Porosity Evaluation of the Geneva Dolomite (Marion County, Illinois) Using Petrophysical Data

Summers, B. R.  and Carpenter, P. J.

Northern Illinois University

Standard methods for evaluation of porosity in carbonate strata require modern porosity logs, i.e. density, neutron and sonic.  However in mature reservoirs these logs are often rare or lacking.  Such is the case in the Geneva Dolomite of the Grand Tower Formation (Devonian) in Marion County, Illinois.  Although the Geneva has been a target of exploration for over 65 years, detailed studies of porosity distribution in this unit have not been undertaken.  The lack of surface exposures of the Geneva has also limited such studies. 

A petrophysical technique has been developed that permits determination of qualitative porosity in carbonate strata from a partial suite of porosity logs. The technique classifies porosity in carbonate sediments and ranks them from intergranular to vuggy.  An investigation of porosity distribution in the Geneva Dolomite in Marion County, Illinois, was undertaken using this technique. Porosity determinations made by the technique show good agreement with core data from the Geneva, and with published data from other reservoirs.  Results indicate a southwest to northeast trending zone of vuggular porosity across the study area.  This technique has potential for the study of carbonates in both petroleum and groundwater exploitation.