Kerogen Maturity and Type by Reflected Light Microscopy Applied to Petroleum Exploration
Wallace G. Dow, Dolores I. O’ Connor, 1987. "Kerogen Maturity and Type by Reflected Light Microscopy Applied to Petroleum Exploration", How to Assess Maturation and Paleotemperatures, F. L. Staplin, W. G. Dow, C. W. D. Milner, D.I. O’Connor, S.A.J. Pocock, P. van Gijzel, D.H. Welte, M.A. Yükler
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One of the best ways to study disseminated organic matter in sedimentary rocks is with reflected light microscopy. The technique includes the familiar vitrinite reflectance method for maturity determination as well as kerogen type identification, which more commonly is carried out under transmitted light. Reflected light analysis can be applied over the entire maturity range from Recent sediments to metamorphic rocks whereas transmitted light analysis loses effectiveness in the higher maturity ranges.
The principal advantage of optical methods of kerogen analysis is the ability to discriminate the various components of sedimentary organic matter and to provide the dimension of source bed history to geochemical studies. The various chemical methods of analysis reveal only the present condition of the average kerogen mixture. The best results are obtained when subjective optical studies are used in combination with objective chemical data.
Maturity analysis is essential in revealing whether oil and gas have been generated in source rocks and preserved in reservoirs. It can also be used effectively to help understand the geologic and tectonic history of basins. Kerogen type analysis is required to identify whether a source bed is capable of generating oil or gas. Other applications include determining depositional environments, diagenetic history, provenance of sedimentary debris, and the presence of contamination from caving or drilling mud additives.
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The application of organic matter studies in petroleum exploration had its start with the recognition of “rank” in coal. During the period of 1900–1925, both physical and chemical methods were developed for determination of the degree of low grade metamorphism of particulate organic materials, or palynodebris in coals and other sediments. Measurements of the relative metamorphism (maturation level) which are based on physical properties are generally quick, cheap, and qualitative to semiquantitative. Those based on chemical analyses are less rapid and tend to be more quantitative. Each method has advantages and disadvantages and they often are combined. Papers included in this course consider methods based on particulate organic matter, reflectance, fluorescence, and geochemistry. A method for integrating the data into a three-dimensional model is included.