Multiscale synchrotron microtomography imaging of kerogen lenses in organic-rich shales from the Norwegian Continental Shelf

Abstract

Kerogen lens shape and size distribution control how organic-rich shales may behave as either source or seal rocks. Prior to thermal conversion, kerogen is a brittle, load-bearing constituent of the shale matrix. During thermal maturation, kerogen lenses become more ductile, and hydrocarbon expulsion may lead to the creation of microfractures, a process controlled not only by temperature and pressure but also by the size and shape of kerogen lenses and their total content in the rock. Here, we use high-resolution multiscale synchrotron microtomography imaging of centimeter-scale shale rock samples collected in two boreholes at different depths in the North Sea and the Barents Sea, respectively. From these three-dimensional microtomography data, we quantify the various shapes of kerogen lenses and discuss how each step of a kerogen lens's life-cycle (i.e. original biological structure, deposition, degradation, and diagenesis) impacted its shape before catagenesis. We quantify the relationship between kerogen volume and the number of kerogen lenses in a given rock volume. The relationship between total organic carbon (TOC) content and the average kerogen lens volume is also measured. For a given rock volume, results show that organic content increases with the number of kerogen lenses up to a point (~8–12 wt% TOC) above which TOC continues to increase, but the number of kerogen lenses decreases. These results combined with kerogen lens orientation may control microfracturing during kerogen maturation.