Dielectric constant and electrical resistivity of natural-state cores

Abstract

As part of a program to obtain basic data on the physical properties of the rock in and near uranium-mineralized zones, measurements of electrical resistivity and dielectric constant were made on parts of 27 cores from the Morrison formation in the Colorado Plateau uranium province. For frequencies between 50 cycles and 30 megacycles per second, resistivity ranged from 102 to 1012 ohm-centimeters, and the dielectric constant ranged from 4.0 to more than 10fl . The water content of the cores seemed to be the controlling factor: the high resistivities were associated with low water content and the high dielectric constants with high water content. INTRODUCTION The electrical properties of the Morrison formation in the Uravan mineral belt of the Colorado Plateau have been measured in place by electric logging and in the laboratory on cores with restored water saturation (Keller, 1959). The resistivity of different members of the Morrison formation in drill holes ranged from 8 to 2,000 ohmmeters. The resistivity of the sandstone members of the Morrison was found to be highest in areas that were considered most favorable for the occurrence of ore, probably because of the lower water saturation or lower salinity of the water in the favorable areas. During 1952 a program of natural-state coring was begun by the U.S. Geological Survey in the Long Park area (Manger, Gates, and Cadigan, 1954) to obtain basic data on physical properties of rocks in and near uranium-mineralized zones. Drilling was done using air and oil-base drilling mud so that the water content of the cores could be preserved as nearly as possible in its original condition. These cores offered an opportunity to obtain truly representative values oi the resistivity. Parts of 27 cores of sandstones and mudstones of the Morrison formation, taken from 2 drill holes in the Long Park area, were used in 257 258 EXPERIMENTAL AND THEORETICAL GEOPHYSICS this study. Both dielectric constant and electrical resistivity were measured from frequencies of 50 cycles to 30 megacycles per second.