MDOIL LIMITED GUIDE TO OPTICAL MICROSCOPY PETROLEUM GEOCHEMISTRY CHRIS MATCHETTE-DOWNES

Transcription

1 MDOIL LIMITED GUIDE TO OPTICAL MICROSCOPY IN PETROLEUM GEOCHEMISTRY BY CHRIS MATCHETTE-DOWNES

2 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

3 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

4 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.

5 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.

6 Figure 3 The Macerals COAL PETROGRAPHIC MACERALS VITRINITE (WOOD) TELINITE COLLINITE cellular structure (mummification in a reducing atmosphere) structureless (humid acid gels) INTERTINITE (MICROBIAL? DEGRADATION OF WOOD) SEMIFUSINITE cellular (temperature?) FUSINITE cellular (temperature?) MICRINITE structureless (aerobic) SCLEROTINITE (fungal) EXTINITE (LIPTINITE) SPORINITE CUTINITE SUBERNITE RESINITE ALGINITE spore exines cutin bark exudate (amber, jet) algae 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.

7 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

8 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.

9 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.

10 Figure 5 Vitrinite Reflectance Histograms

11 Figure 6 Ro% vs. Depth Plot

12 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 Figure 6 Vitrinite reflectance populations. 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.

13 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 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 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.

14 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

15 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-marine Liptinite-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.

16 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 1. Green-yellow 2. Yellow 3. Light orange 4. Mid orange 5. Dark orange 6. Red 7. Brown 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.

17 Figure 11 Photomicrographs of Fluorescing Organic Material Whole Rock Kerogen Concentrate

18 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.

19 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).

20 Figure 13 Correlation Table: Optical and Molecular Parameters

21 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, Collins, Mackenzie, Powell, 1988 e.mail contact: cjmd@mdoil.co.uk