Evaluating Controls on the Formation and Reservoir Architecture of Niagaran Pinnacle Reefs (Silurian) in the Michigan Basin: A Sequence Stratigraphic Approach

Sandomierski, A.E., Grammer,G.M. and  Harrison, W.B., III

Carbonate Sedimentology Laboratory, Western Michigan University, Kalamazoo, MI

Silurian-aged, Niagaran (or Guelph) pinnacle reefs have been productive in the Michigan Basin for more than 60 years, but extensive lateral and vertical heterogeneity has limited primary production efforts, resulting in estimates of stranded hydrocarbons as high as 75%. Enhanced recovery efforts are generally focused on water and CO2 floods, along with horizontal drilling, but the connectivity of the reefs in both lateral and vertical dimensions has been unpredictable.  Evaluating the pinnacle reefs with a sequence stratigraphic approach has provided a framework for understanding and predicting the vertical compartmentalization of these reefs.

Based on reservoir-scale observations, restriction of water circulation related to changes in relative sea level was a major controlling mechanism on the vertical facies distribution. Microbial mounds at the base of the reefs are interpreted as forming in a dysoxic environment related to initial flooding of the basin, while stromatoporoid reefs flourished during highstands when normal marine circulation was established.

Vertical facies patterns indicate a general shoaling upward sequence from microbial mound to reef core, followed by tidal and supratidal environments typically capped by impermeable exposure surfaces. The discovery of these vertical facies successions, each bounded by exposure surfaces and evidence of subsequent flooding, has challenged the current dogma of pinnacle reefs as being the result of continuous long-term growth in favor of a more likely episodic growth model. By identifying these cycle and sequence-scale vertical patterns, facies distribution and the resulting reservoir architecture can be better predicted in pinnacle reefs in the Michigan Basin and may also provide insight into similar structures in the geologic record.