Changes

The Appalachian Basin has been an important shale-gas-producing province since the early 1800s, with an estimated 3.0 tcf already produced from Devonian black shale.<ref name=Dw1986>de Witt Jr., W., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0000/0001.htm Devonian gas-bearing shales in the Appalachian Basin], in C. W. Spencer and R. F. Mast, eds., Geology of tight-gas reservoirs: AAPG Studies in Geology 24, p. 1–8.</ref> Until recently, the organic-rich Marcellus Shale, although recognized as a major source rock, was not a significant hydrocarbon producer in the Appalachian Basin. This has changed greatly in the last few years.

The Appalachian Basin has been an important shale-gas-producing province since the early 1800s, with an estimated 3.0 tcf already produced from Devonian black shale.<ref name=Dw1986>de Witt Jr., W., 1986, [http://archives.datapages.com/data/specpubs/resmi1/data/a066/a066/0001/0000/0001.htm Devonian gas-bearing shales in the Appalachian Basin], in C. W. Spencer and R. F. Mast, eds., Geology of tight-gas reservoirs: AAPG Studies in Geology 24, p. 1–8.</ref> Until recently, the organic-rich Marcellus Shale, although recognized as a major source rock, was not a significant hydrocarbon producer in the Appalachian Basin. This has changed greatly in the last few years.

The modern era of Marcellus Shale production in the Appalachian Basin began in October 2004 when the Range Resources 1 Renz Unit well in Mount Pleasant Township of Washington County, Pennsylvania, was completed using a large Barnett Shale style slick-water frac treatment. This completion established the production rates needed to encourage both industry and public interest in the Marcellus Shale. As of September 2009, more than 70 companies have acquired lease positions and drilled numerous horizontal and vertical discovery wells, establishing a broad play area for the Marcellus Shale, encompassing more than 28 million acres across Pennsylvania, West Virginia, New York, Maryland, and Ohio. The play has attracted the attention of independents, major oil companies, and international partners, as discoveries continue to expand the scope of the play and shed light on the formation's reservoir characteristics and economic potential. Production from the Marcellus Shale is expected to exceed 1 billion cubic feet (gas equivalents) per day (bcfepd) in 2010.

The modern [[era]] of Marcellus Shale production in the Appalachian Basin began in October 2004 when the Range Resources 1 Renz Unit well in Mount Pleasant Township of Washington County, Pennsylvania, was completed using a large [[Barnett Shale]] style slick-water frac treatment. This completion established the production rates needed to encourage both industry and public interest in the Marcellus Shale. As of September 2009, more than 70 companies have acquired lease positions and drilled numerous horizontal and vertical discovery wells, establishing a broad play area for the Marcellus Shale, encompassing more than 28 million acres across Pennsylvania, West Virginia, New York, Maryland, and Ohio. The play has attracted the attention of independents, major oil companies, and international partners, as discoveries continue to expand the scope of the play and shed light on the formation's reservoir characteristics and economic potential. Production from the Marcellus Shale is expected to exceed 1 billion cubic feet (gas equivalents) per day (bcfepd) in 2010.

The potential limits of the Marcellus Shale play can be defined to the north, south, and east by the outcrop belt of the unit and to the west by thinning of the Marcellus Shale or its removal by a Middle Devonian unconformity surface. Significant geologic parameters controlling the productive capability of the Marcellus Shale include thickness, total organic content, porosity, permeability, thermal maturity, pore pressure, depth, gas show characteristics, rock mechanics, natural fracturing, and structural complexity. A key component of the play is the large area where the Marcellus Shale has an overpressured profile, extending from northern West Virginia into most of southwestern and central Pennsylvania and into New York's southern tier.<ref name=Wrghtstn2008>Wrightstone, G. R., 2008, [http://www.papgrocks.org/wrightstone.pdf Marcellus Shale: Regional overview from an industry perspective (abs.)]: AAPG Eastern Section Meeting.</ref><ref>Wrightstone, G. R., 2009, [http://www.searchanddiscovery.com/abstracts/html/2009/annual/abstracts/wrightstone.htm Marcellus Shale: Geologic controls on production (abs.)]: AAPG Search and Discovery article 10206.</ref>

The potential limits of the Marcellus Shale play can be defined to the north, south, and east by the outcrop belt of the unit and to the west by thinning of the Marcellus Shale or its removal by a Middle Devonian unconformity surface. Significant geologic parameters controlling the productive capability of the Marcellus Shale include thickness, total organic content, porosity, permeability, thermal maturity, pore pressure, depth, gas show characteristics, rock mechanics, natural fracturing, and structural complexity. A key component of the play is the large area where the Marcellus Shale has an overpressured profile, extending from northern West Virginia into most of southwestern and central Pennsylvania and into New York's southern tier.<ref name=Wrghtstn2008>Wrightstone, G. R., 2008, [http://www.papgrocks.org/wrightstone.pdf Marcellus Shale: Regional overview from an industry perspective (abs.)]: AAPG Eastern Section Meeting.</ref><ref>Wrightstone, G. R., 2009, [http://www.searchanddiscovery.com/abstracts/html/2009/annual/abstracts/wrightstone.htm Marcellus Shale: Geologic controls on production (abs.)]: AAPG Search and Discovery article 10206.</ref>

