Relation Between Hydraulic Conductivity and Texture in a Carbonate Aquifer: Observations


Based on analysis of over 300 injection‐pressure tests and their assignment to logged stratigraphic intervals, it is argued that the Silurian dolomite aquifer in Milwaukee County, Wisconsin, behaves as both a fractured and porous medium. This paradox is resolved by examining the aquifer's individual stratigraphic units and their respective sedimentologic characteristics. Like many carbonates, the aquifer contains shoaling upward sequences and cycling through a variety of depositional environments and textures. Most of the stratigraphic units are fine‐grained mudstones, but interbeds with porous coarsegrained packstones and grainstones are also present. The packstone and grainstone units have unimodal distributions of hydraulic conductivity with geometric means between 10 4 and 10 3 cm/sec. The high matrix porosity masks any effects of jointing in these deposits, and they behave as porous high conductivity media. In contrast, hydraulic conductivities within the mudstones are bimodal, with one mode of very low matrix conductivities and a second higher mode controlled by jointing. Mudstone conductivities increase logarithmically with joint frequency, but never attain the magnitude of the values for the coarse‐grained units. Because the geometric mean hydraulic conductivities of the porous units are up to 1000 times greater than the thicker mudstones, the aquifer's bulk hydraulic conductivity is controlled by the thinner porous zones. Therefore, as a whole, the aquifer behaves as a porous medium even though most of the section is fracture‐dominated. Only the large number of hydraulic conductivity measurements isolated within specific stratigraphic units has allowed recognition of the importance of porous medium conditions in this aquifer. Because upward shoaling sequences are common within carbonates, a similar combination of porous and fractured medium conditions may be present, but unnoticed, in other carbonate aquifers. Investigators working in dolomites with minimal tectonic deformation should be particularly alert for porous medium effects.


Geography, Geology, and Planning

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