Petroleum generating source rocks can be lacustrine, marine shale, marine
carbonate or terrigenous/coaly, generally corresponding to Type I, Type II, Type II-S
and Type III kerogen based on source rock screening data (Rock-Eval pyrolysis and
TOC determination) plotted in modified van Krevelen diagrams. Rock-Eval and similar
pyrolysis techniques are relatively cheap and allow analysis of many samples.
However, the pyrolysis method can be problematic as it provides bulk geochemical
data of the kerogen composition, which may lead to incorrect kerogen typing. This is
particularly an issue for rocks with mixed kerogen compositions or for rocks with a
deteriorated source potential due to, for example, slight oxidation of organic matter.
Organic petrography in incident white light and fluorescing-inducing blue light of the
macerals in source rocks adds granularity to the bulk geochemical characterization and
is thus a strong supplementary tool for characterizing kerogen. Petrographic examination
of source rocks can inter alia: (1) enhance kerogen characterization by providing
qualitative or quantitative information on the maceral composition, including the
proportions of oil-prone sapropelic and refractory kerogen, (2) identify mixtures of
organic matter types or even slight oxidation of the sapropelic kerogen due to reduced
fluorescence intensity and thereby prevent incorrect interpretations of kerogen, (3)
document lateral and vertical organic facies variations within source rocks, and (4) provide
evidence for petroleum generation by identifying oil droplets, oil films, solid
bitumen, exsudatinite, micrinite or pyrolytic carbon. Despite the fact that reflected light
microscopy is more expensive, time-consuming and complex than classic geochemical
kerogen typing examination of representative or problematic source rock samples may
provide just the missing piece that is required to better understand source rock composition
and quality.
Keywords: Source rock, Maceral, Kerogen type, Organofacies, Petroleum, Oil,
Organic petrology, Rock-Eval.
