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Prediction of Diagenesis and Reservoir Quality Using Well Logs, Williams Fork Formation, Mamm Creek Field, Piceance Basin

Ozkan, Aysen 1; Cumella, Steve 2; Milliken, Kitty 3; Laubach, Steve 3
1 Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX.
2 Bill Barret Corporation, Denver, CO.
3 Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, TX.

Most gas production in the Piceance Basin is from discontinuous fluvial deposits of the Williams Fork Formation. Understanding the controls and distribution of reservoir quality is important for the economic success of these tight gas reservoirs in which diagenesis, along with depositional systems and texture, exerts a strong control on pore networks and rock mechanics.

Sandstone in the Williams Fork consists of feldspathic litharenite to litharenite cemented by varying amounts of quartz, calcite, Fe-dolomite, illite-smectite, chlorite, and sparse siderite, pyrite, Fe-calcite, kaolinite, sphene, and gypsum. Framework grain composition is a major control on compaction and on the authigenic phases precipitated. Grain-coating clays are more common in the volcanic grain-rich Upper Williams Fork where alteration of these grains caused the precipitation of authigenic clays. Fe-dolomite cement is found only in the deeper marine-influenced intervals in which dolostone fragments are present.

Twelve lithofacies are identified based on cement types, grain populations, and clay matrix content. Most common lithofacies are chlorite-cemented sandstones, illite/smectite-cemented sandstones, tightly calcite-cemented sandstones, tightly Fe-dolomite-cemented sandstones, clay matrix- or pseudomatrix-rich sandstones, quartz-cemented sandstones with mica-rich laminations, sandstones with thin detrital clay coats and abundant quartz cement, and quartz- & Fe-dolomite-cemented, hydrocarbon-stained sandstones. The sandstones with the poorest reservoir qualities are tightly cemented with carbonates and quartz or rich in clay matrix. The best reservoir quality sandstones are those with grain-coating clays which inhibited precipitation of quartz cement and preserved primary pores.

The lithofacies were correlated to log responses in order to predict the reservoir quality directly from well logs. Sandstones with the best reservoir quality can easily be identified based on low bulk density log values. Intervals cemented with carbonates are identified by high bulk densities. Clay matrix- and mica-rich samples have high gamma-ray and bulk density values. Presence of abundant K-feldspars in the upper intervals results in high gamma-ray readings even in the clean (clay-matrix free) sandstones. This study suggests that careful petrographic assessment of lithofacies heterogeneity can be up-scaled by correlation with log properties to yield tools for field-scale reservoir quality prediction.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009