Accurate estimation of oil yields in Green River Formation oil shale requires knowledge of the presence or absence of cavities (vugs) left after the natural removal of the mineral nahcolite (NaHCOD3U). Conventional well logs have not provided a means for detecting the vugular porous zone. A 3-arm caliper logging system, logging technique, and mathematical correction method used by the U.S. Bureau of Mines to locate and compensate for the vugular porosity are described. The caliper defines the zone by sensing a high frequency of minor diameter variations as small as 1/10 in. Estimations of oil yields and oil-shale reserves from density logs can be significantly improved when the vugular zones's thickness is known. A porosity-density correction method is presented.