AAPG ANNUAL MEETING
Denver, Colorado
June 12-15, 1994

Significant gas is produced from low permeability sandstones of the Upper Cretaceous Mesaverde Group in the Washakie Basin. Low productivity, very fine grained intervals are dominated by mudstones and silty shales, lenticular and wavy bedded very shaly sandstones, and wavy to ripple cross-laminated shaly sandstones that range in porosity from 0-8% with effective in situ Klinkenberg permeabilities (Keg) below 0.0005 md at irreducible water saturation (Swi). Productive, very fine to fine grained sandstone lithologies include ripple cross-laminated, cross-bedded, low angle cross-laminated to planar-laminated, and massive bedded sandstones, which range in porosity from 7-14% and have Keg values from 0.001-2 md.

A five digit rock classification system keyed to lithofacies, grain size, consolidation, sedimentary structure, and cementation mineralogy provided a quantitative database for information concerning both traditional rock description features as well as important petrophysical and logging properties. In general, average and maximum porosity and permeability increase with increasing rock type number. "Irreducible" brine saturations (Swi) decrease with increasing rock number resulting in increasing Keg at Swi with increasing rock number. Correlation of log suites with the digital rock type and petrophysical data allowed the determination of optimal log signatures which can help identify lithofacies.

Utilizing predictive petrophysical equations, approximate in situ petrophysical properties can be determined based upon log porosities calibrated to core, rock type, and the petrophysical characteristics exhibited by each rock type. Cumulative storage analysis indicates that 70-80% of reservoir storage capacity is in rocks with porosity >10%. Cumulative flow analysis indicates that nearly 95% of flow capacity is from rocks with permeability >0.01 md.

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