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Scientific Investigations Report 2005–5082
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Estimation of Gas-Production Potential Using Well Logs, Cretaceous of Northern Montana

By Timothy C. Hester

thumbnail image of figure 17 within reportAbstract

Cretaceous sandstones of east-central Alberta and southern Saskatchewan, Canada, and eastern Montana, western North Dakota, and parts of South Dakota and Wyoming, United States, are recognized as a major source of natural gas. Although the potential for production exists, the lack of commercial development is due to three major factors: (1) the lack of pipeline infrastructure, (2) the lack of predictable and reliable rates of production, and (3) the difficulty in recognizing and selecting potentially productive gas-charged intervals. Unconventional (tight), continuous-type reservoirs, such as those in the Cretaceous of the northern Great Plains, are not well suited for formation evaluation by conventional methods. To aid in the identification and selection of potential gas-producing intervals, an index is developed here that empirically links the “gas effect” (a geophysical-tool response to gas in a formation) to gas production. The index combines the effects of porosity, water saturation, and clay into a single value—the “gas-production index” (GPI)—that relates in situ rock and fluids to gas-production potential. The fundamental method for isolating the gas effect for calibration is to use a crossplot of neutron porosity minus density porosity (N – D) versus gamma-ray intensity (GR). In addition, an algorithm is developed to allow generation of the GPI by computer using digital well logs.

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Posted January 2006

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