettensohn<ref name=Etnsn85a /><ref name=Etnsn85b /> linked basin [[deformation]] and subsidence to fold-belt orogeny, where a migrating foreland basin (i.e., proximal trough) was created cratonward (westward) of the orogen with a forebulge, the Cincinnati arch, located even farther cratonward to the west. This model suggests that major orogenic highlands (Acadian Highlands) were located to the east of the Marcellus depositional basin from which clastic sediments were derived. These highlands also contributed to deformational loading, providing the accommodation space for accumulating sediments within the subsiding basin.

During deposition of the Marcellus Shale, the central Appalachian Basin is interpreted to have been located between 15 and 30degS latitude<ref name=Etnsn1992 /> with an associated dry tropical or savanna-like climate where rainfall was seasonal with extended dry conditions. In addition, the area was likely to have been subjected to significant seasonal storm activity.<ref>Woodrow, D. L., F. W. Fletcher, and W. F. Ahrnsbrak, 1973, Paleogeography and paleoclimate at the deposition sites of the Devonian Catskill and Old Red Facies: Geological Society of America Bulletin, v. 84, p. 3051–3063, doi:10.1130/0016-7606(1973)842.0.CO;2.</ref> Reconstructions place the basin in the path of southeasterly trade winds, which would have carried moisture from the Iapetus Ocean westward across the Acadian Highlands located east of the basin.<ref name=Etnsn85b /> Ettensohn<ref name=Etnsn85b /> proposed that the Acadian Highlands created a rain shadow effect on the western slopes of these highlands that would have contributed to the arid conditions. The arid conditions and prevailing trade winds are likely to have introduced eolian siliciclastics into the Marcellus depositional basin from lands to the east. Werne et al.<ref name=Wrnetal /> reported the presence and enrichment of eolian silt grains in the organic-rich facies of the Oatka Creek and directly related this to a decrease in carbonate and noneolian siliciclastic sediments. In addition, Sageman et al.<ref name=Sgmn>Sageman, B. B., A. E. Murphy, J. P. Werne, C. A. Ver Straeten, D. J. Hollander, and T. W. Lyons, 2003, A tale of shales: The relative role of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian Basin: Chemical Geology, v. 195,  p. 229–273.</ref> reported a direct relationship between increasing eolian silts and increasing total organic carbon in the Marcellus Shale.

During deposition of the Marcellus Shale, the central Appalachian Basin is interpreted to have been located between 15 and 30degS latitude<ref name=Etnsn1992 /> with an associated dry tropical or savanna-like climate where rainfall was seasonal with extended dry conditions. In addition, the area was likely to have been subjected to significant seasonal storm activity.<ref>Woodrow, D. L., F. W. Fletcher, and W. F. Ahrnsbrak, 1973, Paleogeography and paleoclimate at the deposition sites of the Devonian Catskill and Old Red Facies: Geological Society of America Bulletin, v. 84, p. 3051–3063, doi:10.1130/0016-7606(1973)842.0.CO;2.</ref> Reconstructions place the basin in the path of southeasterly trade winds, which would have carried moisture from the Iapetus Ocean westward across the Acadian Highlands located east of the basin.<ref name=Etnsn85b /> Ettensohn<ref name=Etnsn85b /> proposed that the Acadian Highlands created a rain shadow effect on the western slopes of these highlands that would have contributed to the arid conditions. The arid conditions and prevailing trade winds are likely to have introduced eolian siliciclastics into the Marcellus depositional basin from lands to the east. Werne et al.<ref name=Wrnetal /> reported the presence and enrichment of eolian silt grains in the organic-rich facies of the Oatka Creek and directly related this to a decrease in carbonate and noneolian siliciclastic sediments. In addition, Sageman et al.<ref name=Sgmn>Sageman, B. B., A. E. Murphy, J. P. Werne, C. A. Ver Straeten, D. J. Hollander, and T. W. Lyons, 2003, A tale of shales: The relative role of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian Basin: Chemical Geology, v. 195,  p. 229–273.</ref> reported a direct relationship between increasing eolian silts and increasing total organic carbon in the Marcellus Shale.

# Open natural fractures are rarely observed in whole cores of the Marcellus Shale or in Fullbore Formation MicroImager (FMI™, manufactured by Schlumberger) logs. Most observed natural fractures are calcite cemented and healed.

# Open natural fractures are rarely observed in whole cores of the Marcellus Shale or in Fullbore Formation MicroImager (FMI™, manufactured by Schlumberger) logs. Most observed natural fractures are calcite cemented and healed.

# On a broad scale, most of the current activities in southwestern Pennsylvania and northern West Virginia are situated in relatively uncomplicated structural regions.

# On a broad scale, most of the current activities in southwestern Pennsylvania and northern West Virginia are situated in relatively uncomplicated structural regions.

# The overpressured part of the Marcellus Shale play can be partially attributed to its position north of the Salina Salt pinch-out, where the ductile beds of the deeper Salina Formation are pervasive. Here, during Alleghenian thrusting, much of the early and basinal accommodation was provided by the deformation of the Salina and then to a lesser extent the overlying Devonian shales. In the no-salt areas south of the Burning Springs feature and especially in southern West Virginia and Kentucky, the organic Devonian shale acted as the most ductile beds and hence experienced much more deformation and natural fracturing. This is believed to be one of the causal mechanisms for the productive capabilities of the shale in the Big Sandy field and the underpressured profile of the Marcellus Shale and other upper Devonian shales.

# The overpressured part of the Marcellus Shale play can be partially attributed to its position north of the Salina Salt pinch-out, where the ductile beds of the deeper Salina Formation are pervasive. Here, during Alleghenian thrusting, much of the early and basinal accommodation was provided by the [[deformation]] of the Salina and then to a lesser extent the overlying Devonian shales. In the no-salt areas south of the Burning Springs feature and especially in southern West Virginia and Kentucky, the organic Devonian shale acted as the most ductile beds and hence experienced much more deformation and natural fracturing. This is believed to be one of the causal mechanisms for the productive capabilities of the shale in the Big Sandy field and the underpressured profile of the Marcellus Shale and other upper Devonian shales.

# The original EGSP studies clearly distinguished the more fractured nature of the Big Sandy field as compared with the northern Appalachian areas and associated Marcellus Shale play.

# The original EGSP studies clearly distinguished the more fractured nature of the Big Sandy field as compared with the northern Appalachian areas and associated Marcellus Shale play.