Basin-centered Gas Systems and the Jonah Field
Published:January 01, 2004
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.
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Jonah Field: Case Study of a Tight-Gas Fluvial Reservoir
The discovery of a giant natural gas field within a mature petroleum province is a significant event. Understanding the factors that control such an accumulation is important if the oil and gas industry is to continue to develop natural gas resources. Jonah field, in the Greater Green River basin of southwest Wyoming, is the largest natural gas discovery in the onshore United States in the last 10-15 years with recoverable reserves ranging from 8 to 15 tcf natural gas. Since beginning widespread field development in August 1992, Jonah has produced approximately 1 tcf gas, 10.3 million barrels of oil, and 3.7 million barrels of water. Field production is still increasing with daily production presently at 666 MMCFGPD, 5800 BOPD, and 4000 BWPD from approximately 600 wells. Active drilling continues within the field as operators consider widespread downspacing. By virtue of being a tight-gas field, Jonah is, in many respects, nontraditional. Recent assessments of natural gas potential, for both the U.S. and the world, strongly suggest that most future gas resources will come from low-permeability sandstones in the deeper portions of sedimentary basins, and from fields that will undoubtedly share characteristics with Jonah. The subtle structure, the low-permeability nature of the reservoir, the challenging petrophysics, and the environmental sensitivity surrounding Jonah may foreshadow what explorationists have to look forward to as the demand for natural gas increases, not only in the United States, but throughout the world. This volume brings together previously unpublished material on Jonah field and attempts to integrate all aspects including geology, geophysics, reservoir engineering, drilling and completion, and regulatory affairs. As such, this is a definitive collection that provides a truly integrated perspective of this giant field.