MDOIL - OPTICAL MICROSCOPY

Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009

MDOIL LIMITED

GUIDE TO OPTICAL MICROSCOPY

IN

PETROLEUM GEOCHEMISTRY

BY

CHRIS MATCHETTE-DOWNES

http://www.mdoil.co.uk


CONTENTS

1. Introduction 2. Vitrinite Reflectance (Ro%) 3. Spore Colouration (SCI) 4. Thermal Alteration Index (TAI) and Visual Kerogen Analysis 5. Spore Fluorescence (SF) 6. Other Techniques 7. Photomicrography 8. Selective References FIGURES & TABLES Figure 1 - Optical Procedure Flowchart Figure 2 - Photomicrograph Figure 3 - The Macerals Figure 4 - Reflectance Microscope Figure 5 - Vitrinite Reflectance Populations Display Figure 6 - Annotated log Ro% vs. Depth Plot Figure 7 - SCI Table Figure 8 - TAI Table Figure 9 - Major Maceral Types Figure 10 - Spore Fluorescence Colour Index Figure 11 - Photomicrographs of Fluorescing Organic Material Figure 12 - Transmitted light vs. UB/blue light excitation of spores Figure 13 - Correlation Table



GEOCHEMICAL METHODOLOGY

OPTICAL GEOCHEMISTRY

1. INTRODUCTION

This manual has been written to cover most of the Optical techniques used in petroleum

geochemistry.

It must be remembered

that for successful

geochemical

interpretation all

techniques must be used

in conjunction. A fully

integrated approach is

needed.

Figure 1

Geochemical

Procedure Flowchart

The flowchart (Figure 1)

displays the main optical

techniques involved and

their inter-relationships.

The main techniques are

VITRINITE

REFLECTANCE, SPORE

FLUORESCENCE, SPORE

COLOURATION, VISUAL

KEROGEN ANALYSIS and



PHOTOMICROGRAPHY. Perhaps less well known are the optical techniques such as GRAPTOLITE

REFLECTANCE and CONODONT ALTERATION.

BIOSTRATIGRAPHY, PALYNOLOGY, MICROPALEAONTOLOGY are closely related and should

where possible be integrated with petroleum geochemistry.

The sample materials received by the geochemical laboratories for optical work will be solids in

the form of CUTTINGS, SIDEWALL CORES (SWC), CONVENTIONAL CORES and OUTCROP

samples. Some sample preparation prior to receiving the sample in the Lab may have taken

place.

Some of the initial preparative work is similar to that for molecular geochemistry. Differences in

the preparative techniques emerge from the crushing stage onwards. Principally the sample

should not be crushed to the same degree as for molecular geochemistry (1μm diameter for

molecular geochemistry and ±1mm diameter for optical work).

Optical geochemistry is undertaken for similar reasons as molecular geochemistry; namely to

establish the TYPE (PALAEOENVIRONMENT), AMOUNT and MATURITY of the organic matter. The

two techniques should be used to complement each other. If there are differences then these

should be rigorously investigated, until they are satisfactorily explained.

For ease of explanation this manual has been divided into sections covering each of the optical

techniques.

2. VITRINITE REFLECTANCE (Ro%)

This is an optical technique for determining MATURITY. It is industry accepted and generally

regarded as the most reliable technique for measuring maturity. There are however many

situations where this technique cannot be used, situations where the data could be unreliable

and there are many pit-falls with the technique. It is for this reason that no one technique should

be used exclusively.



Figure 2 Photomicrograph of sample preparation for vitrinite reflectance (Kerogen

concentrate)

COAL is the most suitable rock type for vitrinite determination as it is most likely to contain the

most VITRINITE particles/MACERALS. However it is not always possible to used coal. The next

best material is sediment containing abundant CARBONACEOUS material.

Vitrinite determination then becomes a skilled operation to both distinguish maceral types (See

Figure 3 – The Macerals) and then to distinguish between AUTOCHTHONOUS and

ALLOCHTHONOUS vitrinite. It is the AUTOCHTHONOUS (PRIMARY VITRINITE) that

measurements should be made on and it is values derived from primary vitrinite that should be

recorded. False values of maturity will result if unsuitable particles are measured.



Figure 3 The Macerals

It is not necessary to measure values down the entire well bore, just use carefully selected

samples, every 30 to 100ft. Closer spacing is needed for detailed Basin Modelling especially

where faults and unconformities are expected.

