SCANNING ELECTRON MICROSCOPE OBSERVATIONS OF THE NATURE OF PORE-CONTAINING ORGANIC MATTER IN THERMALLY MATURE MUDROCKS
Controversy exists over whether the pore-containing organic matter is kerogen, solid bitumen, pyrobitumen or possibly any of the three in different cases. Researchers using X-ray-absorption-near-edge-structure (XANES) spectra from scanning transmission X-ray microscopy (STXM) have definitively identified some pore-containing organic matter in gas-window-maturity rocks as being pyrobitumen. However, pores are also observed in organic matter in oil-window-maturity rocks which are too thermally immature to contain pyrobitumen, but do contain solid bitumen. Thus far, solely SEM-based microanalysis cannot conclusively differentiate between kerogen, bitumen, and pyrobitumen, although textural clues provide some insight.
Pores are found in larger, micrometer-scale, grain-like masses of organic matter. It is unlikely based on their size that these represent anything other than kerogen grains. Moreover, patterned arrangement of pores in some organic-matter grains is not consistent with bitumen or pyrobitumen, so it must indicate pore formation in kerogen.
Other textural clues to organic matter type can be ambiguous. Organic matter is commonly seen surrounding grains, in what could be described as a pore-filling morphology. Although this would seem to indicate previously fluid phases such as solid bitumen or pyrobitumen; kerogen can be ductile and has been found in this morphology in very low-thermal-maturity rocks. In another troublesome textural occurrence, micrometer-scale patches of organic matter surround partly euhedral crystals of quartz and/or albite. The euhedral faces of these minerals would seem to require that the crystals grew into a void, which was then filled by bitumen. However, the size of pore this would require is unlikely to be present in a mudrock at conditions where quartz and albite would form or bitumen would be present.