Laboratory Characterization of Anisotropic Rocks

Abstract

ABSTRACT A number of layered rock samples with large permeability contrasts has been collected mostly from the western United States. The collection provides samples from a variety of depositional geological environments with bedding thicknesses ranging from millimeters to decimeters. The objective of this study was to investigate the origin of the large permeability anisotropy. To characterize these rocks, conventional laboratory methods have been used such as macroscopic petrographic description, mineralogical analysis using Fourier transform infrared spectroscopy, as well as permeability, porosity, conductivity and acoustic velocity measurements on Hassler cores orthogonal and parallel to the bedding surfaces. Additionally, novel techniques consisting of permeability scans on the rock surfaces with an experimental minipermeameter setup, petrographic image analysis from thin sections, and density profiles obtained from a computerized tomographic (CT) scanner were used. These provided detailed maps of vertical density, porosity and permeability changes within these rocks. Results of CT scanning at two different energies allowed us to simultaneously determine densities and estimate photoelectric factors. All of these rocks are layered and exhibit large porosity changes over very short vertical distances of sometimes as little as one millimeter. Some sandstone samples as well as an interlayered sand/shale sample were found to exhibit permeability anisotropies of more than one-hundred and vertical porosity variations of up to a factor of three. Conductivity anisotropies of these samples, however, were generally less than 1.6, and acoustic anisotropies less than 1.2. The sedimentological origin of these anisotropic rocks is attributed to an interaction of two or more depositional processes during lateral acceration or vertical aggradation, producing vertical variations in mineralogy, grain size and other petrographic characteristics.