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Two basic types of producible shale resource systems exist: gas- and oil-producing systems with overlap in the amount of gas versus oil. Although dry gas resource systems produce almost exclusively methane, wet gas systems produce some liquids and oil systems produce some gas. These are commonly described as either shale gas or shale oil, depending on which product predominates production. Although industry parlance commonly describes these as shale plays, these are truly mudstone; nonetheless, the term shale is used herein. It is important, however, to view these as a petroleum system,<ref>Magoon, L. B., and W. G. Dow, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0000/0003.htm The petroleum system], in L. B. Magoon and W. G. Dow, eds., The petroleum system: From source to trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 3–24.</ref> regardless of reservoir lithofacies or quality, because all the components and processes are applicable.

Two basic types of producible shale resource systems exist: gas- and oil-producing systems with overlap in the amount of gas versus oil. Although dry gas resource systems produce almost exclusively methane, wet gas systems produce some liquids and oil systems produce some gas. These are commonly described as either shale gas or shale oil, depending on which product predominates production. Although industry parlance commonly describes these as shale plays, these are truly mudstone; nonetheless, the term shale is used herein. It is important, however, to view these as a petroleum system,<ref>Magoon, L. B., and W. G. Dow, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0000/0003.htm The petroleum system], in L. B. Magoon and W. G. Dow, eds., The petroleum system: From source to trap: [http://store.aapg.org/detail.aspx?id=1022 AAPG Memoir 60], p. 3–24.</ref> regardless of reservoir lithofacies or quality, because all the components and processes are applicable.

Given this definition of shale resource systems, these plays are not new with production from fractured mudstone reservoirs ongoing for more than 100 yr.<ref>Curtis, J. B., 2002, [http://archives.datapages.com/data/bulletns/2002/11nov/1921/1921.htm Fractured shale gas systems]: AAPG Bulletin, v. 86, no. 11, p. 1921–1938.</ref> Gas from Devonian shales in the Appalachian Basin and oil from fractured Monterey Shale, for example, have had ongoing long-term (100+ yr) production. The paradigm shift in the new millennium is the pursuit of tight mudstone systems, and although fractures may be present, they are usually healed with minerals such as calcite. Of course, having a brittle rock typically with a high silica content is also very important. These systems are organic-rich mudstones or calcareous mudstones that have retained gas or oil and have also expelled petroleum. The close association of source and nonsource intervals has sometimes made it difficult to ascertain which horizon is the actual source rock, for example, Austin Chalk and interbedded Eagle Ford Shale (Grabowski, 1995). Of course, in addition to retained or juxtaposed expelled petroleum, most of these organic-rich source rocks have expelled petroleum that has migrated, typically longer distances, into conventional reservoirs.

Given this definition of shale resource systems, these plays are not new with production from fractured mudstone reservoirs ongoing for more than 100 yr.<ref>Curtis, J. B., 2002, [http://archives.datapages.com/data/bulletns/2002/11nov/1921/1921.htm Fractured shale gas systems]: AAPG Bulletin, v. 86, no. 11, p. 1921–1938.</ref> Gas from Devonian shales in the Appalachian Basin and oil from fractured Monterey Shale, for example, have had ongoing long-term (100+ yr) production. The paradigm shift in the new millennium is the pursuit of tight mudstone systems, and although fractures may be present, they are usually healed with minerals such as calcite. Of course, having a brittle rock typically with a high silica content is also very important. These systems are organic-rich mudstones or calcareous mudstones that have retained gas or oil and have also expelled petroleum. The close association of source and nonsource intervals has sometimes made it difficult to ascertain which horizon is the actual source rock, for example, Austin Chalk and interbedded Eagle Ford Shale.<ref>Grabowski, G. J., 1995, Organic-rich chalks and calcareous mudstones of the Upper Cretaceous Austin Chalk and Eagleford Formation, south-central Texas, U.S.A., in B. J. Katz, ed., Petroleum source rocks: Berlin, Springer-Verlag, p. 209–234.</ref> Of course, in addition to retained or juxtaposed expelled petroleum, most of these organic-rich source rocks have expelled petroleum that has migrated, typically longer distances, into conventional reservoirs.

