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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
Figure 5 Vitrinite Reflectance Histograms
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
Figure 6 Ro% vs. Depth Plot
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
Figure 11 Photomicrographs of Fluorescing Organic Material
Whole Rock
Kerogen Concentrate
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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.
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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).
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
Figure 13 Correlation Table: Optical and Molecular Parameters
Guide to Optical Microscopy in Petroleum Geochemistry 01/02/95 updated 14/01/2009
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
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