PRELIMINARY RESULTS OF A FLUID-INCLUSION STUDY ON FRACTURED CARBONATE RESERVOIRS OF THE DUNDEE FORMATION, CENTRAL MICHIGAN BASIN, U.S.A

The Middle Devonian Dundee Formation is the most prolific oil producing unit in the Michigan basin, with more than 350 million barrels of oil produced to date. Reservoir types can be fracture controlled or facies controlled, and each type may have been diagenetically modified. Although fracture controlled reservoirs produce more oil than facies controlled reservoirs, little is known about the process by which they were formed and were diagenetically modified.

In parts of the Dundee, preexisting sedimentary fabrics have been strongly overprinted by medium to coarse grained dolomite. Dolomitized intervals contain planar and saddle dolomite, with minor calcite, anhydrite, pyrite, and rare fluorite. Fluid-inclusion analyses of aqueous two-phase inclusions in dolomite and calcite suggest that some water-rock interaction in these rocks occurred at temperatures as high as 120 to 150°C. These data, in conjunction with published organic maturity data, are not easily explained by a long term burial model and have important implications for the thermal history of the Michigan basin. The data are best explained by a model involving short duration transport of fluids and heat from deeper parts of the basin along fault and fracture zones connected to structures in the Precambrian basement. These data give new insight into the hydrothermal processes responsible for the formation of these reservoirs.