Application of micro-FTIR mapping and SEM to study compositional heterogeneity of siltstones: Example from the Late...

This paper explores the applicability of micro-FTIR mapping to study heterogeneity of organic matter-lean siltstones. Closely spaced samples of Late Devonian dolomitic siltstones of the Middle Bakken Member were analysed with micro-FTIR, powder X-ray diffraction, and scanning electron microscopy (SEM) to explore the distribution and chemical properties of organic matter (OM), muscovite/feldspar/clay group, carbonates, and quartz, and their influence on porosity and permeability of these rocks. Our results show that quartz is the dominant component of the samples, and the main mineralogical differences between the samples are reflected in the abundance of carbonate minerals. Organic matter content is usually far below 1 wt. % and dominantly represented by terrestrially derived vitrinite and inertinite. Micro-FTIR mapping demonstrates that the more spatially connected quartz and muscovite/feldspar/clays become, the larger permeability in the rock develops, and these correlations are especially strong for planes parallel to bedding. In contrast, carbonate connectivity shows a strong negative correlation with permeability. No correlations between connectivity of components and porosity have been detected. These observations suggest that micro-FTIR not only can document compositional heterogeneity of siltstones, but also has potential to help understanding their permeability systems.