The Appalachian Basin has a well-established history of shale-gas development ([[:File:M97Ch4FG1.jpg|Figure 1]]). The discovery and commercial use of gas from Devonian shales in the early 1820s in Fredonia, New York, is generally recognized as the birthplace of the natural gas industry. This significantly predates the Drake oil discovery in Titusville, Pennsylvania, in 1859. By 1860, a series of shallow shale-gas fields were developed in a fairway along the Lake Erie shoreline extending from Fredonia, New York, southwest toward the city of Sandusky, Ohio.<ref>Harper, J. A., 2008, The Marcellus Shale: An old “new” gas reservoir in Pennsylvania: Pennsylvania Geology, v. 38, no. 1, p. 2–13.</ref> Accurate data for these wells are scarce, but the likely black shale formations produced include the Dunkirk, Rhinestreet, Middlesex, and to a lesser extent the Marcellus. The initial reported gas rates were commonly high, but actual production rates and pressures were low and are not considered commercial by today's standards. These shallow wells were used mainly for domestic and light industrial purposes and were extensively developed from the 1860s through the mid-1900s.

The Appalachian Basin has a well-established history of shale-gas development ([[:File:M97Ch4FG1.jpg|Figure 1]]). The discovery and commercial use of gas from Devonian shales in the early 1820s in Fredonia, New York, is generally recognized as the birthplace of the natural gas industry. This significantly predates the Drake oil discovery in Titusville, Pennsylvania, in 1859. By 1860, a series of shallow shale-gas fields were developed in a fairway along the Lake Erie shoreline extending from Fredonia, New York, southwest toward the city of Sandusky, Ohio.<ref>Harper, J. A., 2008, The Marcellus Shale: An old “new” gas reservoir in Pennsylvania: Pennsylvania Geology, v. 38, no. 1, p. 2–13.</ref> Accurate data for these wells are scarce, but the likely black shale formations produced include the Dunkirk, Rhinestreet, Middlesex, and to a lesser extent the Marcellus. The initial reported gas rates were commonly high, but actual production rates and pressures were low and are not considered commercial by today's standards. These shallow wells were used mainly for domestic and light industrial purposes and were extensively developed from the 1860s through the mid-1900s.

The first major shale discovery in the Appalachian Basin was in 1921 in northeastern Kentucky, which established the Big Sandy field. To date, a total of more than 21,000 wells have been drilled in the Big Sandy field in eastern Kentucky, southern West Virginia, southern Ohio, and southwestern Virginia. The primary target in the Big Sandy field is the Upper Devonian Huron Shale, with contributions from the Cleveland, Rhinestreet, and Marcellus Shale intervals. Two characteristics of the Big Sandy field are its significant underpressured profile and a well-established open natural fracture network. This distinguishes the Big Sandy field from modern shale plays such as the Barnett, Fayetteville, and Haynesville shales, which have higher pressure gradients combined with lower density open natural fracture networks. These modern shale-gas plays rely more on the creation of induced artificial fractures to achieve commercial production rates than production from existing open natural fractures. To date, more than 2.5 tcf has been produced from the Big Sandy field,<ref name=Dw1986 /> and it still represents one of the top 100 gas fields in the United States. The development of the Big Sandy field continues using both vertical and horizontal drilling.<ref>Morris, L. J., 2008, [http://www.papgrocks.org/morris_p.pdf Horizontal development in the Appalachian Basin Devonian shale]: AAPG 2008 Eastern Section Meeting (abs.).</ref>

The first major shale discovery in the Appalachian Basin was in 1921 in northeastern Kentucky, which established the Big Sandy field. To date, a total of more than 21,000 wells have been drilled in the Big Sandy field in eastern Kentucky, southern West Virginia, southern Ohio, and southwestern Virginia. The primary target in the Big Sandy field is the Upper Devonian Huron Shale, with contributions from the Cleveland, Rhinestreet, and Marcellus Shale intervals. Two characteristics of the Big Sandy field are its significant underpressured profile and a well-established open natural [[fracture]] network. This distinguishes the Big Sandy field from modern shale plays such as the Barnett, Fayetteville, and Haynesville shales, which have higher pressure gradients combined with lower density open natural fracture networks. These modern shale-gas plays rely more on the creation of induced artificial fractures to achieve commercial production rates than production from existing open natural fractures. To date, more than 2.5 tcf has been produced from the Big Sandy field,<ref name=Dw1986 /> and it still represents one of the top 100 gas fields in the United States. The development of the Big Sandy field continues using both vertical and horizontal drilling.<ref>Morris, L. J., 2008, [http://www.papgrocks.org/morris_p.pdf Horizontal development in the Appalachian Basin Devonian shale]: AAPG 2008 Eastern Section Meeting (abs.).</ref>

===Development of the Marcellus Shale Play===

===Development of the Marcellus Shale Play===

Additional experimental tests of the Marcellus Shale were conducted by Belden and Blake in 1997 in Bradford County, Pennsylvania, in Stagecoach field, which produces from the Oriskany Sandstone. These wells were also completed using a carbon dioxide treatment that also failed to establish commercial production rates.

Additional experimental tests of the Marcellus Shale were conducted by Belden and Blake in 1997 in Bradford County, Pennsylvania, in Stagecoach field, which produces from the Oriskany Sandstone. These wells were also completed using a carbon dioxide treatment that also failed to establish commercial production rates.

