Many aeolian sandstone reservoirs contain significant volumes of recoverable hydrocarbons in intervals where the average lamina thickness is well below the resolution of any logging tool. The variability in petrophysical properties of the laminations increases uncertainties and in turn can lead to an underestimation of the hydrocarbon in place. To date estimates of the Archie exponents m and n in thinly laminated sand reservoirs have been based on simplified model structures. Here we illustrate an ability to visualize the anisotropy in aeolian sands at the pore scale via digital microtomographic imaging, and to measure the anisotropy in resistivity via direct calculation of resistivity on the resultant images.
In this study, 3D pore scale imaging of an aeolian core plug exhibiting fine scale laminae (laminations at the scale of 1–2 mm is undertaken via high resolution micro-CT. The full 3D image is obtained at 3.5 micron resolution. The composite image is made up of a 2000 squared voxel cross section (5 mm squared parallel to the bedding planes and a continuous 3 cm length perpendicular to the bedding plane (9,000 voxels. Strong variation in lamina porosity is observed along the length of the core and more than 20 distinct bedding planes are evident. Individual lamina are analysed for Archie?s cementation exponent m and saturation exponent n. A composite m and n is then calculated both parallel and perpendicular to the bedding planes across varying numbers of lamina.
The values of m and n are found to strongly depend on the relative volume fractions of the different lamina and the orientation of the conductivity measurements. Estimates of m and n based on simple averaging are extremely poor. Predictions based on idealized layering (Kennedy and Herrick, 2003 are shown to underestimate the value of m perpendicular to the bedding plane. Extensions to the measurements at varying relative angles between borehole and bedding planes are given and the anisotropy in the permeability of the core is also measured and reported. The results highlight how digital imaging of core material at the pore scale can be used to obtain important petrophysical trends crucial to accurate formation evaluation.