TRAVERSE GROUP RESERVOIRS IN THE MICHIGAN BASIN: A SECOND LOOK

Traverse Group reservoirs have been a prolific source of hydrocarbons in the Michigan Basin since the 1930’s. Early exploration targeted structural traps in these relatively shallow reservoirs (300 to 900 meters). The reservoirs in these fields consists of dolomitized, vuggy carbonates sealed by argillaceous and organic shales of the overlying Antrim Shale.

The Traverse Group in the subsurface of Michigan includes the argillaceous shales of the Bell Shale and shales, dolomites and limestones of the Traverse Limestone. The facies of the Traverse Limestone reflect a shallow water carbonate bank present over much of the Lower Peninsula of Michigan. Facies include grainy oolitic and skeletal sand shoals, patch reefs and reef-associated rubble, muddy lagoonal carbonates, and open shelf deposits consisting of interbedded tempestites and bioturbated, cherty carbonates. Overlying the Traverse Limestone are argillaceous carbonates and dolomitic shales of the Squaw Bay Formation. The contact between the Traverse Limestone and the Squaw Bay Formation is a hardground with pyrite mineralization marking a period of relative sea level rise in the basin. The Squaw Bay Formation was deposited in the outer shelf under more reducing conditions. Up section, the Squaw Bay Formation becomes more argillaceous and exhibits higher gamma ray signatures. This zone transitions into the overlying Antrim Shales.

In productive reservoirs, dolomitization preceded up to the Squaw Bay Formation, which acted as a partial seal to these fluids. Dolomitization generated significant secondary porosity including vuggy and intercrystalline porosity (up to 12% in the Smith-Gerard #1). Grainy carbonates (reef rubble; skeletal, pelletal and oolitic sands) provided permeable pathways for dolomitizing fluids to migrate through the Traverse Limestone if not cemented early.

Historic Production in Traverse Group reservoirs through 1986 was 108 million barrels of oil. Renewed interest in overlooked hydrocarbons is already driving exploration and speculation on the underlying Dundee-Rogers City Formations. These Middle Devonian Reservoirs were exploited prior to modern advances in technology and geologic principles – perhaps it is time to look at Traverse Group reservoirs again as well!