Using optical-electron correlative microscopy for shales of contrasting thermal maturity

Identifying dispersed organic matter (DOM) types present in shales with the help of a standalone scanning electron microscope (SEM) is often difficult to accomplish. Although reflected light microscopes are useful in identifying the organic matter type, they typically achieve magnification of 500× but cannot magnify beyond 2500×. Optical-electron correlative microscopy, a new technique that has recently gained popularity, involves first identifying the type of organic matter under an optical microscope before finding the same particle under SEM for a closer look at its microstructure. In this study, marginally mature in the early oil window (n = 2) and late mature at the boundary of oil window and condensate wet gas window (n = 2) shales from the Rajmahal (RM) and Jharia (Jh) Basins of India, respectively, were compared using the correlative technique. The source rock properties and the mineralogical composition of the shale samples were first evaluated using Rock-Eval 6 and X-ray diffraction (XRD) analysis, respectively. During electron microscopy analysis, SEM parameters such as brightness/contrast levels, acceleration voltage, and working distance were adjusted to make the DOM and mineral matter present in the shale samples clearly distinguishable from one another. At the acceleration voltage of 15 kV under backscattered (BSE)scanning mode and 10 kV under secondary electron (SE2) scanning mode the DOM and mineral matter were clearly discernible in both RM and Jh shale samples. However, without the correlative technique it was challenging to distinguish between the individual maceral types present in the shale samples, as all appear dark under SEM. Mineral matrix characterization using the correlative technique and energy dispersive spectroscopy (EDS) analysis enabled clay minerals in cell lumens of semifusinite to be identified in both RM and Jh shales. Under SEM, pyrite was associated with vitrinite in Jh shales, which was difficult to notice under the optical microscope. The correlative technique enabled the analysis of the organic matter-hosted porosity under SEM at higher magnifications. The sporinite in the RM shale samples was found to have SEM visible pores, whereas no pores were observed to be present within the sporinite in Jh shales. Both vitrinite and inerinite macerals observed in RM and Jh shale samples lacked any SEM-visible pore structure. The pores were found to be more obvious at a 5 kV acceleration voltage than at 10 kV in SE2 scanning mode.