BEST OF AAPG: INTEGRATED RESERVOIR CHARACTERIZATION OF SILURIAN (NIAGARAN) “PINNACLE” REEFS IN THE MICHIGAN BASIN–NEW INSIGHTS INTO A MATURE REEF PLAY (Invited Presentation)

Silurian (Niagaran) reefs are significant hydrocarbon reservoirs in the Michigan Basin, having produced over 490 MMBO and 2.9 TCF of gas. Primary production from these reservoirs is typically low, averaging about 25%, due in large part to the complex internal heterogeneity of the reservoir. Incorporating a detailed sequence stratigraphic framework into the reservoir characterization and geostatistical modeling of these reefs provides an enhanced understanding of the complex lateral and vertical variability of reservoir facies often observed in the subsurface, and should lead to better reservoir prediction at both exploration and production scales.

The sequence stratigraphic hierarchy within the reefs is manifested by 4^th order high frequency sequences (10’s of meters thick), and thinner 5^th order cycles (few meters thick) resulting from a combination of eustatic and 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. In addition to a predictable facies stacking pattern that controls vertical reservoir heterogeneity, the reef complexes show distinct differences between windward and leeward margins, 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 characterized by more gently dipping slopes made up of finer-grained facies that are of poorer reservoir quality.

Reservoir quality in the reef core and capping grainstones are best developed at 4^th order boundaries due to extensive dissolution and resulting porosity development. Capping grainstones in 5^th order cycles that are not associated with 4^th 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.