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At the same time it was recognised that the maceral system that had been developed for coals could be applied with minor changes and extensions, to the organic matter in all post-Silurian sequences. These extensions lead to a change in name for the study of organic matter by petrological methods from coal petrology to organic petrology. This newer term also embraces coke petrology as well as the petrology of organic matter in rocks other than coal.Vitrinite is not present in sequences that are Silurian, or older, because land plants had not yet evolved. However, the remains of graptolites and chitinozoans provide a substitute for vitrinite in rocks of Silurian, Ordovician and Middle and Upper Cambrian age. Alginite is present back into the Precambrian and where these are at low levels of maturation, fluorescence properties can be used to assess maturation levels. Additionally, many occurrences of lamalginite develop measurable reflectances and these can be used to assess maturation even if they cannot be directly correlated with vitrinite reflectance. Bitumen reflectances can also be used, so that it is possible to use organic petrology techniques for rocks of Precambrian age.Organic petrology studies for the oil industry greatly increase the range of rank or maturity that are commonly studied. The coal industry is still dominated by production of coals of bituminous rank although coals of higher and lower rank are also mined. Even within this rank range, the coal industry is dominated by coals of medium to low bituminous rank. By contrast, many studies for petroleum span a range from low rank brown coals to meta- anthracite. The development of exploration for shale gas has placed a marked emphasis on exploration of sedimentary sections lying within the anthracitic range of rank.Extension of the techniques to oil industry studies has resulted in a need to develop the techniques to cope with a much greater range of vitrinite types and to treat a very wide range of rank as a normal outcome. It has also lead to a greater understanding of the three dimensional distribution of rank within sedimentary basins. Modelling of maturation, has also lead to an understanding of the timing of coalification. Techniques such as apatite fission track analysis (AFTA) provide directly determined data on the timing of some specific temperature markers. Integration of various types of data has also become extremely important.2.3 PETROGRAPHIC COMPOSITION OF ORGANIC MATTER 2.3.1 TYPEType is determined by the vegetation and the nature of plant, and to a lesser extent animal material contributed to the sediment and to the extent of early diagenetic alteration. This early digenetic alteration has been most intensively studied in relation to peat. Organic matter type is described in terms of the entities that can be distinguished with an optical microscope.
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