Date of Award

12-2008

Degree Name

Master of Science

Department

Geosciences

First Advisor

Dr. G. Michael Grammer

Second Advisor

Dr. William B. Harrison III,

Third Advisor

Dr. Rob Gillespie

Access Setting

Masters Thesis-Campus Only

Abstract

Detailed sequence stratigraphic analysis of Silurian (Niagaran) pinnacle reefs in the Michigan Basin provides insight into lateral and vertical variability of reservoir facies observed in the subsurface. Detailed core and wireline log analysis show that reefs consist of at least two orders of cyclicity. This sequence hierarchy is manifested by 3rd order sequences (10's of meters thick) controlled by globally recognized sea level changes, and 4th order high-frequency sequences (few meters thick) driven by relative sea level variations. Local changes in relative sea level are likely controlled by higher frequency eustatic variations along with subsidence and autocyclic mechanisms related to reef growth. Facies close to 3rd order boundaries have higher porosity and permeability due to preferential dissolution and porosity enhancement associated with well-developed exposures. In contrast, facies close to 4th order HFS boundaries have lower porosity and permeability due to early cementation and porosity occlusion. Understanding of the sequence hierarchy provides additional insight into the episodic growth of reefs relative to sea level fluctuations, and better explains, and predicts, vertical heterogeneity observed in these carbonate reservoirs.

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