Porosity Preservation and Development in Prairie du Chien Sandstone, Michigan Basin

Craig J. Peck, R. Douglas Elmore, Peter N. Gale, Bruce Carpenter

The Prairie du Chien Sandstone (also known as St. Peter, Bruggers) in the Michigan basin is an excellent gas and condensate reservoir. A model for porosity preservation in the unit is based on grain-coating clay which apparently prevented the formation of early quartz overgrowths. The model is supported by the presence of prismatic overgrowths on non-clay-coated areas of otherwise coated grains. Areas with clay-coated grains usually have significant interparticle and intraparticle porosity, although other cements (e.g., carbonates) are present. Some interparticle pores result from dissolution of potassium feldspar or cements. The absence of clay is generally associated with abundant quartz overgrowths. In highly silica-cemented rocks few other diagenetic features are obse ved, because silica cement reduced permeability and prevented other diagenetic events from occurring.

Numerous correlative horizontal U-shaped porosity profiles are found on open-hole well logs from the unit and represent increasing followed by decreasing porosity. These profiles are related to regressive depositional sequences, which coarsen and/or clean upward from shaly carbonates to clean sandstones. We interpret the lower part of these sequences to contain relatively abundant clay which occludes pores. Vertically, the clay content decreases until there is only enough clay to coat grains. This coating prevented quartz overgrowths from forming, thereby preserving porosity. At the top of the sequences very little clay is present; grains are not coated, and the sandstone is highly silica-cemented.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.