C014 Geoscience Laboratory Techniques: Vitrinite Reflectance

What is Vitrinite?A Background to Coal Geology and Organic matter in sediments

The bulk of organic matter in sediments is derived from plants (phytoclasts) and the thermalalteration of organic matter from plants over geologic time leads to the generation of oil and gas.

Accumulation of organic matter occurs in regions undergoing subsidence where the rate ofdeposition is greater than the rate of erosion. Additionally, the organic matter needs to be stored inthe sediment under anoxic conditions. organic matter is generally in the form of peat, coal, organicshales and dispersed organic matter (DOM).

Hydrocarbons begin to be generated above the temperature threshhold of 60°C. Thisprocess, where peats and lignites become dehydrated and lose other volatiles and kerogen splits intoits four distinctive types is known as the 'carbonization jump'. The 'oil window' lies betweentemperatures of ~ 60-120°C; the gas window between ~ 120-150°C. At temperatures greater than150°C, the organic matter is said to be post mature and is no longer reactive to the development ofhydrocarbons. At temperatures of 200°C, organic compounds are reduced to graphite and methane.

The processses of coalification is driven by increasing temperature and the phytoclastmaterial (especially vitrinite) alter diagenetically. In the early stages, whilst in the peat phase,phytoclasts are altered by micro-organisms such that sugars and proteins are hydrolysed andoxidised. Increasing temperature and burial leads to a the formation of lignites, in which plantmaterial is well preserved (soft brown coals). Peat and lignite are both fiable, porous structuresknown as porous humites. At the Carbonization Jump, the materials lose porosity and volatiles andthe structures of the organic compounds undergo reordering and become aligned parallel tobedding. This process is called 'diagenetic gelification' in coal geology parlance; gels and methaneare expelled from the organic compounds as they reorder. The appearrance of the coals goes fromsoft, brown and dull to hard, black and lustrous; so called 'dense vitrinites' have been formed.Increase in coal rank follows the classification sub-bituminous coal > High-volatile bituminous coal> Medium-volatile bituminous coal > Low-volatile bituminous coal. These materials form in whatis the equivalent of the oil window. An increase in the loss of Hydrogen heralds the formation of theanthracites in the 'gas window'; semi-anthracite > anthracite. Meta anthracite is formed in postmature sediments (at the equivalent of prehnite-pumpellyite facies) and graphite at equivalent ofgreenschist facies.

A maceral is an elementary microscopic constituent of coal that can be recognised by itsshape, morphology, reflectance and fluorescence (Stopes, 1935), broadly the term is equivalent tominerals in rocks. Morphology is the main factor determining the classification of macerals. See thehandout for classification of the maceral groups.

Liptinite ('Exinite') Maceral Group (Type I & II Kerogen)UV Fluorescence = strong yellow or greenReflectances = low• Type I Kerogen; waxy, lipid-rich and resinous parts of plants.• Type II Kerogen; green algae and blue-green algae; common in marine anoxic shales wherevitrinite is very rare.

Liptinite-rich rocks have a high oil and gas producing potential

Vitrinite Maceral Group (Type III Kerogen)

UV Fluorescence = none or poorReflectance = moderate• the most common maceral (organic component) in most humic coals• a common consitituent of organic source rocks.• remains of cell lumens (cell walls), woody tissue of stems, branches, leaves and roots of plantsand the precipitated gels from these materials.

Vitrinite-rich rock tends to be prone to gas generation

Inertinite Maceral Group (Type IV Kerogen)UV Fluorescence = noReflectance = very high• peats that have been oxidised early in their formation• bark, stems, leaves, roots

inertinites are not prone to oil and gas generation.

Vitrinite Reflectance (VR) MeasurementsVitrinite Reflectance (VR) is the most commonly used organic maturation indicator used in

the petroleum industry. This is mainly because it is accurate, quick, non-destructive andinexpensive. Vitrinite, because it is not strongly prone to oil and gas formation, is common as aresidue in source rocks.

