Image analysis and estimation of porosity and permeability of Arnager Greensand, Upper Cretaceous, Denmark

Arnager Greensand consists of unconsolidated, poorly sorted fine-grained, g lauconitic quartz sand, often silty or clayey, with a few horizons of cemen ted coarse-grained sand. Samples from the upper part of the Arnager Greensa nd were used for this study to estimate permeability from microscopic image s. Backscattered Scanning Electron Microscope images from polished thin-sec tions were acquired for image analysis with the software PIPPIN(R). Differe nces in grey levels owing to density differences allowed us to estimate por osity, clay and particle content. The images were simplified into two phase s, pores and particles, and the specific surface of the solid phase was cal culated. We used the Kozeny Equation to calculate the permeability. The pet rophysical properties, porosity and permeability obtained from image analys is were compared to results using laboratory methods. The 150x magnificatio n of the image can not resolve the microporosity within the clay fraction, so we suggest that the imaged porosity at 150x magnification is close to th e effective porosity for permeability assessment. The Heporosity, however, represents the total porosity of the Arnager Greensand. For permeability es timation, a local permeability was calculated for each image. For calculati on of the plug scale permeability, we compare three different averaging met hods: arithmetic, harmonic, and geometric mean. In every case the calculate d permeability overestimates the measured permeability. Only the lowest cal culated local permeabilities corresponds to the measured permeabilities, su ggesting that the overall permeability for these samples is governed by the least permeable parts. (C) 1999 Elsevier Science Ltd. All rights reserved.