The modern era of Marcellus Shale gas development began in late 2004 with the completion of the Range Resources Corporation 1 Renz Unit well test located in Washington County, Pennsylvania. The well originally was planned as a structural test of the Westland Dome, a structural closure associated with a left-lateral strike-slip fault system. The 1 Renz Unit well was drilled in 2003, targeting the Lower Devonian Huntersville Chert and Oriskany Sandstone along with the deeper Silurian Lockport Dolomite. Although several significant shows were encountered in the Lockport interval, attempts to complete this zone failed. During drilling, several large gas shows were observed and recorded in the Marcellus Shale, and a subsequent study indicated favorable comparisons of the Marcellus Shale to the Barnett and Fayetteville shales. On October 23, 2004, a large Barnett-style water frac consisting of 3,569,500 L (943,000 gal) of water and 167,800 kg (370,000 lb) of sand was used to treat a 27 m (90 ft) gross interval of the Marcellus organic shale in the Range Resources 1 Renz Unit. A flow test of approximately 0.400 mmcfepd and an overpressured gradient was observed. This prompted two further offsets by the operator in 2005 that achieved similar encouraging results.

The modern era of Marcellus Shale gas development began in late 2004 with the completion of the Range Resources Corporation 1 Renz Unit well test located in Washington County, Pennsylvania. The well originally was planned as a structural test of the Westland Dome, a structural closure associated with a left-lateral strike-slip fault system. The 1 Renz Unit well was drilled in 2003, targeting the Lower Devonian Huntersville Chert and Oriskany Sandstone along with the deeper Silurian Lockport [[Dolomite]]. Although several significant shows were encountered in the Lockport interval, attempts to complete this zone failed. During drilling, several large gas shows were observed and recorded in the Marcellus Shale, and a subsequent study indicated favorable comparisons of the Marcellus Shale to the Barnett and Fayetteville shales. On October 23, 2004, a large Barnett-style water frac consisting of 3,569,500 L (943,000 gal) of water and 167,800 kg (370,000 lb) of sand was used to treat a 27 m (90 ft) gross interval of the Marcellus organic shale in the Range Resources 1 Renz Unit. A flow test of approximately 0.400 mmcfepd and an overpressured gradient was observed. This prompted two further [[offset]]s by the operator in 2005 that achieved similar encouraging results.

Play activity started to slowly accelerate in 2006 within the high-pore-pressure area of the play with the drilling of 28 Marcellus wells located along a wide fairway stretching from north-central West Virginia, through most of Pennsylvania and into southern New York. Several companies completed Marcellus wells, including discoveries by Range Resources Corporation, Cabot Oil and Gas, Dominion Energy, Fortuna/Talisman, M amp M Energy, Atlas Energy, and Pennsylvania General Energy. A key vertical Marcellus discovery well was drilled in 2006 by Cabot Oil and Gas that brought the northeastern part of Pennsylvania to its modern era of shale-gas development. The discovery well, the Cabot Oil and Gas 5 Teel, was the first new oil and gas well drilled in Susquehanna County in 34 yr and was the fifth well ever drilled in that county. It reported a maximum production test rate to atmosphere of 7 mmcfpd.<ref>Cabot Oil and Gas, 2009, [http://www.slideshare.net/plsderrick/cabot-enercom-2009-oil-gas-conference Cabot Oil and Gas Corporation discussion handout]: Enercom 2009 Oil and Gas Conference.</ref> The first operationally successful horizontal test of the Marcellus Shale was drilled in 2006 by Range Resources Corporation in Washington County, Pennsylvania.

Play activity started to slowly accelerate in 2006 within the high-pore-pressure area of the play with the drilling of 28 Marcellus wells located along a wide fairway stretching from north-central West Virginia, through most of Pennsylvania and into southern New York. Several companies completed Marcellus wells, including discoveries by Range Resources Corporation, Cabot Oil and Gas, Dominion Energy, Fortuna/Talisman, M amp M Energy, Atlas Energy, and Pennsylvania General Energy. A key vertical Marcellus discovery well was drilled in 2006 by Cabot Oil and Gas that brought the northeastern part of Pennsylvania to its modern era of shale-gas development. The discovery well, the Cabot Oil and Gas 5 Teel, was the first new oil and gas well drilled in Susquehanna County in 34 yr and was the fifth well ever drilled in that county. It reported a maximum production test rate to atmosphere of 7 mmcfpd.<ref>Cabot Oil and Gas, 2009, [http://www.slideshare.net/plsderrick/cabot-enercom-2009-oil-gas-conference Cabot Oil and Gas Corporation discussion handout]: Enercom 2009 Oil and Gas Conference.</ref> The first operationally successful horizontal test of the Marcellus Shale was drilled in 2006 by Range Resources Corporation in Washington County, Pennsylvania.

