A STATISTICAL APPROACH FOR CONSTRAINING FACIES VOLUMES IN THE NIAGARAN PINNACLE REEFS, MICHIGAN BASIN, USA

Niagaran (Silurian) pinnacle reefs in the Michigan Basin have produced over 500 million barrels of oil and 2.9 trillion cubic feet of gas. The reefs are currently being targeted for gas storage, CO₂ sequestration, and enhanced oil recovery. A newly published facies model, based on core and well log observations from a series of Niagaran reefs, suggests that facies distributions are strongly controlled by paleo-wind direction. The volumetric and geometric variability of reef facies within the basin is not well understood, however. Evidence from modern reefs suggests that facies distributions are controlled by biological, chemical, and physical processes. This means that the facies geometries and volumes are not random, and should fall within a limited range of values. To test the applicability of this model in the Michigan Basin, internal facies distributions were mapped and 3-D geologic models were created for seventeen Niagaran reefs. From these models, geometries and volumes were extracted and used to populate a database. Statistical analysis of the database shows that facies occupy a relatively narrow range of the overall reef volume. For example, the reef core facies (~10% porosity, 88 mD) represents 20-25% of a reef’s total volume, the bioherm facies (~4%, 5 mD) represents 25-35%, back reef facies (~8%, 12 mD) represents 10-15%, and restricted tidal flat facies (~3%, 3 mD) represents 10-12%. In contrast, the fore-reef facies (~18%, 250 mD), which typically represents the best reservoir quality, occupies only 2-3% of the total reef volume. The data also show that larger reefs are more commonly found in shallower water, and that these larger reefs exhibit a greater degree of heterogeneity than deeper reefs. Although the diagenetic overprint significantly impacts porosity and permeability of these carbonates, the ability to constrain facies volumes can help to reduce geological uncertainty in fields with poor core and log control.