The impact of grain-coating chlorite on the effective porosity of sandstones

Abstract

Chlorite grain coatings in sandstones can inhibit quartz overgrowths and preserve porosity in hot reservoirs, but coatings may also introduce high irreducible water saturation and therefore reduce effective porosity. In order to derive a quantitative model to predict microporosity and effective porosity, five sets of sandstone samples with well-developed chlorite grain coatings were studied. Scanning electron microscopy and X-ray diffraction results on each set indicate various types of chlorite morphology and a broad range of chlorite abundance, though the ratio of the average size to the average thickness of chlorite platelets is constant in all samples. Microporosity, defined in this study as the product of total porosity and irreducible water saturation, is correlated to chlorite by volume content and chemical composition: φmicro = 7.08 × Vchl/(0.39 × Vchl + 2.26) + 0.927 for iron-rich chlorite, and φmicro = 7.08 × Vchl/(1.88 × Vchl + 7.83) + 0.927 for magnesium-rich chlorite, where φmicro is microporosity in porosity units (p.u.) and Vchl is volume percentage (%) of chlorite in the bulk sample. When the total porosity for a sample is known, its effective porosity is the difference between the total porosity and the microporosity.