Basin-centered Gas Systems and the Jonah Field
The giant Jonah gas field, located in western Wyoming, is a gas chimney rooted in a regionally pervasive, direct-type, basin-centered gas accumulation (BCGA). The field is an excellent example of a structural sweet spot in a BCGA. Basin-centered gas systems (BCGSs), of which BCGAs are products, are potentially one of the more economically important, unconventional gas systems in the world; in the United States, they contribute as much as 17% of the total annual gas production. These regionally pervasive gas accumulations are different from conventional accumulations in several respects. The BCGAs associated with BCGSs are typically characterized by regionally pervasive reservoirs that are gas saturated, abnormally (high or low) pressured, 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. Based on these criteria, gas in the Jonah field is interpreted to have been sourced from gas-prone, type III kerogen and is therefore a direct type of BCGA.