The Eagle Sandstone of Late Cretaceous age is an important conventional reservoir for biogenic gas (isotopically light methane) near the Bearpaw Mountains. A petrologic examination of cores from producing wells was undertaken to establish the petrologic features which significantly affect reservoir quality in a conventional shallow gas reservoir, and to provide a framework for the investigation of lower quality reservoirs on the east.

Petrologic studies indicate that the quality of most Eagle Sandstone reservoirs in the Bearpaw Mountains area is controlled by a consistent sequence of postdepositional events. Most sandstones were tightly sealed early in their burial history by authigenic calcite, which filled intergranular pores and partially replaced some framework grains. In some sandstones, minor quartz cement partly preceded the precipitation of calcite. Subsequent to calcite precipitation, siderite or ankerite formed as numerous patches which were localized by altering biotite. This iron-rich carbonate may occupy as much as 3 or 4% of the rock volume. Later, most calcite was removed through dissolution that resulted in abundant intergranular and intragranular porosity. The newly developed porosity further facil tated the movement of interstitial waters, which produced extreme dissolution effects in susceptible framework grains such as andesine. Later in the burial history of the Eagle Sandstone, clay minerals were formed in intergranular pores and to a lesser extent in intragranular pores. Although kaolin is the dominant clay, iron-rich chlorite and mixed-layer mica-smectite are locally important.

The following conclusions can be made. (1) The highly porous and permeable nature of the Eagle Sandstone in conventional reservoirs is due predominantly to the dissolution of authigenic and detrital components. (2) The formation of dissolution porosity occurred at relatively shallow depths in thermally immature rocks and was not directly related to burial diagenesis of clays in associated shale sequences. (3) The common occurrence of acid-soluble iron-rich phases should be considered when using acid treatments. (4) In some conventional Eagle reservoirs, migration and/or expansion of clay-size components may cause formation damage if clays are not stabilized.

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