GUY, WILLARD J., ALAN P. BYRNES, JOHN
H. DOVETON, EVAN K. FRANSEEN
Kansas Geological Survey, Lawrence, Kansas
Abstract: The Prediction of Effective Porosity and Permeability in Mississippian Carbonates, Kansas
The integration of saturated and desaturated NMR responses and airmercury and air-brine capillary pressure analyses with more conventional petrophysical techniques allows the exploration and development geologist to better predict the effective porosity and permeability of producing reservoirs. The Schaben Field in western Kansas, which has produced over 9 million barrels of oil primarily from the Mississippian Osage carbonate, has been extensively studied as a Class 2 USDOE project.
The primary reservoir is a coarsening upward spicule-rich dolomite wackestone-packstone-grainstone deposited on a shallow southwestward dipping ramp. The dominant grain type is sponge spicules and their molds intermixed with a dolomite mudstone. The porosity is primarily moldic, intercrystalline, and intergranular, but may contain a significant number of vugs. Grain or crystal sizes are fine (<100 micrometers to <2 micrometers) resulting in very fine microcrystalline pores. Determination of effective porosity requires additional data than that available through typical well log suites.
The oil column in the Schaben Field is between 35-50 ft and is very discontinuous due to a very heterogeneous reservoir. The integration of NMR responses, which predominantly measures the size of the pores, and the capillary pressure data, which predominantly measures the size of the pore throats allows an excellent evaluation of the pore geometry of the reservoir. The critical component of the NMR evaluation is the T2 relaxation time cutoff which divides the effective and ineffective pore sizes. The T2 cutoff within the Schaben Field is typically about 2025 ms as defined by the point of divergence of the desaturated and saturated cumulative porosity curves. As the T2 values increase, there is an increase in pore size and permeability.
AAPG Search and Discovery Article #90921©1999 AAPG Mid-Continent Section Meeting, Wichita, Kansas