The drilling levels increased significantly in 2007, with 153 wells completed during that year. Atlas Energy began its successful development of the Marcellus Shale in southwestern Pennsylvania. In 2007, the first significant horizontal completion was reported in Washington County, Pennsylvania, by Range Resources Corporation, with a reported initial potential (IP) of 3.9 mmcfepd. Also in 2007, Cabot Oil and Gas drilled its confirmation well, offsetting the 2006 discovery of the 5 Teel well in Susquehanna County. Several other key step-out and exploration wells were drilled in Butler, Elk, Greene, Clarion, and Lycoming counties of Pennsylvania. Significant drilling programs were initiated by Atlas Resources, Chief Oil and Gas, EOG Resources, Eastern American Energy, Pennsylvania General Energy, Range Resources Corporation, Rex Energy, Texas Keystone, and others. During 2007, the focus was still clearly on vertical well testing, and only a limited number of horizontal wells were drilled and completed.

The drilling levels increased significantly in 2007, with 153 wells completed during that year. Atlas Energy began its successful development of the Marcellus Shale in southwestern Pennsylvania. In 2007, the first significant horizontal completion was reported in Washington County, Pennsylvania, by Range Resources Corporation, with a reported initial potential (IP) of 3.9 mmcfepd. Also in 2007, Cabot Oil and Gas drilled its confirmation well, offsetting the 2006 discovery of the 5 Teel well in Susquehanna County. Several other key step-out and exploration wells were drilled in Butler, Elk, Greene, Clarion, and Lycoming counties of Pennsylvania. Significant drilling programs were initiated by Atlas Resources, Chief Oil and Gas, EOG Resources, Eastern American Energy, Pennsylvania General Energy, Range Resources Corporation, Rex Energy, Texas Keystone, and others. During 2007, the focus was still clearly on vertical well testing, and only a limited number of [[horizontal well]]s were drilled and completed.

The year 2008 was the breakout year for the Marcellus Shale play, with more than 360 completed wells reported for that year. Several key vertical and horizontal discoveries were established. These include discoveries reported by CNX Gas and EQT in Greene County, Pennsylvania, and Epsilon Resources in Susquehanna County, Pennsylvania. Encouraging vertical test results were reported by Atlas Energy in Fayette and Westmoreland counties of Pennsylvania after developing effective two-stage fracturing techniques for vertical wells, with average test rates after completion of 2 mmcfpd. Range Resources reported several strong horizontal wells with reported IPs ranging from 4 mmcfepd to more than 26.0 mmcfepd in Washington County, Pennsylvania.<ref>Range Resources Corporation, 2010, [http://b2icontent.irpass.cc/790%2F112383.pdf?AWSAccessKeyId=1Y51NDPSZK99KT3F8VG2&Expires=1282674243&Signature=T%2FrJ6ggrDsxTUsOJXDNyXkKvHbI%3D Range Resources Company Presentation, August 2010].</ref>

The year 2008 was the breakout year for the Marcellus Shale play, with more than 360 completed wells reported for that year. Several key vertical and horizontal discoveries were established. These include discoveries reported by CNX Gas and EQT in Greene County, Pennsylvania, and Epsilon Resources in Susquehanna County, Pennsylvania. Encouraging vertical test results were reported by Atlas Energy in Fayette and Westmoreland counties of Pennsylvania after developing effective two-stage fracturing techniques for vertical wells, with average test rates after completion of 2 mmcfpd. Range Resources reported several strong horizontal wells with reported IPs ranging from 4 mmcfepd to more than 26.0 mmcfepd in Washington County, Pennsylvania.<ref>Range Resources Corporation, 2010, [http://b2icontent.irpass.cc/790%2F112383.pdf?AWSAccessKeyId=1Y51NDPSZK99KT3F8VG2&Expires=1282674243&Signature=T%2FrJ6ggrDsxTUsOJXDNyXkKvHbI%3D Range Resources Company Presentation, August 2010].</ref>

[[File:M97Ch4FG3.jpg|400px|thumb|{{figure number|3}}Middle Devonian paleogeography showing the restricted Marcellus Shale depositional basin. Modified from Blakey.<ref name=Blky>Blakey, R., 2005, [http://jan.ucc.nau.edu/~rcb7/globaltext2.html Global paleogeography].</ref>]]

[[File:M97Ch4FG3.jpg|400px|thumb|{{figure number|3}}Middle Devonian paleogeography showing the restricted Marcellus Shale depositional basin. Modified from Blakey.<ref name=Blky>Blakey, R., 2005, [http://jan.ucc.nau.edu/~rcb7/globaltext2.html Global paleogeography].</ref>]]

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.

Based on petrographic analysis of preserved organic matter from the Marcellus in western New York, Sageman et al.<ref name=Sgmn /> reported that the Marcellus black mudstone contained 100% marine material. The organic-rich facies of the Marcellus are dominated by short alkanes, whereas the reverse is true for the non-organic-rich facies that are dominated by long alkanes.<ref>Murphy, A. E., 2000, Physical and biochemical mechanisms of black shale deposition, and their implications for ecological and evolutionary change in the Devonian Appalachian basin: Ph.D. dissertation, Northwestern University, Evanston, Illinois, 363 p.</ref> This indicates that terrestrial input of organic material into the basin was dominant during those periods when non-organic-rich muds and carbonate were deposited and that algal marine phytoplankton were dominant during the deposition of the Marcellus organic-rich black mudstones.

