Date of Award


Degree Name

Master of Science


Geological and Environmental Sciences

First Advisor

Dr. Donald M. Reeves

Second Advisor

Dr. William B. Harrison III

Third Advisor

Dr. Peter Voice


A-2 carbonate, reservoir, characterization, structure, stratigraphy

Access Setting

Masters Thesis-Open Access


The Overisel and Salem gas storage fields of southwest Michigan annually store 34 Bcf of natural working gas in the upper dolomitized portion of the Silurian A-2 Carbonate. Porosity and permeability in these fields is thought to be enhanced by fracturing and dolomitization associated with the dissolution and collapse of underlying salt units. This study utilizes wire-line logs from 383 wells to explore the structural and stratigraphic controls on the deposition and diagenesis of the A-2 Carbonate in southwest Michigan. Core and thin sections from 4 wells serve as the primary data for investigating the depositional and diagenetic history of the A-2 Carbonate. A three-dimensional static reservoir model of the Overisel field is generated from petrophysical porosity and permeability values from 77 wells. Production and well test data are used to investigate larger scale permeability trends and the potential role that fractures may have on the Overisel reservoir. Structure of the A-2 Carbonate fields in southwest Michigan is likely controlled by a combination of basement features and dissolution of the underlying A-1 Evaporite. Facies analysis of core and thin sections suggest that the A-2 Carbonate was deposited in an arid tidal flat, sabkha environment with diagenesis varying on a regional scale. The 3D static reservoir model indicates three zones in the structurally higher parts of the Overisel reservoir with favorably higher permeability and porosity. Estimated permeability from transient gas flow well tests are, on average, five times greater than core permeability values suggesting that fractures contribute to enhanced permeability in the Overisel reservoir. Analysis of the production and well test data indicate that the structurally higher sections of the A-2 Carbonate are the most productive, and acidizing and hydraulic fracturing of the A-2 Carbonate are key components of reservoir production.