Sequence Stratigraphic Control of the Reservoir Architecture in Silurian (Niagaran) Reefs in the Michigan Basin
Detailed sequence stratigraphic analysis of Silurian (Niagaran) reefs in the Michigan Basin provides insight into the lateral and vertical variability of reservoir facies observed in the subsurface. The sequence hierarchy is manifested by 3rd order sequences (10’s of meters thick) controlled by eustatic sea level changes, and thinner 4th order high frequency sequences (few meters thick) driven by relative sea level variations. Incorporating the sequence stratigraphic framework into a 3-D stratigraphic model illustrates the episodic nature of reef growth as exhibited by the stacked nature of framework reef and capping grainstones. This relationship is often characterized by well-developed exposure horizons as well as distinct differences between windward and leeward margins of the reef complex, illustrated in both the reef geometry and the resulting distribution of reservoir facies. Windward margins are steeper due to higher rates of aggradational growth and typically contain better reservoir quality in both the reef core and fore reef facies. In contrast, leeward margins are typically more gently sloping due to progradational growth of finer-grained facies and are generally considered poorer reservoir intervals. The stratigraphic hierarchy plays a major role in controlling the overall quality and vertical heterogeneity of the reservoir units. Reservoir quality in reef and capping grainstones are best developed at 3rd order boundaries due to extensive dissolution and resulting porosity development. Capping grainstones in 4th order high frequency sequences not associated with 3rd order sequence boundaries, however, generally exhibit poor reservoir quality due to extensive cementation and porosity occlusion. Understanding of the sequence hierarchy in these Silurian reefs provides additional insight into the episodic growth of the reefs relative to sea level fluctuations, and provides a means to better predict the lateral and vertical reservoir heterogeneity.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009