Based on petrographic analysis of preserved organic matter from the Marcellus in western New York, Sageman et al.<ref name=Sgmn /> reported that the Marcellus black [[mudstone]] contained 100% marine material. The organic-rich facies of the Marcellus are dominated by short alkanes, whereas the reverse is true for the non-organic-rich facies that are dominated by long alkanes.<ref>Murphy, A. E., 2000, Physical and biochemical mechanisms of black shale deposition, and their implications for ecological and evolutionary change in the Devonian Appalachian basin: Ph.D. dissertation, Northwestern University, Evanston, Illinois, 363 p.</ref> This indicates that terrestrial input of organic material into the basin was dominant during those periods when non-organic-rich muds and carbonate were deposited and that algal marine phytoplankton were dominant during the deposition of the Marcellus organic-rich black mudstones.

Like other organic-rich shales, the creation, deposition, and preservation of the organic Marcellus sediments was controlled by three factors: (1) primary photosynthetic production, (2) bacterial decomposition, and (3) bulk sedimentation rate.<ref name=Sgmn /> The traditional interpretation of deposition of the organic-rich members of the Marcellus Shale is a preservation model of organic enrichment, where a permanently stratified water column with anoxic or euxinic (anoxic-sulfidic) bottom water conditions allowed for the preservation of organic material.<ref>Demaison, G. J., and G. T. Moore, 1980, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0064/0008/1150/1179.htm Anoxic environments and oil source bed genesis]: AAPG Bulletin, v. 64, p. 1179–1209.</ref> This preservation model is best reflected in the proposed model of a nearly permanent pycnocline by Ettensohn.<ref name=Etnsn1992 />

Like other organic-rich shales, the creation, deposition, and preservation of the organic Marcellus sediments was controlled by three factors: (1) primary photosynthetic production, (2) bacterial decomposition, and (3) bulk sedimentation rate.<ref name=Sgmn /> The traditional interpretation of deposition of the organic-rich members of the Marcellus Shale is a preservation model of organic enrichment, where a permanently stratified water column with anoxic or euxinic (anoxic-sulfidic) bottom water conditions allowed for the preservation of organic material.<ref>Demaison, G. J., and G. T. Moore, 1980, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0064/0008/1150/1179.htm Anoxic environments and oil source bed genesis]: AAPG Bulletin, v. 64, p. 1179–1209.</ref> This preservation model is best reflected in the proposed model of a nearly permanent pycnocline by Ettensohn.<ref name=Etnsn1992 />

[[File:M97Ch4FG4.jpg|400px|thumb|{{figure number|4}}A map showing the primary structural features of the Appalachian Basin. Modified from Shumaker.<ref name=Shmkr1996>Shumaker, R. C., 1996, Structural history of the Appalachian Basin, in J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: Morgantown, West Virginia, West Virginia Geological and Economic Survey Publication V-25, p. 8–10.</ref> Decollement trends are from Colton,<ref name=Cltn>Colton, G. W., 1970, The Valley and Ridge and Appalachian plateau; stratigraphy and sedimentation; the Appalachian Basin; its depositional sequences and their geologic relationships, in G. W. Fisher, F. J. Pettijohn, and J. C. Reed Jr., eds., Studies of Appalachian geology, central and southern: New York, Interscience Publishers, p. 5–47.</ref> Frey,<ref name=Fry>Frey, M. G., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0006/1000/1027.htm Influence of Salina Salt on structures in New York-Pennsylvania part of the Appalachian Plateau]: AAPG Bulletin, v. 57, p. 1027–1037.</ref> and Sanford.<ref name=Snfrd>Sanford, B. V., 1993, St. Lawrence platform: Economic geology, in D. F. Stott and J. D. Aitken, eds., Sedimentary cover of the craton in Canada: Boulder, Colorado, Geological Society of America, The Geology of North America, v. D-1, p. 787–798.</ref> VT = Vermont; CT = Connecticut.]]

[[File:M97Ch4FG4.jpg|400px|thumb|{{figure number|4}}A map showing the primary structural features of the Appalachian Basin. Modified from Shumaker.<ref name=Shmkr1996>Shumaker, R. C., 1996, Structural history of the Appalachian Basin, in J. B. Roen and B. J. Walker, eds., The atlas of major Appalachian gas plays: Morgantown, West Virginia, West Virginia Geological and Economic Survey Publication V-25, p. 8–10.</ref> Decollement trends are from Colton,<ref name=Cltn>Colton, G. W., 1970, The Valley and Ridge and Appalachian plateau; stratigraphy and sedimentation; the Appalachian Basin; its depositional sequences and their geologic relationships, in G. W. Fisher, F. J. Pettijohn, and J. C. Reed Jr., eds., Studies of Appalachian geology, central and southern: New York, Interscience Publishers, p. 5–47.</ref> Frey,<ref name=Fry>Frey, M. G., 1973, [http://archives.datapages.com/data/bulletns/1971-73/data/pg/0057/0006/1000/1027.htm Influence of Salina Salt on structures in New York-Pennsylvania part of the Appalachian Plateau]: AAPG Bulletin, v. 57, p. 1027–1037.</ref> and Sanford.<ref name=Snfrd>Sanford, B. V., 1993, St. Lawrence platform: Economic geology, in D. F. Stott and J. D. Aitken, eds., Sedimentary cover of the craton in Canada: Boulder, Colorado, Geological Society of America, The Geology of North America, v. D-1, p. 787–798.</ref> VT = Vermont; CT = Connecticut.]]

