ABSTRACT: Silica Budget of Diagenetically Banded Tight Zones, St. Peter Sandstone, Michigan Basin, U.S.A.

DRZEWIECKI, PETER A., L. D. SHEPHERD, J. M. BAHR, A. SIMO, and B. WINTER, University of Wisconsin-Madison, Madison, WI

Diagenetically banded sandstones from low porosity and permeability seals in the deeply buried (2 to 3 km) Middle Ordovician St. Peter Sandstone of the Michigan basin are characterized by alternating centimeter- to decimeter-scale layers of silica-cemented sandstone and porous sandstone. Bands are often localized by textural heterogeneities, but they also occur in homogeneous sandstone, without apparent control by grain size or other sedimentary fabrics. Bands occasionally cut cross-bedding.

The amount of quartz cement and the amount of silica released by intergranular pressure solution (IPS) was determined by point counting (600 points per thin sections) and modeling of chemical compaction. Based on our study, the silica released by IPS can account for all of the quartz cement present in the St. Peter. For

example, the St. Foster 1-12 core averages 4.7% quartz cement and 8.4% dissolved silica, and the Sun Mentor "C" 1-29 core averages 5.9% quartz cement and 5.7% dissolved silica.

Diagenetic processes can result in the creation of tight zones without a stratigraphic template. A self-amplifying feedback model has been proposed as an explanation of diagenetic banding through local redistribution of quartz from zones of high IPS to zones of low IPS. Our research implies that a local source of silica was available to form low permeability banding in an initially homogeneous, porous sandstone. However, oxygen isotope data from quartz overgrowths suggest that the quartz-precipitating fluids were meteoric in origin and infiltrated into the St. Peter.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)