Basin-centered gas systems (BCGSs) are potentially one of the more economically important unconventional gas systems in the world; in the United States they contribute as much as 15% of the total annual gas production. These regionally pervasive gas accumula tions are different from conventionally trapped accumulations in several respects. The basin-centered gas accumulations (BCGAs) associated with BCGSs are typically characterized by regionally pervasive accumulations that are gas saturated, abnormally pres sured, commonly lack a downdip water contact, and have low-permeability reservoirs. The accumulations range from single, isolated reservoirs a few feet thick to multiple, stacked reservoirs several thousand feet thick. Two types of BCGSs are recognized; a direct type, characterized by having gas-prone source rocks, and an indirect type, characterized by having liquid-prone source rocks. During the burial and thermal histories of these systems, the source rock differences between the two types of BCGSs result in strikingly different characteristics that impact exploration strategies. The majority of known BCGAs are the direct type. Exploration activity for BCGAs is in the early stages and thus far has been focused in North America. In other parts of the world, concepts of basin-centered gas systems are poorly known, and exploration activity focused on basin-centered gas accumulations is minimal.

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