Trapping Styles Associated with Paleovalleys and Interfluves in Unconformity-Bounded Sequences; Glauconitic Sandstone Member (Lower Cretaceous), Southeastern Alberta, Canada

James M. Wood, John C. Hopkins

Recognition of unconformity-bounded sequences in the Glauconitic Sandstone Member (Mannville Group) provides important geologic input for the future development of this oil-bearing unit in southeastern Alberta. Each sequence is thickest where a ribbon deposit lies at its base. The ribbons are up to 45 m thick and 7 km wide, truncate laterally adjacent strata, and cut down from various stratigraphic levels. Each ribbon is interpreted as a paleovalley incised during a drop in relative sea level and infilled with fluvial and/or estuarine deposits during a subsequent rise in relative sea level. In interribbon locations, the sequences are less than 8 m thick, commonly capped by paleosols, and contain sheet sandstones interpreted as fluvial, deltaic, paralic, and shoreface deposits laid down in interfluvial areas during relatively highstands of sea level.

Reservoirs occur in interfluvial sheets and sandstone bodies within paleovalley ribbons. Thick ribbon sandstones are much better reservoirs than thin sheet sandstones. Five trapping styles are recognized in the Turin, Retlaw, Enchant, Little Bow, and Badger fields: (1) ribbon-sandstone reservoir sealed by updip facies change to ribbon shale; (2) ribbon-sandstone reservoir sealed updip by younger, crosscutting ribbon infilled with shale or tight sandstone; (3) ribbon-sandstone reservoirs occurring as a series of erosional remnants below a tight ribbon (younger valley followed trend of older valley); (4) sheet-sandstone reservoir sealed updip by tight ribbon, and (5) sheet-sandstone reservoir draped over a differentially compacted ribbon.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.