Reservoir Characterization of the South Buckeye Oil Field, Dundee Limestone, Michigan Basin

By

BARNES, DAVID A., GILLESPIE, ROBB, and HARRISON, WILLIAM B., III

Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008; [email protected]

Commercial accumulations of oil were first discovered in the South Buckeye Oil field, Gladwin County, Michigan, in 1935. To date, the field has produced nearly 5.5 million barrels of oil from carbonate rocks of the Middle Devonian Dundee Limestone Formation. Secondary oil recovery is currently in progress through a water flood program involving water injector and oil producer wells throughout the field.

     Drilling technology during the initial “boom” years in the ‘30’s and ‘40’s (cable tool drilling) involved simple percussion of the subsurface rock formations using a drill bit at the end of a wire line without blow-out prevention apparatus nor effective sampling procedures in use. This technology resulted in the termination of well drilling within the top few feet of an oil-bearing rock stratum with little geological information recovered from the oil reservoir. In general little was known about the geological properties of the reservoir rock formation, even in oil fields like South Buckeye that were highly productive and economically significant. The urgency of oil resource needs during the war years of the 1940’s further de-prioritized the collection of scientific data necessary to understand and efficiently produce the South Buckeye reservoir and other Michigan oil fields. The efficiency of oil recovery was probably well below 30% of the original oil in place in South Buckeye up to the 1970’s when only stripper wells were in operation.

     Wiser Oil Company initiated an enhanced oil recovery (water flood) program in the early 1970’s in South Buckeye. In the course of this undertaking, Wiser drilled a number of new wells using modern mud-rotary drilling technology and acquired modern wire-line logs (about 40) and core samples (about 25) from the field. Careful study of this material in the formulation of an enhanced recovery program was never fully completed and the field ownership subsequently changed in the 1980’s. A reservoir study was contracted by the new operators, Cronus Energy, at this time in order to more carefully evaluate the spatial distribution and reservoir properties. Results of this study suggested an isotropic and radial distribution of reservoir properties. Enhanced (water flood) recovery was generally not successful and termination of production from the field was considered.

     This study evaluated the sedimentary facies present in the Dundee Limestone in South Buckeye field on the basis of core sample material and wire line log analysis. Five sedimentary rock facies were identified from core.  The boundstone, grainstone, and dolomitic, silty wackestone facies (in decreasing significance) were found to possess sufficient porosity and permeability to be considered reservoir facies. These reservoir facies were found to be laterally and vertically discontinuous over the field area consistent with an initial distribution controlled by sedimentary depositional environments in a shallow marine, carbonate shelf or ramp environment in the Michigan basin in Middle Devonian time.

     The spatial distribution of the boundstone reservoir facies (stromatoporoid “reefal” strata) was considered to be the most significant factor in the reconfiguration of the location of water injector and oil producer wells. Both elevation of the oil bearing strata (geometry and elevation of the Dundee Limestone top surface and position of the oil water contact) and the thickness of this sedimentary facies are important in evaluation of water flood production procedures in the field. Higher elevation and greater thickness of the boundstone facies are the key factors indicating the potential for addition oil production through water flood. Lateral continuity of the main reservoir rock type (boundstone facies) is also fundamental in the configuration of the water flood program.

     The main conclusion of the study is that the boundstone bodies have a laterally discontinuous geometry between the northern lobe to the southern higher elevation area of the field. Initial distribution of reefs during deposition of the Dundee Limestone was laterally discontinuous with formation of a probable windward, arcuate reefal body to the west. Cross sections indicate that the northern lobe (despite generally lower initial production and lower structural elevation) may have the greatest potential for enhanced recovery and that the location of injector wells near the periphery of the boundstone facies “thick” in the northern lobe and producer wells in the middle of the lobe would be most effective.