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>

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 />

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* 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.

* 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]]

[[Category:Memoir 97]]