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The microscopic particulate organic matter occurring in sedimentary rocks is referred to as visual kerogen when examined by use of strew slides prepared from a kerogen concentrate. Examination under a high-powered microscope in transmitted light yields information on both the organic matter type present and the level of organic metamorphism (LOM). This presentation concentrates on the LOM aspects of visual kerogen and addresses it from a utilization point of view.
The color of the kerogen, preferably plant cuticle fragments or pollen and spores, is used to determine the level of organic metamorphism. Various scales have been proposed to reflect this change in coloration. The TAI scale is most commonly used. Visual kerogen assessment is considerably less precise than vitrinite reflectance. It is a subjective call made by the analyst. Additionally, the equivalent reflectance range broadens as higher LOMs are attained. However, the ability to visually discern differences in the suite of organic material present can override its drawbacks in precision. Caved versus indigenous populations can be recognized, as can recycled versus primary vitrinite. Thermal history can also be established in sections that are barren of vitrinite. As is the case with early all organic geochemical techniques, reliable interpretations can be make if the limitations of the method are considered and the results are cross-correlated with other methods.
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