Subsurface Distribution of Silurian Reef/Bank Facies in East Central Illinois

Zohreh Askari Khorasgani, Zakaria Lasemi, and Donald G. Mikulic
Illinois State Geological Survey, Institute of Natural Resource Sustainability, University of Illinois, Champaign, Illinois, [email protected]

Drill cuttings and geophysical logs from 21 wells in Champaign, Coles, Cumberland, Ford, and Douglas Counties were examined to determine the presence, thickness and distribution of reef and non-reef facies within the Bainbridge Group (Silurian) rocks in east central Illinois. Reef or carbonate bank facies in the Silurian have been documented previously from cores and quarry exposures in the Villa Grove Quadrangle, Douglas County. The reef/bank facies in the Villa Grove Quadrangle consists of a massive, porous and fossiliferous dolomite that is capped by a highly porous, pale brown dolomite. The cuttings in the study area indicate that the reef/bank facies can also be recognized outside the Villa Grove Quadrangle. The reef/bank facies typically is a light gray, vuggy to very vuggy, pure dolomite that contains fossil-moldic porosity, and is comprised of bryozoans, crinozoans, and other calcareous organisms. It typically lacks chert and is a very high purity dolomite. The reef/bank facies is not present in all the wells examined and where present, the thickness is variable. It appears to be better developed along a line running north and south from the Tuscola quarry in the Villa Grove Quadrangle, but thins or is absent in areas to the east of this line. Locally, the reef/bank facies is quite thick; resembling an individual reef structure, as opposed to the more blanket-like carbonate platform rocks, but further work is needed to verify this conclusion. This reef distribution pattern may be useful for delineation of other reefs in the known petroleum-producing areas in west central and southern Illinois.

AAPG Search and Discovery Article #90095©2009 AAPG Eastern Section Meeting, Evansville, Indiana, September 20-22, 2009