As coal rank increase, and the chemical composition of the vitrinite correspondinglychanges, the vitrinite macerals become increasingly reflective. Therefore, the percentage reflectionof a beam of normal incident white light from the surface of polished vitrinite is a function of therank (maturity) of the maceral.

The reflectivity (R) may either be recorded as as Rv max% or Ro%. Both are measurementsof the percentage of light reflected from the sample, calibrated against a material which shows~100% reflectance (i.e. a mirror). Because vitrinite is 'anisotropic'; reflectance will be greatest onthe bedding parallel surfaces and least on surfaces cut orthogonal to the bedding. Surfaces cut atangles between these two extremes will have intermediate reflectance. Consequently, under (cross)polarised light, the reflectance of the vitrinite maceral observed will depend upon its positionrelative to the plane of polarisation of the light. In cross polars, the vitrinite will, in a 360° rotationof the stage, have two reflectance maxima and two reflection minima. It is the average % reflectionof the two reflectance maxima which provides analysts with the value Rv max%. Thismethodology is that of choice in Australia. In the USA and Europe, Ro% is measured. This issimply the reflection off macerals from a normal incident beam of non-polarised light.

Samples are separated and washed, and then mounted in resin. These resin blocks are thenground and polished to a high standard. Poor polishing will lead to spurious reflectionmeasurements. Sample preparation will take ~ 1 day. The blocks will obviously contain particles ofvitrinite plus other macerals (i.e. liptinites and inertinites) which will need to be recognised anddiscarded {NB reflectance of these macerals may be recorded as RL% or RI%}. The number ofindividual reflection measurement will be dependent on the abundance of vitrinite in the sample,but should be in the order of 30 - 100 vitrinite measurements. A skilled analyst can make thesemeasurements in, say, 30 minutes.

Measure average of Rv max%~ 30 measurements per block; 50x magnification, oil immersion, XPL ('bireflectance'), record twomaxima through rotation of the stage.

or Ro% …~ 30 measurements per block; 50x magnification, oil immersion, PPL

NB when reflectance < 1%, Rv max% = Ro%

Advantages and Disadvantages of VRAlthough the below lists show far more disadvantages than advantages, be aware that the

advantages far outweigh the limitations of the technique.

Advantages• VR analysis has worldwide exceptance as a technique capable of producing precise measurementof maximum palaeotemperatures in hydrocarbon-bearing basins.

• VR is applicable over a wide range of maturity temperatures and its behaviour is rigorouslymodelled.

• The technique is simple, cheap and quick.

• Vitrinite is common in post-Silurian terrestrial basins

Disadvantages• Analysis is subject to human error - you have to be able to distinguish your vitrinite from othermaceral groups (however, this is not a problem to the experienced analyst).

• Vitrinite is rare in marine sediments and Type II Liptinites are the best oil and gas generators.

• Where vitrinite DOES occur in marine sediments, reflectance values are often suppressed as aresult of high hydrogen contents.

• Vitrinite is absent in pre-Silurian rocks (but these rocks are also somewhat lacking in otherhydrocarbon-producing materials).

• Coal macerals cannot be dated. Timing of maximum palaeotemperature is therefore not possiblewith this technique.

• Macerals may be damaged by reworking or poor polishing may give spurious reflectionmeasurements.

• Does chemical variation affect reflectance? This is a subject under debate, but chemistry isassumed to have negligible effects.

Selected ReferencesYes, I know - most of these will be impossible to get hold of in our library - don't worry, at leastyou know where to go if one day you really need them. Also see

http://www.tsop.org/refs/refs.htm (coal & vitrinite)

http://www.geotrack.com.au/bibliog.htm (mostly AFTA)

http://www.le.ac.uk/geology/map2/pander/pander96/apapers.html (conodonts)

American Society for Testing and Materials (ASTM), 1994, Standard test method for microscopicaldetermination of the reflectance of vitrinite in a polished specimen of coal: Annual book of ASTMstandards: gaseous fuels; coal and coke, sec. 5, v. 5.05, D 2798-91, p. 280-283.

