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U.S. Geological Survey, Denver, CO

Abstract: The "Gas Effect" as an Indicator of Gas-Production Potential, Cretaceous of North-Central Montana

The "gas effect" is a geophysical-tool response to gas in a formation, and occurs whenever gas is present. On well logs, the gas effect is often manifested as a visual "crossover" of neutron and density-porosity curves. As clay volume and associated bound water increase in a formation, neutron- and density-porosity curves separate until the crossover no longer occurs and the gas effect, although still present, is no longer detectable. An alternative, empirical approach uses a crossplot of neutron porosity minus density porosity vs gamma-ray intensity to detect and measure the gas effect regardless of clay content.

The magnitude of the gas effect is directly related to the bulk volume of gas present. Therefore, the gas effect can substitute for water-saturation determinations as a qualitative measure of gas concentration and an indicator of production potential. This system has an advantage over conventional log analysis in areas, such as the Cretaceous of the northern Great Plains, where clay volume makes water-saturation determination unreliable.

The crossplot is used to separate the effects of gas and claybound water, and to isolate the gas effect for calibration. An overlay is used in combination with the crossplot to establish a baseline from which to measure and scale the gas effect into 12 indexed levels of magnitude. The index links the gas effect to actual gas production, and provides an estimate of production potential for a broad range of reservoir conditions.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana