Porosity in crystalline rocks – A matter of scale

Abstract

The pore space (porosity) in a crystalline rock is the sum of microfractures, intragranular pores and fluid inclusions. The total porosity can be divided into connected (characterized by flow and diffusion) and unconnected porosity (due to isolated pores). The different porosities are distinguished by different methods of measurement but are dependent on sample size and shape. The present study presents porosity measurements carried out on samples from a drill-core from the porphyritic Äspö quartz monzodiorite, SE Sweden. Water saturation measured on eleven 60-mm-long core pieces has been used to calculate the connected porosity. This ranges between 0.32 and 0.44 vol.%. Some of the 60-mm samples were subsequently cut into 35 slices of various thicknesses (3 mm, 6 mm, 10 mm and 20 mm), and water saturation was measured on all these. On average, higher connected porosity values were measured in the thin slices compared with the 60-mm-thick samples. This is mainly because the thinner samples have a higher relative portion of pores created by mechanical failure than the thicker samples; this is also due to a higher portion of microfractures which have lengths exceeding the thicknesses of the samples. However, the variation is much larger for the thin sample (values between 0.45 and 0.68 vol.% were measured for the 3-mm slices). This larger variation is also valid for the density measurements indicating less relative homogeneity in the thin slices. Eleven of the samples were ground to powder in order to measure grain densities and bulk and grain densities were used to calculate total porosity. The total porosity varied from 0.9 to 1.5 vol.% (∼70% between 0.98 and 1.23 vol.%). The portion of connected pores in relation to total porosity amounts to 33–60% of which the lower values represent 60-mm-thick samples. Most microfractures are orientated along the foliation and are mostly related to K-feldspar and quartz grains.