The major structural features of the Appalachian Basin, key shale production trends, and structural provinces of the Appalachian Basin are depicted in [[:File:M97Ch4FG4.jpg|Figure 4]]. Key structural elements of the Appalachian Basin from west to east include the Waverly arch and Cincinnati arch to the west, the Cambridge arch and Burning Springs anticline farther eastward, the Rome trough, and the anticlinal fold belts in the Appalachian Plateau and Valley and Ridge province. To date, most economic productive Marcellus wells are located within the Appalachian Plateau physiographic province. This province is marked by generally gentle structures and a lack of intense faulting in the western parts of the province. Structural complexity increases to the east toward the structural front, where high-amplitude, detached, salt-cored anticlines are present trending northeast–southwest. Structural complexity may also occur in the synclines within the eastern parts of this province. The structural front represents the boundary between the plateau and the Valley and Ridge province, where the Devonian section rises quickly to the surface and crops out. The Valley and Ridge province represents the most structurally challenging area in which the Marcellus Shale is present. The area is structurally complex, with high-amplitude detached folds, repeated and overturned beds, and multiple thrust faults.

The major structural features of the Appalachian Basin, key shale production trends, and structural provinces of the Appalachian Basin are depicted in [[:File:M97Ch4FG4.jpg|Figure 4]]. Key structural elements of the Appalachian Basin from west to east include the Waverly arch and Cincinnati arch to the west, the Cambridge arch and Burning Springs anticline farther eastward, the Rome trough, and the anticlinal fold belts in the Appalachian Plateau and Valley and Ridge province. To date, most economic productive Marcellus wells are located within the Appalachian Plateau physiographic province. This province is marked by generally gentle structures and a lack of intense faulting in the western parts of the province. Structural complexity increases to the east toward the structural front, where high-amplitude, detached, salt-cored anticlines are present trending northeast–southwest. Structural complexity may also occur in the synclines within the eastern parts of this province. The structural front represents the boundary between the plateau and the Valley and Ridge province, where the Devonian section rises quickly to the surface and crops out. The Valley and Ridge province represents the most structurally challenging area in which the Marcellus Shale is present. The area is structurally complex, with high-amplitude detached [[fold]]s, repeated and overturned beds, and multiple thrust faults.

===Controls Caused by Basement Faulting===

===Controls Caused by Basement Faulting===

# Basement faulting and structural complexity

# Basement faulting and structural complexity

# Degree of natural fracturing

# Degree of natural fracturing

# Lateral landing target and orientation

# [[Lateral]] landing target and orientation

# Ability to hydrofracture

# Ability to hydrofracture

Present-day TOC content in the Marcellus Shale ranges from less than 1% to more than 15% (wt. %), establishing the Marcellus Shale as a world-class source rock. Analysis of data from proprietary whole core and sidewall programs from 15 wells drilled by Range Resources in Pennsylvania indicates that TOC values in the Marcellus Shale range from less than 1% to more than 15%. Recent work in the Marcellus Shale play in New York<ref>Nyahay, R., J. Leone, L. B. Smith, J. P. Martin, D. J. Jarvie, 2007, [http://www.searchanddiscovery.com/documents/2007/07101nyahay/*05 Update on regional assessment of gas potential in the Devonian Marcellus and Ordovician Utica shales of New York]: Search and Discovery Article 10136.</ref> indicates that TOC values in the Marcellus Shale average 6.5% and range from less than 1% to 11%. The EGSP studies analyzed TOC contents for the various Devonian shales, identifying a range from 1 to 27% from all Devonian shales in the EGSP study.<ref name=ZM>Zielinski, R. E., and R. D. McIver, 1982, Resources and exploration assessment of the oil and gas potential in the Devonian gas shales of the Appalachian Basin: U.S. Department of Energy, Morgantown Energy Technology Center, DOE/DP/0053-1125, p. 326.M</ref> However, most of these data are only readily available as interval averages. These earlier studies did indicate that the Marcellus Shale, in general, had higher TOC contents in the thermogenic areas of the Appalachian Basin as compared with the other organic-rich Devonian shales. Minimum threshold values for good source rocks and prospective shale-gas plays are typically 2.0% TOC or higher.<ref>Jarvie, D. M., R. J. Hill, and R. M. Pollastro, 2005, Assessment of the gas potential and yields from shales: The Barnett Shale model, in Unconventional energy resources in the southern midcontinent, 2004 symposium: Oklahoma Geological Survey Circular 110, p. 37–50.</ref> As such, the Marcellus Shale has some of the highest TOC contents of thermogenic-style shale plays. Studies by Reed and Dunbar<ref>Reed, J. R., and D. Dunbar, 2008, [http://www.papgrocks.org/reed_p.pdf Using ArcGIS to estimate thermogenic gas generation volumes by Upper and Middle Devonian shales in the Appalachian Basin (abs.)]: AAPG Eastern Section meeting.</ref> suggest calculated original TOC contents in the Marcellus Shale to be in the 4 to 20% range.

