Visual kerogen & vitrinite reflectance
Information about the type and maturity of organic material present in rock samples can be obtained by optical examination of maceral populations under a microscope, making use of properties such as reflectance, colour, shape and relief (or polishing hardness). Studies may be confined to measurement of vitrinite reflectance for maturity evaluation, or make more extensive use of the additional information gained about kerogen type from a general visual inspection of macerals.
For coals, the whole sample comprises macerals (small amounts of pyrite, ash and clay may also be present), but for dispersed kerogen some concentration of the organic matter may well be required in order to obtain adequate representation of the macerals present. Polished sections are used according to ICCP (International Committee for Coal and Organic Petrology) standards (Stach et al. 1982).
Vitrinite reflectance measurements can be made on the same samples to provide maturity assessment. This is particularly useful where calibration of thermal modelling is required, and in establishing maturity/generation maps.
Other useful maturity indications can be obtained from liptinite fluorescence and the colour changes that occur as palynomorphs experience progressive maturation (e.g. thermal alteration index, TAI).
The best samples for vitrinite reflectance, and petrology in general, are those where one can be sure that the macerals represent in-situ organic matter, so SWC or core samples are preferred. In practice, however, often only cuttings samples are available, and picking may be necessary.
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Because vitrinite is derived from woody particles and is most prominent in coals and proximal settings, the optimum lithologies for vitrinite reflectance measurements are coals or claystone/shale where there is a relatively high TOC and the facies suggest a considerable proportion of terrigenous organic material is present.
For coals ~30 g are preferred, and for shales 30–500 g, depending upon organic content.
Sampling should be relatively evenly distributed throughout a drilled sequence to obtain an optimal maturity-depth trend, but this may be modified depending upon conditions (e.g. distribution of organic matter) and project aims.
The samples (cuttings, SWC, outcrop or core samples) are prepared either as “whole rock”, if maceral density is expected to be sufficiently high (e.g. from TOC and Rock-Eval results), or are treated with hydrochloric and hydrofluoric acid to concentrate macerals prior to further preparation.
The acid treatment also avoids incorporation of soft and expanding mineral phases that can hinder good polishing quality. The whole rock or the kerogen resulting from the acid treatment is embedded in an epoxy resin to make briquettes, which are ground flat and then polished (using 0.25mm diamond paste and MgO).
Maceral identification employs incident light microscopy (20x and 50x magnification) with oil immersion of objectives. Additional aids to identification and maturity assessment include fluorescence wavelength and intensity.
Vitrinite reflectance is measured using a photometer microscope (Zeiss MPM 03), again with oil immersion objective (Epiplan-Neofluar 40/0.90; oil refractive index 1.515 at 18°C) (e.g. Ward 1984). A constant 2.5mm diameter field is used for reflectance measurements at 546 nm, without polarizer. This yields random reflectance (%Rm). The photometer response is calibrated daily against a 0.488% Rm standard and checked against 0.879 and 1.696% Rm standards (deviations of <±0.01 and 0.02, respectively are considered acceptable).
Further checks are made at least hourly during use (acceptable deviation <±0.005). Where possible, at least 20 (ideally 50) point measurements should be made. However, measurements can be affected by factors considered in the next section.
Pitfalls for maceral analysis in general and vitrinite reflectance in particular include:
- reworked vitrinite and general difficulty identifying indigenous vitrinite
- vitrinite particle size and/or surface quality
- bitumen/oil based mud staining
- influence of pyrite
Although isolated kerogen may be needed to obtain adequate vitrinite reflectance measurements, it can be more difficult to identify the true indigenous population because of the lack of context: how different macerals relate to one another and minerals in a bulk rock sample. There is some operator subjectivity involved in the identification of the true vitrinite population, which can be problematical when vitrinite is sparse. Comparison with other maturity measurements (e.g. Rock-Eval Tmax, liptinite fluorescence and TAI) is recommended.
Some suppression of vitrinite reflectance can occur where there are significant amounts of indigenous bitumen. In contrast, artificial heating from turbo drilling can enhance reflectance. Pyrite has high reflectance, and if close to vitrinite, can cause elevated reflectance readings. Various mud additives can also influence the measurement, as can pitting of vitrinite, which affects how incident light is reflected.
Woody plants became abundant during the Devonian, so vitrinite reflectance is not applicable to older samples and the reflectance of other identifiable bodies within kerogen may potentially be used, such as bituminite or graptolites (although maturity calibration is less well constrained than for vitrinite).
Dow W. (1977) Kerogen studies and geological interpretations. Journal of Geochemical Exploration 7, 79–99.
ICCP (1963) Handbook of Coal Petrology. International Committee for Coal and Organic Petrology, http://www.iccop.org/
McCartney J.T., Teichmüller M. (1972) Classification of coals according to degree of coalification by reflectance of the vitrinite component. Fuel 51, 64–68.
Stach E., Mackowsky M.Th., Teichmüller M., Taylor G.H., Chandra D., Teichmüller R. (eds) (1982) Stach’s Textbook of Coal Petrology. Gebrüder Borntraeger, Berlin.
Wilkins R.W.T., Wilmshurst J.R., Russell N.J., Hladky G., Ellacott M.V., Buckingham C. (1992) Fluorescence alteration and the suppression of vitrinite reflectance. Organic Geochemistry 18, 629–640.