Monday, March 31, 2008 : 2:40 p.m.
Secondary Porosity Development in the Karstic Galena/Platteville (Trenton/Black River) Dolomite on the Wisconsin Arch : Implications for Triple Porosity Ground Water Flow
Secondary porosity in the karstic Galena/Platteville 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 implaced as a result of tectonic 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 first, second, third, fourth, and fifth order fractures and conduits; with apertures, diameters, and spacings increasing from one order to the next logarithmically.
All three types of secondary porosity in the Galena/Platteville dolomite have been enhanced by karst (dissolution) processes, either meteoric or hydrothermal. Meteoric karstification occurred post-Galena as well as post-Paleozoic, while hydrothermal karstification occurred predominantly during the Pennsylvanian. Vertically, maximum dissolution occurred above and below argillaceous layers in the Galena/Platteville 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.
Triple breaks on spring hydrographs and pump drawdown curves seem to indicate a sequential flow of fluid from the three porosity types. Initial flow to wells and springs comes almost exclusively from karst conduits until a certain pressure differential between the conduits and fractures is met. Flow from the fractures is then initiated and continues until a certain pressure differential between the fractures and matrix is reached, at which point flow starts coming from the matrix storage.
Dean W. Ekberg, Northern Illinois University Dean W. Ekberg is a Petroleum Geologist and Hydrogeologist with the Illinois State Geological Survey. Mr. Ekberg is a PhD Candidate in Hydrogeology at Northern Illinois University; and has received his M.S. degree in Geological Engineering from the University of Missouri – Rolla, and his B.S. degree in Geology from Wheaton College. Prior to joining the ISGS, Mr. Ekberg worked as a hydrogeologist for Southern Baptists in Africa, as a Research Assistant in Hydrogeology at Fermilab, and as a hydrogeologic consultant for Vulcan Materials in northeast Illinois. Mr. Ekberg’s research has centered around fluid flow in karst carbonates and fractured metamorphics.