Silica Budget for a Diagenetic Seal

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Diagenetic banding commonly occurs in association with zones of abnormal fluid pressures that have been identified as pressure compartments. Although diagenetically banded intervals may contain layers of moderately high porosity, the bands act collectively as a low-permeability unit and are therefore important as potential low-permeability seals for pressure compartments. This study focuses on a diagenetically banded interval in the Middle Ordovician St. Peter Sandstone of the Michigan basin. This interval is located within a large area of anomalous pressures identified by Bahr et al. (this volume) in the deep Michigan basin and is composed of a seal-forming lithology. The banded interval is characterized by millimeter- and centimeter-scale diagenetic banding, with alternating quartz-cemented bands, pressure solution-dominated bands, and porous bands. Point counting techniques and an image analysis system were used to quantify porosity, textural properties, and quartz cement. A theoretical model was used in conjunction with these data to estimate the amount of silica dissolved by intergranular pressure solution. Porosity variations in the St. Peter Sandstone are controlled by the combined effects of quartz cementation and intergranular pressure solution. A silica budget calculated for the banded interval indicates that more silica was dissolved by intergranular pressure solution than is present as quartz cement, suggesting that pressure solution alone could have produced enough silica to account for the banded quartz cement. On a local scale, the banded interval served as an exporter of silica. However, a larger-scale silica budget analysis computed for another well in the same region of the basin indicates that the St. Peter may actually be balanced on a regional scale. Results of this study were used to investigate the controls on diagenetic band formation. No significant correlation exists between porosity and grain size or porosity and sorting in the banded interval, suggesting that depositional textural parameters are not important in controlling the distribution of porosity and cement within the banded interval itself. However, original