Bostick, N.H., 1979, Microscopic measurement of the level of catagenesis of solid organic matter insedimentary rocks to aid exploration for petroleum and to determine former burial temperatures -- areview, in P.A. Scholle and P.R. Schluger, eds., Aspects of diagenesis: SEPM Special Publication26, p. 17-43.

Bray, R.J., Green, P.F. and Duddy, I.R. (1992). Thermal History Reconstruction using ApatiteFission Track Analysis and Vitrinite Reflectance: A Case Study from the UK East Midlands andSouthern North Sea. In: Hardman, R.F.P. (ed.), Exploration Britain: Geological Insights for theNext Decade. Geological Society Special Publication, 67, 3-25.

Burnham, A. K., and J. J. Sweeney, 1989, A chemical kinetic model of vitrinite maturation andreflectance: Geochimica et Cosmo-chimica Acta, v. 53, p. 2649–2657.

Cook, A.C., and D.G. Murchison, 1977, The accuracy of refractive and absorptive indices derivedfrom reflectance measurements on low-reflectaing materials, Journal of Microscopy, v. 109. pt. 1,p. 29-40.

Cook, A.C., and N.R. Sherwood, 1991, Classification of oil shales, coals, and other organic-richrocks: Organic Geochemistry, v. 17, p. 211-222.

Davis, A., 1978, The reflectance of coal, in C. Karr, Jr., ed., Analytical methods for coal and coalproducts, v. 1: New York, Academic Press, p. 27-81.

Dean, M. T., and N. Turner. 1995. Conodont Colour Alteration Index (CAI) values for theCarboniferous of Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences.85:211-220.

Duddy, I.R., Green, P.F., Hegarty, K.A., and Bray, R. (1991). Reconstruction of Thermal History inBasin Modelling using Apatite Fission Track Analysis: What is Really Possible? Offshore AustraliaConference Proceedings Vol. I, 111-49 - 111-61.

Green, P.F., Duddy, I.R., and Bray, R.J., (1995). Applications of Thermal History Reconstruction ininverted basins. From Buchanan, J.G. and Buchanan, P.G (eds), Basin Inversion, GeologicalSociety Special Publication No. 88, 149-165.

Harris, A. G., 1979, Conodont color alteration, an organo-mineral metamorphic index and itsapplication to Appalachian Basin Geology., SEPM Special Publication 26, 3-16.

Harris, A. G., J. E. Repetski, and J. A. Grow. 1995. Exploring for hydrocarbons in geothermally andhydrothermally complex areas--A southern Nevada example [abs.]. American Association ofPetroleum Geologists Bulletin. 79:918-919.

Harris, A. G., J. E. Repetski, N. R. Stamm, and D. J. Weary. 1995. Conodont Age and CAI Data forNew Jersey. United States Geological Survey Open-File Report 95-557, 31 p.

Khorasani, G.K., and J.K. Michelsen, 1994, The effects of overpressure, lithology, chemistry andheating rate on vitrinite reflectance evolution, and its relationship with oil generation: APEAJournal, v. 34, pt. 1, p. 418-434.

Sweeney, J. J., and A. K. Burnham, 1990, Evaluation of a simple model of vitrinite reflectancebased on chemical kinetics: AAPG Bulletin, v. 74, p.1559–1570.

Teichmüller, M., and R. Teichmüller, 1981, The significance of coalification studies to geology:Bulletin des Centres de Recherches Exploration -- Production, Elf-Aquitaine, v. 5, p. 491-534.

Thomas, L., 1992, Handbook of Practical Coal Geology: John Wiley & Sons, Chichester, 338 pp.

Waples, D. W., 1979, Simple method for oil source bed evaluation: AAPG Bulletin, v. 63, p.239–245.

Waples, D. W., 1994a, Maturity modeling: thermal indicators, hydrocarbon generation, and oilcracking, in L. B. Magoon and W. G. Dow, eds., The petroleum system—from source to trap:AAPG Memoir 60, p. 285–306.

Waples, D. W., 1994b, Modeling of sedimentary basins and petroleum systems, in L. B. Magoonand W. G. Dow, eds., The petroleum system—from source to trap: AAPG Memoir 60, p. 307–322.