--> Abstract: Stratigraphy, Depositional Environments, and Reservoir-Seal Relationships of the Prairie du Chien and St. Peter Formations (M. Ordovician) in the Michigan Basin, by J. W. Harris, E. R. Goter, M. S. McElhaney, and C. A. Sternbach; #91012 (1992).

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ABSTRACT: Stratigraphy, Depositional Environments, and Reservoir-Seal Relationships of the Prairie du Chien and St. Peter Formations (M. Ordovician) in the Michigan Basin

HARRIS, J. W., and E. R. GOTER, Shell Western E&P Inc., Houston, TX, M. S. MCELHANEY, Pecten International, Inc., Houston, TX, and C. A. STERNBACH, Shell Western E&P Inc., Houston, TX

During the 1980s, the Michigan basin deep gas play targeted previously ignored Middle Ordovician objectives in the Prairie du Chien and St. Peter formations. This sequence consists of up to 1300 ft (400 m) of shallow water, interbedded carbonate and clastic sediments in the basin center that thin to a zero edge along the southern margin owing to erosional truncation and onlap. The major Knox (Sauk) Unconformity, identified by conglomeratic lags in core, can be correlated throughout the basin by gamma-ray markers, and the St. Peter and Glenwood/Black River limestones onlap this unconformity. Stratigraphic, sedimentologic, and petrographic studies of over 300 well penetrations and 125 conventional cores by SWEPI exploration staff between 1981 and 1990 provide a stratigraphic framework f r reservoir and seal prediction as well as lithofacies-based reservoir characterization.

The Prairie du Chien in the north contains 30-100 ft (10-30 m) upward-shoaling and coarsening sequences with shaly, parallel-bedded sandstones overlain by skolithos-burrowed and cross-bedded sandstones interpreted to represent interbar, bar flank, and bar crest environments of shelf sand bars. Upward-shoaling and fining sequences 3-30 ft (1-10 m) thick, common within the Prairie du Chien to the south, consist of wavy to flaser-bedded sandstones grading upward into tan, thinly laminated shaly dolomites. The wavy and

flaser-bedded sandstones were deposited in a variety of subtidal channels and shoals, while the thinly laminated dolomites represent the intertidal to supratidal component of mixed clastic and carbonate tidal flats. The overlying St. Peter contains 5-50 ft (1.5-15 m) thick upward-coarsening and shoaling sequences with medium gray shaly carbonates and variegated shales grading upward into shaly, parallel-bedded, churned, and diverse-burrowed sandstones capped by skolithos-burrowed and cross-bedded sandstones. The good lateral continuity of these sequences suggests subregional transgressive events producing deeper marine shale and carbonate, followed by progradation of shelf to shoreface sandstones.

The best reservoirs, cross-bedded and skolithos-burrowed sandstones, typically average 6-10% porosity, 10-20 md permeability, and 25-35% water saturation on structure. Shaly sandstones, including parallel-bedded, churned, and diverse-burrowed facies, although higher in average porosity (8-14%), make poor reservoirs on small structures because of low permeability (0.5-5 md), and high water saturations (55-75%) caused by excessive clay in the pore network. Flaser- and wavy-bedded sandstones also make poor reservoirs except for thin, high-permeability lenses caused by diagenetic enhancement through dissolution of carbonate or quartz overgrowth cements. Both tidal-flat shaly dolomites in the Prairie du Chien, and transgressive shales and carbonates of the St. Peter locally act as top seal for 50-200 ft (15-60 m) gas columns, resulting in multiple pay zones in numerous Prairie du Chien and St. Peter fields.

 

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