Present-day TOC content in the Marcellus Shale ranges from less than 1% to more than 15% (wt. %), establishing the Marcellus Shale as a world-class source rock. Analysis of data from proprietary whole core and sidewall programs from 15 wells drilled by Range Resources in Pennsylvania indicates that TOC values in the Marcellus Shale range from less than 1% to more than 15%. Recent work in the Marcellus Shale play in New York<ref>Nyahay, R., J. Leone, L. B. Smith, J. P. Martin, D. J. Jarvie, 2007, [http://www.searchanddiscovery.com/documents/2007/07101nyahay/*05 Update on regional assessment of gas potential in the Devonian Marcellus and Ordovician Utica shales of New York]: Search and Discovery Article 10136.</ref> indicates that TOC values in the Marcellus Shale average 6.5% and range from less than 1% to 11%. The EGSP studies analyzed TOC contents for the various Devonian shales, identifying a range from 1 to 27% from all Devonian shales in the EGSP study.<ref name=ZM>Zielinski, R. E., and R. D. McIver, 1982, Resources and exploration assessment of the oil and gas potential in the Devonian gas shales of the Appalachian Basin: U.S. Department of Energy, Morgantown Energy Technology Center, DOE/DP/0053-1125, p. 326.M</ref> However, most of these data are only readily available as interval averages. These earlier studies did indicate that the Marcellus Shale, in general, had higher TOC contents in the thermogenic areas of the Appalachian Basin as compared with the other organic-rich Devonian shales. Minimum threshold values for good source rocks and prospective shale-gas plays are typically 2.0% TOC or higher.<ref>Jarvie, D. M., R. J. Hill, and R. M. Pollastro, 2005, Assessment of the gas potential and yields from shales: The Barnett Shale model, in Unconventional energy resources in the southern midcontinent, 2004 symposium: Oklahoma Geological Survey Circular 110, p. 37–50.</ref> As such, the Marcellus Shale has some of the highest TOC contents of thermogenic-style shale plays. Studies by Reed and Dunbar<ref>Reed, J. R., and D. Dunbar, 2008, [http://www.papgrocks.org/reed_p.pdf Using ArcGIS to estimate thermogenic gas generation volumes by Upper and Middle Devonian shales in the Appalachian Basin (abs.)]: AAPG Eastern Section meeting.</ref> suggest calculated original TOC contents in the Marcellus Shale to be in the 4 to 20% range.

One of the best indirect measurements of TOC content in the Marcellus Shale is its gamma-ray count. Schmoker<ref>Schmoker, J. W., 1981a, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0007/1250/1285.htm Determination of organic-matter content of Appalachian Devonian shales from gamma-ray logs]: AAPG Bulletin, v. 65, no. 7, p. 1285–1298.</ref><ref>Schmoker, J. W., 1981b, Organic-matter content of Appalachian Devonian shales determined by use of wire-line logs: Summary of work done 1976–80: U.S. Department of the Interior Geologic Survey, Open-File Report, p. 81–181.</ref> documented a direct correlation between the organic content of Appalachian shales and the wireline log gamma-ray intensity. Significant TOC content (5%) can be identified with gamma-ray counts of 200 API units or greater. In some areas, particularly in southwestern Pennsylvania and northern West Virginia, peak gamma-ray counts in excess of 300 to 400 API are not uncommon and reflect the generally higher TOC contents in the southwestern Marcellus Shale play area when compared with the northeastern parts of the play. Within the Marcellus Shale play, TOC content can be directly related to porosity development resulting from the conversion of kerogen to hydrocarbons.

One of the best indirect measurements of TOC content in the Marcellus Shale is its gamma-ray count. Schmoker<ref>Schmoker, J. W., 1981a, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0007/1250/1285.htm Determination of organic-matter content of Appalachian Devonian shales from gamma-ray logs]: AAPG Bulletin, v. 65, no. 7, p. 1285–1298.</ref><ref>Schmoker, J. W., 1981b, Organic-matter content of Appalachian Devonian shales determined by use of wire-line logs: Summary of work done 1976–80: U.S. Department of the Interior Geologic Survey, Open-File Report, p. 81–181.</ref> documented a direct correlation between the organic content of Appalachian shales and the wireline log gamma-ray intensity. Significant TOC content (5%) can be identified with gamma-ray counts of 200 API units or greater. In some areas, particularly in southwestern Pennsylvania and northern West Virginia, peak gamma-ray counts in excess of 300 to 400 API are not uncommon and reflect the generally higher TOC contents in the southwestern Marcellus Shale play area when compared with the northeastern parts of the play. Within the Marcellus Shale play, TOC content can be directly related to porosity development resulting from the conversion of [[kerogen]] to hydrocarbons.

===Drilling Depth to Base of Marcellus===

===Drilling Depth to Base of Marcellus===

Normal to overpressured gradients are proposed for much of the remaining parts of the Marcellus Shale play in north-central West Virginia and northward into Pennsylvania and the southern tier of New York. Pressure gradients in these areas are projected to range from approximately 0.43 to more than 0.80 psi/ft. It is also noted that a decline in the pressure gradient may exist along the eastern margins of the play, near the structural front, possibly caused by a combination of a lack of overlying seal integrity and/or a decrease in preserved organic material approaching the structural front. Based on the established successes of recent Marcellus Shale horizontal and vertical wells in northeastern Pennsylvania, southwestern Pennsylvania, and northern West Virginia, it is proposed that consistently high production volumes and high ultimate reserves in the Marcellus Shale will be mainly found in the areas that are normal to overpressured. The position of the Rome trough system is closely related to the areas of highest observed pressure gradients in the Marcellus Shale.

