ABSTRACT: Petrophysics of Bimodal Porosity: Lower Cretaceous Rodessa Limestone, Running Duke Field, Houston County, Texas
ASQUITH, G. B., and A. D. JACKA, Center for Applied Petrophysical Studies, Texas Tech University, Lubbock, TX
The Running Duke field in Houston County, Texas, is developed on an east-west-oriented domal structure with production from the Lower Cretaceous Rodessa Limestone. The reservoir rocks consist of ooid grainstone, ooid skeletal grainstone, and skeletal grainstone.
Micritization of the ooids by early freshwater diagenesis has created intragranular microporosity. The ooid grainstone reservoir, therefore, has a bimodal pore system that consists of intragranular microporosity and intergranular megaporosity. The presence of the bimodal pore system in the ooid grainstone results in high water saturations (55-87%) because the intragranular microporosity is water filled.
To determine the true productive potential of these bimodal reservoirs, the effective water saturation in the intergranular megaporosity and the amount of effective intergranular megaporosity were determined by combining mercury injection and petrographic analysis with log analysis. First, the percentages of both the micro- and megaporosities were determined using mercury injection and petrographic analysis. Then, total water saturation was corrected to effective water saturation. Uncorrected total water saturations for the ooid grainstones averaged 69.4%. The corrected effective water saturations averaged 34.9%. The much lower effective water saturations calculated by a combination of log analysis plus mercury injection and/or petrographic analysis indicates the true productive poten ial of these bimodal reservoirs.
AAPG Search and Discovery Article #91014©1992 AAPG GCAGS and GC-SEPM Meeting, Jackson, Mississippi, October 21-23, 1992 (2009)