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

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 (similar to 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.

Automated summary

This study investigates the shape and size distribution of kerogen lenses in shale rock samples using high-resolution multiscale synchrotron microtomography imaging, revealing the relationship between kerogen volume and the number of kerogen lenses in rock volume, as well as the relationship between total organic carbon content and average kerogen lens volume.