PREPARATION

Once the sample is crushed, it is mounted in a RESIN. When the resin solidifies, usually

overnight, it is successively polished using finer and finer grades of ALUMINA powder. The

resulted highly polished RESIN BLOCK/PLUG is then viewed in reflected light. It is important to

use only high grade resin.

A polished plug is mounted on the microscope stage and a drop of oil is placed on the surface of

the plug. Oil is used as this increased the contrast which in turn helps in the identification of

these low-reflectance materials.

As the difference between the refractive index of the substance and that of the immersion

medium (the oil) increases the reflectance increases, but there is a lowing of contrast between

the constituents. It is therefore important to always used the same oil and keep it from

deteriorating. Cedar Oil (Rl 1.515) has the best properties. The oil should be stored and used as

near to 25°C as possible in a tightly sealed container.

COAL PETROGRAPHIC MACERALS

VITRINITE (WOOD) TELINITE cellular structure (mummification in a reducing atmosphere) COLLINITE structureless (humid acid gels) INTERTINITE (MICROBIAL? DEGRADATION OF WOOD) SEMIFUSINITE cellular (temperature?) FUSINITE cellular (temperature?) MICRINITE structureless (aerobic) SCLEROTINITE (fungal) EXTINITE (LIPTINITE) SPORINITE spore exines CUTINITE cutin SUBERNITE bark RESINITE exudate (amber, jet) ALGINITE algae



Water is sometimes used where bitumens may be soluble in the immersion oil.

MONOCHROMIC light from a stablised light source passes through a BEREK PRISM and through

an OIL IMMERSION OBJECTIVE onto the polished surface of the sample (see Figure 4,

Reflectance Microscope).

Figure 4 Reflectance Microscope



Using the eye-piece a suitable particle is selected and focused on. The light is then directed to

the PHOTOMULTIPLIER through a small pre-set aperture. The light reaches the photomultiplier

and is converted to an electrical signal.

The signal is recorded and compared to a standard to determine the reflectance value of the

sample. A standard is chosen that has a reflectance value that is close to that of the material

being examined.

Typically a Geochemist will be looking at vitrinite of reflectances in the range Ro 0.3 to 1.5%. A

useful standard might be an artificial Sapphire with a reflectance of Ro 0.504%. Other standards

should include glass (various values) or Yttrium Aluminium Garnet (0.917%) or diamond

(3.314%).

It is very important that the standards are stored carefully and kept clean and that the linearity

of the photomultiplier is checked daily with a range of standards.

A particular problem in the tropics with microscopy is that fungus may develop almost anywhere

on the instrument. This must be checked for regularly and removed where it occurs. Failure to do

this will result in a reduction of the microscope’s efficiency. Microscopes must be operated in air

condition rooms.

If the signal from the photomultiplier is sent to a chart recorded, then a series of peaks will be

recorded for the sample and the standard corresponding to each measurement.

Ro% = (SAMPLE) = D(SAMPLE)/D(STANDARD) * Ro% STANDARD

Where: Ro% = Reflectance in oil

D = Deflectance of pen recorder

The vitrinite reflectance value obtained is derived from randomly orientated particles so an

average of at least 30 measurements on suitable particles must be made. Vitrinite exhibits

BIREFLECTANCE. Ro% min is recorded perpendicular to the bedding and Ro% max is recorded

parallel to the bedding.



There is a steady increase in Ro min, Ro max and bireflectance up to an Ro max of 4%. This is

the boundary of ANTHRACITE and METAANTHRACITE and as such well beyond the boundary of

petroleum geochemistry.

Sample material selected from cores is the best as viewing this material avoids the possibility of

making false readings on CAVINGS. It also usually avoids the possibility of analyzing oxidised

samples.

Oxidation can usually be spotted as it usually is accompanied by curving microfissures within the

vitrinite particles. Natural weathering does not tend to raise the reflectance.



Figure 5 Vitrinite Reflectance Histograms



Figure 6 Ro% vs. Depth Plot



If heating is involved the vitrinite reflectance will be higher along the microfissures.

MUDLOGGERS may inadvertently oxidise samples in their sample drying ovens for instance – so

beware.

The correct particles to measure usually have a wispy appearance, unlike the ALLOCHTHONOUS

material which is often more highly reflective and abraded. Other macerals may be distinguished

from vitrinite by their shape, colour, relief, texture association and fluorescence.

Figure 5 Vitrinite reflectance populations.

Figure 6 Annotated log Ro% vs. Depth Plot.

