Analysis of sedimentary organic materials by scanning electron microscopy: the application of backscattered electron imagery and light element X-ray microanalysis

Abstract

Whole rock polished thin sections, from a variety of sedimentary environments (Carboniferous and Jurassic coals, Toarcian and Kimmeridgian source rocks, Recent upwelling sediments) have been examined using the scanning electron microscope techniques of backscattered electron imagery and light element X-ray microanalysis. Backscattered images allow the recognition of variations in composition between different materials and within individual particles. Light element X-ray analysis enables the detection of carbon, oxygen, sulphur and chlorine contents within individual sedimentary organic particles. The different types of sedimentary organic material have been distinguished on a morphological basis. The conventional maceral classification is not applicable due to the lack of characteristics such as reflectance and fluorescence in the SEM. Osmium tetroxide staining demonstrates that significant concentrations of organic matter are present in some sediments adsorped by clay minerals. The results show that backscattered electron imagery can be related to the composition of sedimentary organic matter, revealing overall differences in elemental content. Qualitative light element X-ray microanalysis results show that sulphur uptake is related to kerogen type, with a correlation between sulphur and oxygen contents. The latter may be attributable to uptake of sulphur by oxygenated functional groups. The concentration of organic sulphur between homologous particles varies millimetre by millimetre in source rock samples, indicating a lamina scale control on diagenetic processes. This study demonstrates that surface analytical techniques, such as backscattered electron imagery and light element X-ray microanalysis, can offer a new insight into the relationship between the optical petrology of a sample and its geochemistry.