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.