Quantitative estimate of interstitial clays in sandstones using Nomarski differential interference contrast (DIC) microscopy and image analysis

The Nomarski differential interference contrast (DIC) microscopy in the reflected-light visualizes a virtual surface relief of rock components through varying resistance to grinding and polishing of different mineral phases. In the case of sandstones, illustrated here with an example of the Cergowa sandstones ( Lower Oligocene) from the Flysch Carpathians, the used analytical technique depicts a homogeneity with no or very weak relief in the surface morphology of all components besides clay particles, where a virtual relief is observed. Thereby, the Nomarski DIC imaging proved to be very useful in a distinction between an interstitial matrix and other binders of sedimentary rocks. Such separation with standard petrographic examinations is often far from clear which causes usually an overstatement of the matrix content and an incorrect rock classification. Such monotonous picture of sandstones as from the Nomarski DIC microscopy seems to be easy to apply in an image analysis. In the case of the Cergowa sandstones, an automatic extraction of clay matrix in an image analysis was disturbed by numerous replacements of some minerals ( especially feldspars) by clay minerals that were detected as a matrix with an illusory relief. The applied manual approach in the ImageJ analysis programme pointed out: two of six analysed images were overestimated by means of the polarising microscopy. In both cases, the matrix content was estimated >15%, whilst in the Nomarski DIC images showed contents of 10.5% and 10.2%, what classifies them as arenites, not wackes. The presence of clay matrix in sandstones has a great influence on their petrophysical properties, including the most important porosity and permeability, a sufficient amount of which permits to accumulate crude oil or natural gas under adequate trapping conditions. Therefore, their correct extraction and measurement is so substantial.