Log vitrinite Ro% shows generally a liner increase with depth, so it is useful to plot it in this way

to help pick faults and unconformities.

Whilst measuring Ro% it is well worth examining for the maceral content for

palaeoenvironmental purposes. The lithology and the degree of bitumen saturation can also be

assessed. These data will provide important clues as to what is going on that particular rock.

Some geochemical laboratories prefer to concentrate the KEROGEN. This is especially true in

cases where low abundance of vitrinite is suspected. Marine carbonate sequences may often

contain no vitrinite particles whatsoever.

KEROGEN ISOLATION is a messy, lengthy and often hazardous process. First the rock is

dissolved in HCL to remove the carbonates, and then in HF to remove the SILICATES. This may

take several days if done to remove all the rock, however for commercial reasons a swifter

approach is normally used.

Quick methods of kerogen isolation are probably sufficient for optical work, but a more complete

inorganic constituent digestion is needed for ELEMENTAL ANALYSIS and other chemical analysis.

The main advantages of kerogen isolation are that suitable particles are concentrated and more

readings are possible, secondly if additional optical analyses are required then the material is

already prepared.



The main disadvantage is that the vitrinite/ lithological relationships are lost, as is the degree of

bitumen staining etc. There are many other advantages and disadvantages too numerous to

mention but worth some thought before proceeding.

3. SPORE COLOURATION (SC)

Spore colouration is another maturity assessment technique. This is a technique that is preferred

by some organisations for the following reasons: more temperature dependant than Ro%,

cheaper and quicker, spores contain aliphatic material and therefore are closer compositionally to

oil, and can be run in conjunction with biostratigraphy . The sample material may be prepared by

KEROGEN ISOLATION as described above. SPORES (preferably from the same species) are

selected and examined for their colouration. With increasing maturity there is progressive change

in colour and darkening of the spores. The spore colouration index (SCI) is determined using

transmitted light.

Figure 7 SCI Table

For consistency between workers and between different labs, it is important to have a standard

set of spores to refer to. These can be purchased from commercial laboratories e.g. SPT or better

made from your own reference collection.

Spore Colouration Index 1 Colourless 1.5 Colourless – pale yellow 2 Pale yellow 2.5 Pale yellow – lemon yellow 3 Lemon yellow 3.5 Lemon yellow – golden yellow 4 Golden yellow 4.5 Deep yellow 5 Yellow orange 5.5 Light orange 6 Mid orange 6.5 Dark orange 7 Orange brown 7.5 Light brown 8 Mid brown 8.5 Dark brown 9 Very dark brown 9.5 Brown black 10 Black



SCI determination is one of the areas where the “boundary” of Geochemistry and

Micropalaeontology/Palynology cross.

4. THERMAL ALTERATION INDEX (TAI) AND VISUAL KEROGEN ANALYSIS

Again the isolated Kerogen is examined in transmitted light. Instead of attention being confined

to one maceral type, all the organic matter is examined. The organic matter is examined for its

maceral content to establish the environment of deposition and hence its generic potential.

Progressive darkening of organic matter is recorded for maturation purposes.

THERMAL ALTERATION INDEX (TAI) proposed by Staplin (1969) is another spore colouration

alteration index. (See Figure 8, TAI).

Figure 8 TAI

Ro% TAI Ro% TAI 0.30 2.0 1.26 3.15 0.34 2.1 1.30 3.2 0.38 2.2 1.33 3.25 0.40 2.25 1.36 3.3 0.42 2.3 1.39 3.35 0.44 2.35 1.42 3.4 0.46 2.4 1.46 3.45 0.48 2.45 1.50 3.5 0.50 2.5 1.62 3.55 0.55 2.55 1.75 3.6 0.60 2.6 1.87 3.65 0.65 2.65 2.0 3.7 0.70 2.70 2.25 3.75 0.77 2.77 2.5 3.8 0.85 2.8 2.75 3.85 0.93 2.85 3.0 3.9 1.00 2.9 3.25 3.95 1.07 2.95 3.5 4.0 1.15 3.0 4.0 4.0 1.19 3.05 4.5 4.0 1.22 3.1 5.0 4.0



The TAI scale has fewer divisions than the SCI. Values range from 1 to 5 and correspond to

spore colour from yellow to black. 5 indicates the onset of maturity. It is obviously important to

know what scale one is dealing with.

