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Outcrop Study of Secondary Porosity in the Mid-Ordovician Trenton Dolomite of Northern Illinois and Its Implications for
Reservoir Characterization and Development*

Dean W. Ekberg1

Search and Discovery Article #50100 (2008)

Posted November 13, 2008

*Adapted from oral presentation at AAPG Annual Convention, April 20-23, 2008

1 Illinois State Geological Survey, Champaign, IL ([email protected])



Begin Abstract section

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












































































Selected Figures

Bedrock geologic map of Illinois.

Generalized stratigraphic column for northern Illinois.

Map of karst and fractures, North Aurora mine.

Settler's Cave map, illustrating influence of fractures in karstification.

Map of MVT fracture mineralization, northwest Illinois.

Cross-section showing Trenton dolomitization, Michigan Basin.

Karst collapse breccia, Trenton, Michigan Basin.

Faulted karst dome, as a hydrocarbon trap.

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  • Identify secondary porosity features in the Trenton at its outcrop area in north-central Illinois.

  • Characterize the fracture and karst conduit network.

  • Develop a karstification model.

  • Identify potential hydrocarbon source beds and traps.


  • Secondary porosity in the Trenton occurs as conduits, fractures, and matrix vugs.

  • Fractures and conduits are arranged in an orthogonal pattern with conduits forming at the fracture junctions.

  • Karstification process


    • Meteoric karstification in post-Trenton.

    • Hydrothermal karstification in Late Paleozoic.

    • Post-Paleozoic meteoric karstification.

  • Source beds for the Trenton are the Maquoketa Shale and the Guttenberg (Decorah) Shale.

  • Hydrocarbons can be stratigraphically trapped in faulted karst domes and faulted and karst synclines.


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Dershowitz, W.S., and H.H. Einstein, 1988, Characterizing rock joint geometry with joint system models: Rock Mechanics and Rock Engineering, v. 21/1, p. 21-51.

Heyl, A.V., A.F. Agnew, C.H. Behre, Jr., E.J. Lyons, and A.E. Flint, 1959, The geology of the upper Mississippi Valley zinc-lead district [Illinois, Iowa, Wisconsin]: in USGS Professional Paper, Report #P. 0309, p. 310.

McGarry, C.S., 2000, Bedrock geology of Boone and Winnebago counties, Illinois: in Illinois State Geological Survey Open File Series Report #2000-3, 1 sheet.

Willman, H.B., and D.R. Kolata, 1978, The Platteville and Galena groups in northern Illinois: in Illinois State Geological Survey Circular, no. 502, p. 75.

Wood, J.R., and W.B. Harrison, 2002, Advanced characterization of fractured reservoirs in carbonate rocks; the Michigan Basin: in Department of Energy Final Report DE-AC26-98BC15100 Accessed 10-13-2008..


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