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

Anthony Sandomierski, G. Michael Grammer, and William Harrison
Western Michigan University, Kalamazoo, MI

Silurian-aged (Niagaran) “pinnacle” reefs have been productive in the Michigan Basin for 60+ years, but extensive lateral and vertical heterogeneity limits primary production to as little as 25%. Enhanced recovery efforts are generally focused on water and CO2 floods, or horizontal drilling, but the connectivity of the reefs laterally and vertically is poorly understood and unpredictable, leading to marginal success in many reefs. Niagaran “pinnacle” reef growth has previously been described as continuous growth during a single relative sea level rise. In this model, the characteristic shoaling upward sequence varies from a microbial mound facies at the base, with a stromatoporoid-dominated reef core capped by algal laminites and anhydrites that form a regional seal for many of the reefs in the Basin.

Detailed core analysis within a sequence stratigraphic framework, however, indicates that the overall shoaling sequence is made up of higher frequency depositional cycles, each bounded by exposure or flooding surfaces. These tens of meters to meter scale cycles support an episodic reef growth model controlled by multiple fluctuations in relative sea level, and provides a means to predict reservoir quality since porosity and permeability is often related to primary facies in these reefs. Because many cycles contain reservoir facies bounded by low permeability units, the result is often significant vertical compartmentalization. This core-based understanding of the episodic nature of “pinnacle” reef growth, as well as the vertical facies successions and resulting impact on reservoir heterogeneity, should lead to enhanced predictability of reservoir architecture from wireline log signatures alone.