Stratigraphy and Lithofacies of Foster Formation in Michigan Basin

Barbara A. Luneau, Mark W. Longman, John C. Horne

The Lower Ordovician Foster formation lies in apparent conformity on the Umlor formation and is both conformably and unconformably overlain by the St. Peter Sandstone (Bruggers Formation), the primary target of the Michigan basin deep gas play. The Foster is stratigraphically equivalent to the Shakopee Dolomite and New Richmond Sandstone of the Illinois basin.

The Foster formation is composed of up to 500 m of shallow marine and peritidal deposits divisible into three intervals. The basal interval is relatively uniform in thickness throughout the basin and is recognized by its interbedded lithologies above the relatively massive Umlor formation. The basal interval is primarily dolomite and sandstone with minor anhydrite. The middle interval is thickest in the basin center and progressively onlaps the basin margins. This interval is primarily dolomite and anhydrite. The upper unit extends throughout the basin but is thinner and truncated from above along the basin margins. Shaly dolomite and sandstone are the major lithologies.

Stacked shoaling-upward cycles deposited in shallow subaqueous and wind tidal flat environments occur in each Foster interval and demonstrate the remarkable persistence of peritidal depositional environments throughout the basin during the Early Ordovician. Upper Foster sandstones represent laterally discontinuous marine reworked dune deposits up to 10 m thick. Such sandstones form hydrocarbon reservoirs at Fletcher Pond and Santiago fields. Porous sandstones are also present in the basal Foster, but their potential as hydrocarbon reservoirs is relatively untested.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.