Effect of Grain Size on Porosity Revisited

Abstract

Abstract The sand grains of unconsolidated hydrocarbon bearing formations have been found to be well sorted. Porosity which is a very important parameter in estimating hydrocarbon reserves partly depends on the pore spaces between sand grains. In this work, the porosity values of well sorted and moderately sorted sands obtained from three different locations in the Niger Delta were determined. The results showed that porosity increases with decreasing grain size. It was generally observed that porosity increased from about 26% to 42% as the sand grain sizes decreased from 1000microns to 45microns. Theoretically, grain size does not affect porosity for well sorted grains but in nature, sand grain size affects porosity probably because sand grain deformation from a spherical shape increases with decrease in particle size. It is therefore suggested that attempts be made to use naturally deposited sand grain sizes to predict porosity based on the premise that particle shape affects porosity and particle sphericity decreases as grain size decreases and this accounts for the increase in porosity with decreasing grain size.