Facies-Controlled Porosity Distribution in the Silurian Dolomite Aquifer of the Door Peninsula, Northeastern Wisconsin

Mark T. Harris, Kurt R. Waldhuetter

The Silurian (Llandoverian) Dolomite Aquifer is the primary water source for the Door Peninsula of northeastern Wisconsin. Although typically treated as a fracture-controlled flow system, the aquifer contains significant and predictable porosity variations that are directly related to depositional facies.

The Silurian strata consist of four major inner to middle shelf facies: (1) Cyclical Laminite Facies representing tidal flat complexes, (2) Rippled Packstone Fades deposited under high energy shelf conditions above fairweather wave base, (3) Bedded Coral Facies recording similar conditions, and (4) Brachiopod and Burrowed Wackestone Facies representing shelf areas below fairweather wave base.

The component subfacies that comprise each facies have characteristic porosity types and abundance that dictate the pore characteristics of each facies. The Cyclical Laminite Fades has 1-4% fenestral, mudcrack, mat, and vug porosity. The Rippled Packstone Fades has 1-5% vug porosity. The Bedded Coral Fades has <1% fossil moldic and vug porosity. The Brachiopod and Burrowed Wackestone Facies has 5-8% fossil moldic and vug porosity.

These porosity characteristics allow the facies model to be translated into a porosity model. The result is that the aquifer can be zoned into seven stacked hydraulic conductivity zones. High conductivity zones occur in the lower and upper parts of the Mayville, and Engadine formations.

These results indicate that a "double porosity" model is appropriate for modeling the Silurian aquifer. Fracture-flow models should be superimposed upon a layered hydraulic aquifer model, not a homogeneous and undifferentiated aquifer.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995