Petrophysical investigation of porosity distribution in the Geneva Dolomite Member of the Grand Tower Formation in Marion County, Illinois

Porosity distribution, possible mechanisms for dolomitization and secondary porosity development in the Geneva Dolomite Member (Geneva) of the mid-Devonian Grand Tower Formation in Marion County, Illinois, are examined in this study. The Geneva has been an active petroleum reservoir for over sixty years and has recently provided new discoveries. However to date, no systematic study of these strata has been conducted using petrophysical data.; Three hypotheses are tested in this investigation. The first was that qualitative porosity could be classified using a sparse petrophysical data set. The second was that porosity development, such as the presence of an inferred subaerial exposure interval, fractures, and karst could be assessed using petrophysical techniques. The third was that dolomitization mechanisms could be inferred from petrophysical methods combined with geologic history, lithologic relationships and structural geology.; The petrophysical method developed requires spontaneous potential, resistivity/induction and density, neturon or sonic logs. Three methods---the F-&phis;-M, bulk volume water crossplot, and shallow resistivity/mud filtrate ratio---were used to evaluate porosity. Porosity type was determined by calculating the numeric average of the scores determined by each method.; Porosity classifications showed good correlation to core descriptions of a well in adjacent Fayette County. In addition, porosity classifications appear to correlate well with laboratory permeability measurements from cores.; Porosity classifications were mapped across Marion County at four-foot intervals. Low porosity "tight zones" and fractured strata were indicated at the upper contact of the Geneva, and appear to confirm a subaerial exposure. The causal mechanism for fracturing is believed to result from post-depositional tectonic forces.; A review of published methods suggests that dolomitization of the Geneva was initiated by the Simm's model. Following a marine regression, the process continued under the influence of meteoric continental groundwater flow as described by the model of Toth.; Finally, porosity determined by the petrophysical method correlates with hydrocarbon producing intervals. The petrophysical method developed in this investigation uses historic well logs previously regarded as having little or no value to petroleum exploration. The method developed in this study may prove useful in studies of sedimentation and stratigraphy, petroleum exploration and groundwater resource assessment.