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New Insight into the Reservoir Architecture of Silurian (Niagaran) Pinnacle Reefs in the Michigan Basin

G. Michael Grammer1, A.E. Sandomierski2, W.B. Harrison III1, D.A. Barnes, and R. Gillespie1

1Michigan Geological Repository for Research and Education, Western Michigan University, Kalamazoo, MI 49008

2ExxonMobil Production Company, Houston, TX 77002

Silurian-aged (Niagaran) pinnacle reefs have been productive in the Michigan Basin for over 60 years, but extensive lateral and vertical heterogeneity in the reservoirs may limit primary production to as little as 25%. Enhanced recovery efforts have generally been focused upon horizontal or directional drilling and waterfloods, but the internal reservoir architecture is often poorly understood which leads to marginal economic success in many reefs. Recent detailed facies analysis from core suggests that vertical compartmentalization in some pinnacle reefs is the result of complex facies variability, and that the vertical distribution of these facies can be constrained, and therefore predicted, within a sequence stratigraphic framework.

The sequence stratigraphic framework of the Miller Fox 1-11 reef, Oceana Co., MI, is characterized by a tripartite hierarchy of sequences, high frequency sequences, and cycles. Large-scale sequences (90-120 ft) correspond reasonably well to the commonly accepted "pinnacle reef model" in the Basin which describes an overall shoaling from mud mound to coral-stromatoporoid framework reef, to a restricted marine algal/stromatolitic unit which is ultimately capped with supratidal algal mats and evaporites. Smaller scale high frequency sequences (35-50 ft) and cycles (3-10 ft), however, consisting of shoaling upward packages bounded by low permeability facies, result in the potential for vertical permeability baffles or barriers within the overall "pinnacle reef" complex. Because there is a distinct correlation between various facies types and porosity/permeability values within these higher resolution packages, enhanced understanding of how these facies are distributed should result in more effective primary and enhanced production efforts.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York