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Charpentier, Ronald R.1, Troy Cook1
(1) U.S. Geological Survey, Denver, CO

ABSTRACT: Conventional and Continuous Oil and Gas Accumulations - A Spectrum, Not a Dichotomy

For the 1995 U.S. Geological Survey National Assessment, both conventional and continuous (unconventional) oil and gas resources were assessed. Since then, drilling has led to a better geologic understanding of continuous accumulations and corresponding changes in assessment methodology. This better geologic understanding considers the two models—conventional and continuous—as endpoints of a spectrum rather than as two distinct categories.
At the conventional end of the spectrum, resources occur in discrete accumulations with sharp boundaries, generally described by oil-water or gas-water contacts. At the other end of the spectrum, (the classic continuous model), the resources are not in discrete accumulations but are randomly distributed across extensive areas. Oil-water and gas-water contacts are not evident.
Between these end members, there is increasing spatial organization and spatial correlation as one goes from the classic continuous toward the conventional end. This is reflected in the better-producing wells becoming more organized into clusters called sweet spots. The boundaries of sweet spots change from gradational to distinct as one approaches the conventional end.
Because conventional accumulations are discrete entities, they can be counted and have specific sizes. The methodology for assessing conventional resources therefore is based on estimating numbers and sizes of undiscovered accumulations. Because the rest of the spectrum involves sweet spots of varying distinctness, an alternate methodology based on well-, rather than field-level data is more appropriate.
Coalbed gas and fractured-shale resources tend to be closer to the classic continuous end of the spectrum. Tight-gas sands may be near the conventional end.


AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.