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A shale resource system is described as a continuous organic-rich source rock(s) that may be both a source and a reservoir rock for the production of petroleum (oil and gas) or may charge and seal petroleum in juxtaposed, continuous organic-lean interval(s). As such, there may be both primary migration processes that are limited to movement within the source interval<ref name=W&L1984>Welte, D., and Leythauser, 1984, Geological and physicochemical conditions for primary migration of hydrocarbons: Naturwissenschaften, v. 70, p. 133–137, doi:10.1007/BF00401597.M</ref> and secondary migration into nonsource horizons juxtaposed to the source rock(s).<ref name=W&L1984 /> Certainly additional migration away from the resource system into nonjuxtaposed, noncontinuous reservoirs may also occur. In this scheme, [[fracture]]d shale-oil systems, that is, shales with open fractures, are included as shale resource systems.

A shale resource system is described as a continuous organic-rich source rock(s) that may be both a source and a reservoir rock for the production of petroleum (oil and gas) or may charge and seal petroleum in juxtaposed, continuous organic-lean interval(s). As such, there may be both primary migration processes that are limited to movement within the source interval<ref name=W&L1984>Welte, D., and Leythauser, 1984, Geological and physicochemical conditions for primary migration of hydrocarbons: Naturwissenschaften, v. 70, p. 133–137, doi:10.1007/BF00401597.M</ref> and secondary migration into nonsource horizons juxtaposed to the source rock(s).<ref name=W&L1984 /> Certainly additional migration away from the resource system into nonjuxtaposed, noncontinuous reservoirs may also occur. In this scheme, [[fracture]]d shale-oil systems, that is, shales with open fractures, are included as shale resource systems.

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.<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.

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.