Changes

===Total Organic Carbon Content===

===Total Organic Carbon Content===

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.