The production success from shale-gas resource systems in North America has led to an international effort in exploration to identify such systems. This type of resource potential is present wherever a source rock is present, with risk ranging from and including geologic, geochemical, petrophysical, engineering, logistical, and economical to environmental factors. One clear advantage of shale-gas resource systems is the fact that they are the cleanest form of combustible carbon-based energy. Not only are particulate and smog-inducing components minimal, but also carbon dioxide emissions are the lowest for any carbon-based fuel.

The production success from shale-gas resource systems in North America has led to an international effort in exploration to identify such systems. This type of resource potential is present wherever a source rock is present, with risk ranging from and including geologic, geochemical, petrophysical, engineering, logistical, and economical to environmental factors. One clear advantage of shale-gas resource systems is the fact that they are the cleanest form of combustible carbon-based energy. Not only are particulate and smog-inducing components minimal, but also carbon dioxide emissions are the lowest for any carbon-based fuel.

* Faqira, M., A. Bhullar, and A. Ahmed, 2010, Silurian Qusaiba Shale Play: Distribution and characteristics (abs.): Hedberg Research Conference on Shale Resource Plays, December 5–9, 2010, Austin, Texas, Book of Abstracts, p. 133–134.

* Faqira, M., A. Bhullar, and A. Ahmed, 2010, Silurian Qusaiba Shale Play: Distribution and characteristics (abs.): Hedberg Research Conference on Shale Resource Plays, December 5–9, 2010, Austin, Texas, Book of Abstracts, p. 133–134.

* Fuhrmann, A., K. F. M. Thompson, R. di Primio, and V. Dieckmann, 2003, Insight into petroleum composition based on thermal and catalytic cracking: 21st International Meeting on Organic Geochemistry (IMOG), Krakow, Poland, September 8–12, 2003, Book of Abstracts, part I, p. 321–322.

* Fuhrmann, A., K. F. M. Thompson, R. di Primio, and V. Dieckmann, 2003, Insight into petroleum composition based on thermal and catalytic cracking: 21st International Meeting on Organic Geochemistry (IMOG), Krakow, Poland, September 8–12, 2003, Book of Abstracts, part I, p. 321–322.

* Grabowski, G. J., 1995,Organic-rich chalks and calcareous mudstones of the Upper Cretaceous Austin Chalk and Eagleford Formation, south-central Texas, U.S.A., in B. J. Katz, ed., Petroleum source rocks: Berlin, Springer-Verlag, p. 209–234.

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* Horsfield, B., S. Bharati, S. R. Larter, F. Leistner, R. Littke, H. J. Schenk, and H. Dypvik, 1992, On the atypical petroleum-generating characteristics of alginate in the Cambrian Alum Shale, in M. Schidlowski, S. Golubic, M. M. Kimerly, and P. A. Trudinger, eds., Early organic evolution: Implications for mineral and energy resources: Berlin, Springer-Verlag, p. 257–266.

* Horsfield, B., S. Bharati, S. R. Larter, F. Leistner, R. Littke, H. J. Schenk, and H. Dypvik, 1992, On the atypical petroleum-generating characteristics of alginate in the Cambrian Alum Shale, in M. Schidlowski, S. Golubic, M. M. Kimerly, and P. A. Trudinger, eds., Early organic evolution: Implications for mineral and energy resources: Berlin, Springer-Verlag, p. 257–266.

* Horsfield, B., R. Littke, U. Mann, S. Bernard, T. A. T. Vu, R. di Primio, and H-M. Schulz, 2010, [http://www.searchanddiscovery.net/documents/2010/110126horsfield/ndx_horsfield.pdf Shale gas in the Posidonia Shale, Hils area, Germany: Genesis of shale gas: Physicochemical and geochemical constraints affecting methane adsorption and desorption]: AAPG Annual Convention, New Orleans, Louisiana, April 11–14, 2010, Search and Discovery Article 110126, 33 p.

* Horsfield, B., R. Littke, U. Mann, S. Bernard, T. A. T. Vu, R. di Primio, and H-M. Schulz, 2010, [http://www.searchanddiscovery.net/documents/2010/110126horsfield/ndx_horsfield.pdf Shale gas in the Posidonia Shale, Hils area, Germany: Genesis of shale gas: Physicochemical and geochemical constraints affecting methane adsorption and desorption]: AAPG Annual Convention, New Orleans, Louisiana, April 11–14, 2010, Search and Discovery Article 110126, 33 p.