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A petroleum system, as defined by Magoon and Dow<ref name=Magoonanddow_1994>Magoon, L. B., and W. G. Dow, eds., 1994, The petroleum system-from source to trap: AAPG Memoir 60, 655 p., quote on p. 3</ref> "includes all the elements and processes needed for an oil and gas accumulation to exist." In Magoon and Dow's definition, the elements include [[source rock]], [[reservoir rock]], [[seal rock]], and [[overburden rock]]. Relevant processes include trap formation and the generation, expulsion, migration, and accumulation of petroleum. A basin-centered gas system (BCGS) contains all of these components; however, the magnitude and function of some of the components interact to form a unique type of hydrocarbon accumulation.
A petroleum system, as defined by Magoon and Dow<ref name=Magoonanddow_1994>Magoon, L. B., and W. G. Dow, 1994, [http://archives.datapages.com/data/specpubs/methodo2/data/a077/a077/0001/0000/0003.htm Introduction], 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], pp. 3-22</ref> "includes all the elements and processes needed for an oil and gas accumulation to exist." In Magoon and Dow's definition, the elements include [[source rock]], [[reservoir rock]], [[seal rock]], and [[overburden rock]]. Relevant processes include trap formation and the generation, expulsion, migration, and accumulation of petroleum. A basin-centered gas system (BCGS) contains all of these components; however, the magnitude and function of some of the components interact to form a unique type of hydrocarbon accumulation.
In general, BCGAs are regionally pervasive accumulations that are gas saturated, abnormally pressured (high or low), commonly lack a downdip water contact, and have low-permeability reservoirs. In the context of a petroleum system, there are two types of basin-centered gas systems: a direct type and an indirect type.<ref name=Law_2000>Law, B. E., 2000, What is a basin-centered gas system: 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 8 p.</ref> The attributes of these two types of systems are provided in Table 1. Direct and indirect types of BCGSs are distinguished on the basis of source rock quality; a direct BCGS has a gas-prone source rock, and an indirect BCGS has an oil-prone source rock. This fundamental difference, oil-prone vs. gas-prone source rocks, leads to significantly different characteristics, as shown in Table 1. In addition to the two types of systems, there may be hybrid systems in which gas-prone and liquid-prone source rocks have contributed to the development of a BCGA.
In general, BCGAs are regionally pervasive accumulations that are gas saturated, abnormally pressured (high or low), commonly lack a downdip water contact, and have low-permeability reservoirs. In the context of a petroleum system, there are two types of basin-centered gas systems: a direct type and an indirect type.<ref name=Law_2000>Law, B. E., 2000, What is a basin-centered gas system: 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 8 p.</ref> The attributes of these two types of systems are provided in Table 1. Direct and indirect types of BCGSs are distinguished on the basis of source rock quality; a direct BCGS has a gas-prone source rock, and an indirect BCGS has an oil-prone source rock. This fundamental difference, oil-prone vs. gas-prone source rocks, leads to significantly different characteristics, as shown in Table 1. In addition to the two types of systems, there may be hybrid systems in which gas-prone and liquid-prone source rocks have contributed to the development of a BCGA.
|+ {{table number|1}}Attributes of direct and indirect basin-centered gas systems
|+ {{table number|1}}Attributes of direct and indirect basin-centered gas systems
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! Type || Source rocks || Reservoir in-situ permeability (md) || Hydrocarbon migration distance || Reservoir pressure || Pressure mechanism || Seal || Seal quality || Nature of upper boundary || Thermal maturity top of BCGA || Occurrence
! Type || Source rocks || Reservoir in-situ permeability (md) || [[Hydrocarbon migration]] distance || Reservoir pressure || Pressure mechanism || Seal || Seal quality || Nature of upper boundary || Thermal maturity top of BCGA || Occurrence
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| Direct || gas-prone type III kerogen || <0.1 || short || over-/underpressure || hydrocarbon generation || capillary || variable || cuts across stratigraphy || >0.7% R<sub>o</sub> || downdip from water
| Direct || gas-prone type III [[kerogen]] || <0.1 || short || over-/underpressure || hydrocarbon generation || capillary || variable || cuts across stratigraphy || >0.7% R<sub>o</sub> || downdip from water
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| Indirect || liquid-prone types I/II kerogen || <0.1 || short/long || over-/underpressure || thermal cracking of oil to gas || lithologic/capillary || good || bedding parallel || highly variable || downdip from water
| Indirect || liquid-prone types I/II kerogen || <0.1 || short/long || over-/underpressure || thermal [[cracking]] of oil to gas || lithologic/capillary || good || bedding parallel || highly variable || downdip from water
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