Outcrop study of secondary porosity in the mid-Ordovician Trenton Dolomite of northern Illinois and its implications for reservoir characterization and development

Based on an extensive study of roadcuts, quarries, mines, caves, and springs in Northern Illinois, secondary porosity in the Trenton (Galena) dolomite can be subdivided into three types : matrix, fracture, and conduit. Secondary matrix porosity is present as small vugs and vesicles resulting from volume reduction during hydrothermal dolomitization. Fracture porosity occurs in northeast and northwest-trending vertical fracture sets as well as in a horizontal bedding plane fracture set. All three fracture sets are arranged in an orthogonal pattern and were emplaced as a result of orogenic compression and extension. Vertical karst conduits are present at the junctions of the two vertical fracture sets. Horizontal karst conduits are found at the junction of the horizontal fracture set and a vertical fracture set. Study of the fracture and conduit network shows the presence of 1st, 2nd, 3rd, 4th, and 5th order fractures and conduits, ordered in a "logarithmic" base ten arrangement. All three types of secondary porosity in the Trenton have been enhanced by karst processes, either meteoric or hydrothermal. Meteoric karstification occurred post-Trenton as well as post-Paleozoic, while hydrothermal karstification occurred during the Pennsylvanian. Vertically, maximum dissolution occurred above and below minor shale and bentonite layers in the Trenton as well as directly beneath the Maquoketa shale cap. In the horizontal plane, maximum karstification occurred along synclinal axes as well as near major faults and fractures. The best reservoir porosity, therefore, occurs in the top 50 to 100 feet of the Trenton, as well as in linear trends along the fault and fracture zones. This is true for the Michigan Basin and appears to be true for the Illinois Basin as well.