Normal to overpressured gradients are proposed for much of the remaining parts of the Marcellus Shale play in north-central West Virginia and northward into Pennsylvania and the southern tier of New York. Pressure gradients in these areas are projected to range from approximately 0.43 to more than 0.80 psi/ft. It is also noted that a decline in the pressure gradient may exist along the eastern margins of the play, near the structural front, possibly caused by a combination of a lack of overlying seal integrity and/or a decrease in preserved organic material approaching the structural front. Based on the established successes of recent Marcellus Shale horizontal and vertical wells in northeastern Pennsylvania, southwestern Pennsylvania, and northern West Virginia, it is proposed that consistently high production volumes and high ultimate reserves in the Marcellus Shale will be mainly found in the areas that are normal to overpressured. The position of the Rome trough system is closely related to the areas of highest observed pressure gradients in the Marcellus Shale.

The cause of the decrease in pressure gradient in central and southern West Virginia in the Marcellus Shale is likely related to an inadequately developed seal or degradation of seal integrity by natural fracturing. Conversely, the normal and overpressured Marcellus Shale areas of the basin likely will have a well-developed and relatively noncompromised seal or seals, which served to retain most of the natural gas generated during thermal maturation. The seal integrity in northern West Virginia and western Pennsylvania was still not complete, as the Marcellus Shale and possibly other Devonian black shales were the source rock for most of the oil and gas produced in these areas in the Lower Devonian and younger reservoirs.<ref name=M&S2006 />

The cause of the decrease in pressure gradient in central and southern West Virginia in the Marcellus Shale is likely related to an inadequately developed seal or degradation of seal integrity by natural fracturing. Conversely, the normal and overpressured Marcellus Shale areas of the basin likely will have a well-developed and relatively noncompromised seal or seals, which served to retain most of the natural gas generated during [[thermal maturation]]. The seal integrity in northern West Virginia and western Pennsylvania was still not complete, as the Marcellus Shale and possibly other Devonian black shales were the source rock for most of the oil and gas produced in these areas in the Lower Devonian and younger reservoirs.<ref name=M&S2006 />

Two possible theories are proposed to explain the pressure gradient changes. These theories reflect a close correlation between both the Middle Devonian Tully Limestone (C. Patterson, 2009, personal communication) and the deeper Silurian Salina Salt compared with the regional pressure profile. [[:File:M97Ch4FG8.jpg|Figure 8]] shows the outline of the limits of the Silurian Salina Salt overlain onto the regional pressure gradient map and exhibits a good correlation between the presence of thick Salina Salt and the normal- to high-pressure areas. The presence of a regional decollement in the Salina Group evaporites has been proposed<ref>Gwinn, V. E., 1964, Thin-skinned tectonics in the Plateau and northwestern Valley and Ridge provinces of the central Appalachians: Geological Society of America Bulletin, v. 75, no. 9, p. 863–900, doi:10.1130/0016-7606(1964)75[863:TTITPA]2.0.CO;2.</ref><ref name=Shmkr1996 /><ref name=M&S2006 /> for the area underlain by the thick Salina Salt. This Salina decollement created an allochthonous block that was transported generally westward along low-angle detachment thrust faults. Shumaker<ref name=Shmkr1996 /> also proposed that south of the Salina pinch-out, the basal detachment zone occurred within the Devonian interval. It is proposed that this detachment and extensive movement in the Devonian section led to severe compromise of seals above the Marcellus Shale and subsequent loss of hydrocarbons and pressure.

Two possible theories are proposed to explain the pressure gradient changes. These theories reflect a close correlation between both the Middle Devonian Tully Limestone (C. Patterson, 2009, personal communication) and the deeper Silurian Salina Salt compared with the regional pressure profile. [[:File:M97Ch4FG8.jpg|Figure 8]] shows the outline of the limits of the Silurian Salina Salt overlain onto the regional pressure gradient map and exhibits a good correlation between the presence of thick Salina Salt and the normal- to high-pressure areas. The presence of a regional decollement in the Salina Group evaporites has been proposed<ref>Gwinn, V. E., 1964, Thin-skinned tectonics in the Plateau and northwestern Valley and Ridge provinces of the central Appalachians: Geological Society of America Bulletin, v. 75, no. 9, p. 863–900, doi:10.1130/0016-7606(1964)75[863:TTITPA]2.0.CO;2.</ref><ref name=Shmkr1996 /><ref name=M&S2006 /> for the area underlain by the thick Salina Salt. This Salina decollement created an allochthonous block that was transported generally westward along low-angle detachment thrust faults. Shumaker<ref name=Shmkr1996 /> also proposed that south of the Salina pinch-out, the basal detachment zone occurred within the Devonian interval. It is proposed that this detachment and extensive movement in the Devonian section led to severe compromise of seals above the Marcellus Shale and subsequent loss of hydrocarbons and pressure.

# 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.

{{reflist}}

{{reflist}}

* Repetski, J. E., R. T. Ryder, D. J. Weary, A. G. Harris, and M. H. Trippi, 2008, Thermal maturity patterns (CAI and % Ro) in the Ordovician and Devonian rocks of the Appalachian Basin: A major revision of U.S. Geological Survey Map I-917 using new subsurface collections: U.S. Geological Survey Scientific Investigations Map 3006.

[[Category:Memoir 97]]