The integration of saturated and desaturated NMR responses and air-mercury 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 um to <2 um) resulting in very fine microcrystal line 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 20-25 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.