Basin-centered gas systems
| Basin-centered gas systems | |
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| Series | AAPG Bulletin, November 2002 |
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| Author | Ben E. Law |
| Link | Web page |
A petroleum system, as defined by Magoon and Dow[1] "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.[2] 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.
| 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 |
|---|---|---|---|---|---|---|---|---|---|---|
| Direct | gas-prone type III kerogen | <0.1 | short | over-/underpressure | hydrocarbon generation | capillary | variable | cuts across stratigraphy | >0.7% Ro | 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 |
References
- ↑ Magoon, L. B., and W. G. Dow, 1994, Introduction, in L. B. Magoon and W. G. Dow, eds., The petroleum system-from source to trap: AAPG Memoir 60, pp. 3-22
- ↑ Law, B. E., 2000, What is a basin-centered gas system: 2000 basin-centered gas symposium: Rocky Mountain Association of Geologists, 8 p.
See also
- Basin-centered gas
- Basin-centered gas systems: historical development and classification
- Basin-centered gas systems: development
- Basin-centered gas systems: elements and processes
- Basin-centered gas systems: examples
- Basin-centered gas systems: gas resources
- Basin-centered gas systems: global distribution
- Basin-centered gas systems: evaluation and exploration strategies
- Tight gas reservoirs: evaluation
External links
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