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Permeability from Rock Fabric--Log Response Relationships in Carbonate Reservoir Rocks

Jerry F. Lucia

The permeability of carbonate reservoir rocks is related to interparticle porosity and particle (grain or crystal) size in the absence of touching-vug porosity. Total porosity is normally composed of interparticle and separate-vug porosity and can usually be measured by neutron and density logs. Measuring particle size and separate-vug porosity from wireline log response is difficult, and two methods have been investigated.

A shoaling-upward sequence from fossiliferous wackestone to ooid grainstone is present in the Wolfcampian of the Palo Duro basin, Texas. Both the total porosity and the separate-vug porosity increase vertically.

A crossplot of acoustic transit time and porosity has been calibrated for separate-vug porosity. Since the interval is 100% water bearing, the lithologic factor (m) can be calculated from resistivity logs using the Archie equation. The lithologic factor has been related to separate-vug porosity through the ratio of vuggy porosity to total porosity. Permeabilities calculated by both methods match core data reasonably well.

Shallowing-upward tidal flat cycles are common in the San Andres/Grayburg of west Texas. In the Dune field, vuggy porosity is not abundant and the principal control on permeability is particle size and interparticle porosity. Although particle size tends to increase vertically through the subtidal-intertidal sequence, the upper grainstones are generally cemented. Plots of resistivity versus porosity show that oil saturation above the water level is controlled by particle size and interparticle porosity as is the permeability. The permeability profiles which result from this relationship show excellent agreement with core permeability profiles.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.