Sandstone Pore Aspect Ratio Spectra from Direct Observations and Velocity Inversion

Abstract

Measurements of pore shapes from Scanning Electron Microscope (SEM) images for three sandstone samples (the Navajo Sandstone, the Weber Sandstone, and the Kayenta Sandstone) are compared to the aspect ratio spectra obtained from inverting laboratory velocity versus pressure data using the method of Cheng and Toksoz (1979). The results indicate that the inversion method is in very good agreement with the observations at high aspect ratios ( ex > 0.0 1). At low aspect ratios the agreement is very good for the clean Navajo Sandstone sample, but poor for the Weber and Kayenta samples which contain clay. The Navajo sample is composed chiefly of quartz with significant pressure dissolution apparent along grain contacts resulting in smooth, flat cracks between grains. The Weber and Kayenta samples have rougher crack surfaces as well as tapered pore edges, indicating that asperities ,and non-elliptical pore shapes may result in an over estimation of low aspect ratio cracks by velocity inversion. The presence of degraded feldspars may also play a role.