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 thermal alteration 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 of deposition is greater than the rate of erosion. Additionally, the organic matter needs to be stored in the sediment under anoxic conditions. organic matter is generally in the form of peat, coal, organic shales and dispersed organic matter (DOM).

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

The processses of coalification is driven by increasing temperature and the phytoclast material (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 and oxidised. Increasing temperature and burial leads to a the formation of lignites, in which plant material is well preserved (soft brown coals). Peat and lignite are both fiable, porous structures known as porous humites. At the Carbonization Jump, the materials lose porosity and volatiles and the structures of the organic compounds undergo reordering and become aligned parallel to bedding. This process is called 'diagenetic gelification' in coal geology parlance; gels and methane are expelled from the organic compounds as they reorder. The appearrance of the coals goes from soft, 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 what is the equivalent of the oil window. An increase in the loss of Hydrogen heralds the formation of the anthracites in the 'gas window'; semi-anthracite > anthracite. Meta anthracite is formed in post mature sediments (at the equivalent of prehnite-pumpellyite facies) and graphite at equivalent of greenschist facies.

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

Liptinite ('Exinite') Maceral Group (Type I & II Kerogen)
UV Fluorescence = strong yellow or green
Reflectances = 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 where vitrinite is very rare.

Liptinite-rich rocks have a high oil and gas producing potential
Vitrinite Maceral Group (Type III Kerogen)
UV Fluorescence = none or poor
Reflectance = 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 plants and the precipitated gels from these materials.

Vitrinite-rich rock tends to be prone to gas generation
Inertinite Maceral Group (Type IV Kerogen)
UV Fluorescence = no
Reflectance = 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) Measurements
Vitrinite 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 and inexpensive. Vitrinite, because it is not strongly prone to oil and gas formation, is common as a
residue in source rocks.

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

The reflectivity (R) may either be recorded as as Rv max% or Ro%. Both are measurements of 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 on the bedding parallel surfaces and least on surfaces cut orthogonal to the bedding. Surfaces cut at angles between these two extremes will have intermediate reflectance. 

Consequently, under (cross) polarised light, the reflectance of the vitrinite maceral observed will depend upon its position relative to the plane of polarisation of the light. In cross polars, the vitrinite will, in a 360° rotation of the stage, have two reflectance maxima and two reflection minima. It is the average % reflection of the two reflectance maxima which provides analysts with the value Rv max%. This methodology is that of choice in Australia. In the USA and Europe, Ro% is measured. This is simply 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 then ground and polished to a high standard. Poor polishing will lead to spurious reflection measurements. Sample preparation will take ~ 1 day. The blocks will obviously contain particles of vitrinite plus other macerals (i.e. liptinites and inertinites) which will need to be recognised and discarded {NB reflectance of these macerals may be recorded as RL% or RI%}. The number of individual 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 these measurements in, say, 30 minutes.

Measure average of Rv max%
~ 30 measurements per block; 50x magnification, oil immersion, XPL ('bireflectance'), record two maxima 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 VR
Although 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 measurement of maximum palaeotemperatures in hydrocarbon-bearing basins.
• VR is applicable over a wide range of maturity temperatures and its behaviour is rigorously modelled.
• 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 other maceral 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 a result of high hydrogen contents.
• Vitrinite is absent in pre-Silurian rocks (but these rocks are also somewhat lacking in other hydrocarbon-producing materials).
• Coal macerals cannot be dated. Timing of maximum palaeotemperature is therefore not possible with this technique.
• Macerals may be damaged by reworking or poor polishing may give spurious reflection measurements.
• Does chemical variation affect reflectance? This is a subject under debate, but chemistry is assumed to have negligible effects.