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AAPG Bulletin


AAPG Bulletin, V. 86, No. 11 (November 2002), P. 1993-1999.

Copyright ©2002. The American Association of Petroleum Geologists. All rights reserved.

Resource-assessment perspectives for unconventional gas systems

James W. Schmoker1

1U.S. Geological Survey, Mail Stop 939, Denver Federal Center, Denver, Colorado 80225; email: [email protected]


James W. Schmoker, Ph.D., is an emeritus scientist at the U.S. Geological Survey, Denver, Colorado, where he has spent the last decade working on issues of petroleum resource assessment. He has contributed to the methodology used by the U.S. Geological Survey in many of their recent oil and gas assessments and is particularly interested in the geologic nature and approaches to resource assessment of continuous (unconventional) oil and gas accumulations.


I thank members of recent U.S. Geological Survey Petroleum Resource Assessment teams for the data, ideas, and suggestions they contributed. Constructive comments and reviews were provided by Ben E. Law, Ronald R. Charpentier, Brian W. Horn, and Gordon L. Dolton.


Concepts are described for assessing those unconventional gas sys tems that can also be defined as continuous accumulations. Contin uous gas accumulations exist more or less independently of the wa ter column and do not owe their existence directly to the buoyancy of gas in water. They cannot be represented in terms of individual, countable fields or pools delineated by downdip water contacts. For these reasons, traditional resource-assessment methods based on es timating the sizes and numbers of undiscovered discrete fields can not be applied to continuous accumulations. Specialized assessment methods are required.

Unconventional gas systems that are also continuous accumu lations include coalbed methane, basin-centered gas, so-called tight gas, fractured shale (and chalk) gas, and gas hydrates. Deep-basin and bacterial gas systems may or may not be continuous accumu lations, depending on their geologic setting.

Two basic resource-assessment approaches have been em ployed for continuous accumulations. The first approach is based on estimates of gas in place. A volumetric estimate of total gas in place is commonly coupled with an overall recovery factor to nar row the assessment scope from a treatment of gas volumes residing in sedimentary strata to a prediction of potential additions to re serves. The second approach is based on the production perfor mance of continuous gas reservoirs, as shown empirically by wells and reservoir-simulation models. In these methods, production characteristics (as opposed to gas in place) are the foundation for forecasts of potential additions to reserves.

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