Permeability Estimated from Subsurface Data: Grayburg Formation, Dune Field, Crane County, West Texas
F. Jerry Lucia
Permeability estimation from wireline log data is an important means of extending data obtained from cores and pressure-buildup tests. Complicated pore geometries found in many carbonate reservoir rocks often make a simple porosity-permeability correlation unrealistic. Rock fabric studies have shown that permeability is related to the amount of interparticle porosity (grains or crystals) and to the size of the grains or crystals. Vuggy porosity usually does not contribute to permeability unless it is in the form of caverns or fractures.
Dune field contains little vuggy porosity so total porosity and particle size become the determining factors. Three pore-geometry families are distinguished: intergranular porosity between 300-µm grains in grainstones, intercrystalline porosity between 50-µm dolomite crystals in dolomitized wackestones/packestones, and rocks with an intimate mixture of these two end members. Unique porosity-permeability transform functions for these three pore-type families are determined.
The presence of up to 55% gypsum in Dune field complicates calculations of total porosity. Accurate calculations of the amount of gypsum present could not be made from the acoustic, density, and neutron logs, so the acoustic log was calibrated with low-temperature core analysis data and used to provide porosity data.
Particle size was determined by relating particle size to irreducible water saturation. In the oil column, irreducible water saturation is a function of interparticle porosity and particle size. Assuming similar porosity values, rocks with finer pores will have higher water saturations than those with larger pores. In Dune field, intergranular pores are larger than the intercrystalline pores, and saturation values calculated from wireline logs show lower water saturations for the intergranular pore-type families than for the intercrystalline pore-type families. The mixed family is intermediate.
Permeabilities are calculated from sonic and resistivity logs by determining the pore-type family from the saturation values and the permeability from the porosity-permeability relationship for that family. The resulting permeability profiles compare favorably with core analysis profiles. Permeability logs are correlated assuming parallel beds, and permeability cross sections and permeability-thickness maps are prepared. The results show that the reservoir can be divided into upper and lower members separated by a 25-ft permeability barrier. The permeability in either member varies on scales from 600 ft to 1 mi and can be related to depositional and diagenetic facies.
AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.