There are many classifications; however they all reflect the amount of hydrogen contained in the

kerogen which in turn is controlled by the original organic matter input and some cases

controlled by subsequent alteration. There is no useful hydrogen in INERTINITE and abundant

hydrogen in AMORPHOUS FLUORESCING KEROGEN e.g. partially degraded algal remains (the

Type I with very high HI values.)

Figure 9 The Major Kerogen Types

Marine Non-marineLiptinite-rich* KEROGEN TYPE Vitrinite/exinite/inertinite rich Phytoplankton and SOURCE ORGANISMS Higher plants and non-marine Bacteria algae High H/C; low “O” lipids, CHEMICAL Low H/C; high “O”, polysaccharides etc. COMPOSITION polysaccharides, lignin, sporopollen, lipids etc. Major precursors for world HYDROCARBONS Mainly gas, but some liquid H/C oil reserves, Gas at higher GENERATED can be formed maturity. *Note: Typical marine kerogen contains a mixture of autochthonous marine and allochthonous terrigenous kerogen.



5. SPORE FLUORESCENCE (SF)

This is another maturity indicator. The technique is usually performed along with a vitrinite

reflectance determination using the same polished plug. (See Figure 10 Photomicrograph

fluorescing material).

Figure 10 Spore Fluorescence Colour Index

A reflectance type of microscope is fitted with a blue to ultraviolet light source instead of white

light. The light source is usually a Mercury Vapor Lamp. The light passes through an excitor filter

and is directed down onto the sample using a Dichroic mirror.

(See Figure 4: Reflectance Microscope).

The resultant fluorescence passes through a barrier filter (usually 500nm) before it reaches the

eye. The colour and intensity of the fluorescence is then assessed.

Fluorescence can also be used in many other ways. In the bulk sample it is used by mudloggers

and core analysts to assess the amount and type of reservoired/migrated hydrocarbons.

Fluorescence observations made on polished blocks are also very useful to assess the degree of

bitumen staining and the maturity of the hydrocarbons (e.g. dead oil vs. live oil etc.).

Some experience in the recognition of microfossils is useful. Often under fluorescent light it is

possible for instance to recognize organisms such as BOTRYOCOCCUS or TASMANITES. In this

instance and LACUSTRINE or MARINE environment is suggested respectively.

1. Green-yellow

2. Yellow

3. Light orange

4. Mid orange

5. Dark orange

6. Red

7. Brown



Figure 11 Photomicrographs of Fluorescing Organic Material

Whole Rock

Kerogen Concentrate



Figure 12 Transmitted Light vs. UV Light Excitation of Spores

REF: SPT promotional literature

SF technique is particularly useful in the range Ro 0.35 to 0.9% and extremely useful in the

range Ro 0.35 to 0.5% where vitrinite reflectance is not so reliable.



6. OTHER MATURITY RELATED TECHNIQUES

I have included GRAPTOLITE REFLECTANCE in this section as it may become a useful technique

in ancient sections (ORDOVICIAN or SILURIAN). Recent studies have revealed a direct correlation

between Ro% of graptolitic material and vitrinite of the same maturity.

CONODONT ALTERATION INDEX

Again this technique is not commonly used and is confined to ancient sediments of CAMBRIAN to

TRIASSIC age. This technique charts the change in colour and changes in opacity due to

increases maturity in certain microfossils called CONODONTS.

All the optical palaeoenvironmental data can be used to compliment molecular and bulk source

rock data. Optical data may have been the only information available when the chemical

signatures are lost through oil or drilling fluid contamination.

All the maturity palaeoenvironmental and other information gathered from the above techniques

must be used, where possible, in conjunction with molecular data and bulk data (see other

manuals).



Figure 13 Correlation Table: Optical and Molecular Parameters



7. PHOTOMICROGRAPHY

This is merely taking photographs of rocks under high magnification. It is difficult in reflected and

fluorescent light as there is so little light returned (reflected) to the camera/eye. It is therefore

necessary to have ling exposures. The camera replaces the photomultiplier (see Figure 4).

Due to long exposure times needed the microscope table and building housing the microscope

has to be stable. Traffic vibration will for instance cause the pictures to become blurred. It is a

useful technique to demonstrate points and keep records.

8. SELECTED REFERENCES

1. Stach’s textbook of Coal Petrology

2. Tissot & Welte : Petroleum Formation and Occurrence

3. Bustin et al. : Optical Properties of graptolite peridium following simulated masturation

4. Jones, 1986

5. Collins, 1990

6. Mackenzie, 1985

7. Powell, 1988

e.mail contact: [email protected]


MDOIL - OPTICAL MICROSCOPY

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