Characterization and Identification of Kerogen and Bitumen and Determination of Thermal Maturation by Means of Qualitative and Quantitative Microscopical Techniques
Pieter van Gijzel, 1987. "Characterization and Identification of Kerogen and Bitumen and Determination of Thermal Maturation by Means of Qualitative and Quantitative Microscopical Techniques", 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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The kerogen description, determination of the T.A.I. (Thermal Alteration Index) and vitrinite reflectance measurements are widely used to assess the thermal maturation of sedimentary basins, complementary to organic geochemical data. However, their application has also shown some disadvantages. Though not always fully understood, it is often difficult or even impossible to recognize certain constituents, such as alginite and vitrinite, which affects unfavorably the reliability of the microscopical data.
A discussion will be given of the limitations of kerogen description, thermal alteration index and vitrinite reflectance determinations and improvements obtained by applying fluorescence microscopic techniques. These are:
Improved recognition of kerogen constituents and solid crude oil bitumens.
Determination of the maturity of source-beds, particularly of the principle stage of oil generation.
Recognition of kerogen networks of various composition and texture, which play an important role in migration of hydrocarbons from source-rocks.
Determination of geothermal gradient and maturation in the subduction zone in margins of thrustbelt areas.
Special attention will be paid to measuring the fluorescence spectra of sporinite in combination with the vitrinite reflectance which may deliver an exact statement of the thermal maturation. At increasing depth, peaks in the fluorescence spectra gradually shift towards higher wavelengths. A large change occurs in the range between 0.50% and 0.85% vitrinite reflectance (“first coalification jump”), which probably marks the range of principle stage of oil generation. Below this zone, the sporinite fluorescence shifts further into the red and is extinguished at a vitrinite reflectance between 1.20% and 1.35%. This marks the “oil death line”, which is a zone rather than a line.
These quantitative techniques also provide a tool to characterize and identify solid crude oil bitumens and to distinguish these from vitrinite. Further research of all topics mentioned is needed.
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How to Assess Maturation and Paleotemperatures
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.