Play Analysis--The Cornerstone of the National Oil and Gas Assessment

"The U.S. Geological Survey has been conducting oil and gas assessments for many years. The first one I know of was conducted in 1909 by David T. Day. Other assessments will be conducted in the future. In recent assessments, especially at a national scale, the basis for analysis of oil and gas information is the play, which is a means for understanding the interaction of geological factors in the face of the realities of exploration activity. A reliable play analysis yields a reliable National Assessment."

Dr. Don Gautier, U.S. Geological Survey

Knowledge of oil and gas resources is needed for strategic planning, formulating economic and energy policies, evaluating lands in the purview of the Federal government, and developing sound environmental policies. The U.S. Geological Survey (USGS), in collaboration with the Minerals Management Service (MMS), is the only independent body able to conduct such a study. The USGS National Oil and Gas Assessment is a dispassionate and scientifically-based analysis that is unencumbered by vested interests in the outcome. USGS geologists develop hypotheses for occurrence and distribution, rather than conduct inventories of oil and gas resources, as the basis for preparing the National Oil and Gas Assessment. The end result is an estimate of the quantity of oil and gas yet to be discovered and recovered, and of the past and future growth in hydrocarbon reserves.



The USGS works with many Federal and State agencies and industry sources to gather relevant information.

The USGS maintains primary responsibility for the assessment of all onshore lands (including Federal, State, private, and Trust lands) and all State offshore waters. MMS maintains primary responsibility for Federal offshore waters. The USGS and MMS cooperate in those areas of responsibility that are geographically close; however, the agencies employ different techniques to develop their hypotheses largely because of differences in the types of information available. For example, MMS has access to industry-confidential geophysical data used to develop prospects and relies heavily on this source of information to prepare its assessment. The USGS relies primarily on geological interpretations of rock structure, organic geochemistry, and field and reservoir data in preparing its assessment.



Play analysis is the basis for the National Oil and Gas Assessment.

The USGS used "play analysis" in the previous National Oil and Gas Assessment, and since then, MMS has adopted this method. A play is a set of discovered or undiscovered oil and gas accumulations or prospects that exhibit nearly identical geological characteristics. A play is defined, therefore, by the geological properties (such as trapping style, type of reservoir, nature of the seal) that are responsible for the accumulations or prospects. The beauty of play analysis is that it ties statistics of oil and gas exploration and development to geological expertise. Reserve growth and unconventional resources are also being analyzed on a play basis using different analytical methods.



Plays are described through a structured set of information on geology and history of exploration.

About 700 plays are grouped into 72 provinces which in turn are grouped into 8 regions. More than 50 geologists are responsible for gathering relevant geologic information through literature surveys, field work, discussions with colleagues in industry and State and Federal agencies for each play in their respective province. This information is used to clearly define each play using various information sources available to USGS scientists. Each play is described in narrative form in detail sufficient to allow a complete and consistent analysis, and to allow comparison among plays and provinces.



USGS researchers estimate subjective probabilities for the favorable existence of critical play attributes.

Geologists have determined that five play attributes of source rock, timing, migration, reservoir, and trap, control the occurrence of oil or gas in a play. These attributes are thought to be generally independent of each other, and the probability that each is favorable is estimated by geologists from geological and geophysical information. If dependency exists, it is accounted for. The estimated probabilities for each attribute are multiplied together to yield a "play probability." Where a play is known to have at least one discovery of minimum size, the play probability is commonly set to one, that is all five attributes are certain. Therefore, if these attributes are favorable, geologists estimate the probability of occurrence of at least one additional undiscovered accumulation of oil or gas in the play of minimum size or larger.



Estimates of sizes and number of undiscovered fields in each play are used to estimate resources.

A procedure originally developed by John Houghton, now at the Department of Energy, and used by his colleagues at the USGS, fits a so-called truncated shifted Pareto (TSP) distribution to size data for discovered accumulations in a play. Data derived from these fitted distributions are applied to estimates of numbers of undiscovered accumulations in the play. Then, the product of estimated size distribution with the estimated frequency for undiscovered accumulations produces a resource estimate for a chosen play. This resource estimate may be modified to reflect the associated risk (1 minus the play probability).



USGS scientists see benefits to using play analysis in the assessment process.

Play analysis using the TSP distribution method provides a conceptually simple, yet robust, means for geologists to incorporate exploration, discovery, and production data with their geologic judgment to estimate undiscovered resources. Several other benefits accrue to the assessment when play analysis is its basis. The method groups discrete units of the analysis by common geological characteristics; uses all available data in a consistent manner; uses the collective experiences of the petroleum industry as seen in the exploration history; and, yields resource estimates that lend themselves to economic analyses.

For More Information:

Email: dhouse@usgs.gov

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Maintained by L. Friedrich Last updated 20-Feb-1997