Microporosity in the O’Hara oolite zone of the Mississippian Ste. Genevieve Limestone, Hopkins County, Kentucky, and its implications for formation evaluation
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Recent research in carbonate provinces has shown that micritized oolite grainstones may develop a bimodal pore system containing both micro and mega porosities. However, the extent and frequency of bimodal pore system occurrence has not yet been determined. Bimodal pore systems in oolites have been reported in the Silurian Interlake, Jurassic Smackover, and Cretaceous Sligo and Rodessa formations. Recognition of the presence of microporosity has significant implications for accurate assessment of the hydrocarbon potential of oolitic carbonate reservoirs. These reservoirs require specialized petrophysical log analysis because of their unique characteristics
New evidence has identified another formation with a bimodal pore system — the O’Hara oolite zone in the Mississippian Ste. Genevieve Limestone, located in the southeastern portion of the Illinois basin. The Ste. Genevieve Limestone is a major oil reservoir occurring as thin, lenticular oolite grainstone bars. Petrographic and SEM analysis of the O’Hara oolite in Hopkins County, Kentucky, clearly shows micritized oolites with microporosity in the Interstate Drilling Anita Gamm no. 1 well. The intragranular microporosity within the micritized ooids and the intergranular porosity between the ooids form a bimodal pore system. Two stages of cementation occur: marine isopachous fringing calcite cement and fresh water phreatic blocky calcite spar.
Conventional log analysis, employing the Archie water saturation method, indicates high bulk volume water values of .053 in the Anita Gamm no. 1 well. The bulk volume water values suggest that the oolite zone is above irreducible water saturation and will produce water rather than hydrocarbons. However, the opposite happened when the well was produced: initial production was water-free.
The Archie Method produces erroneous data because of the presence of the bimodal pore system. The high bound water content in the oolite’s microporosity causes data distortion. If, however, the Ratio water saturation method is used instead of the Archie Method for determining bulk volume water, the relevant values are significantly reduced. Bulk volume water values become 0.034. Thus, the well can be accurately evaluated as productive. Study in other areas seems to verify that the Ratio Method has greater validity than the Archie Method when microporosity is present, and should be used to avoid bypassing hydrocarbon-productive reservoirs.
KeywordsWater Saturation Ooids Oolite Archie Intergranular Porosity
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