Quantitative palynofacies analysis

Chapter II

Figure 2.6 Stockmarr’s (1971) chart showing the estimated total error (curves) in % when error in the pollen counts (horizontal lines), error on the tablets, and error in the Lycopodium spores are taken into account.

D. Quantitative palynofacies analysis

The quantitative analysis of the bulk organic composition (palynofacies) of these samples has been calculated with the same absolute abundance formula (1) mentioned above. All particulate components of the organic residue have been taken into consideration such as the phytoclasts (tracheids, black wood, cuticles, membranous tissues, etc.), the marine elements (dinoflagellate cysts and microforaminiferal test linings), and selected terrestrial palynomorph groups (pteridophyte spores, saccate pollen, Classopollis pollen, Ephedripites pollen and angiosperm pollen). The conventional count of the first 250 particles was also applied to these samples.

2.2.2 Vitrinite

The petrographic examination of vitrinite is exclusively carried out under reflected light microscopy. This type of study was initially used in coal petrology to evaluate the rank of the coal by measuring the vitrinite reflectivity, as the adsorption index of the vitrinite rises in coal with increasing rank (Stach et al., 1982). This

study requires the preparation of polished surfaces of coal particles embedded in resin moulds. Hillier & Marshall (1988) noted that this method was adopted from the preparation techniques of Stach et al., (1975), American Society for Testing and Materials (1978), and British Standards Institute (1982). Other preparation techniques have been described by Bostick & Alpern (1977), Baskin (1979), Davies

& Avery (1984), and Bertrand et al., (1985) to study the petrographic properties of the organic matter concentrate on thin polished sections which has been recovered from the rock samples by the standard HCl/HF technique. Hillier & Marshall (1988) described a simple preparation technique for thin polished sections to study organic matter concentrates, which takes less time with minimum polishing, and requires only a small amount of the organic concentrate, and is therefore suitable for samples with low concentrations of organic contents, or for small samples. In this study the Hillier & Marshall (1988) preparation technique has been employed as follows:

1. After the recovery of the organic matter from the rock samples by the standard HCl/HF preparation technique, the concentrates of seventeen non-oxidized organic residues known to be rich in phytoclasts with a 300 foot (91.44 m) interval between each sample were selected for the preparation of thin polished sections.

2. Sixteen standard cover slips (22 x 22 mm) were coated on one side with the releasing agent polytetrafluoroethylene (PTFE), and left for few minutes until the carrier evaporated.

3. A small quantity of the organic matter concentrate of each sample was pipetted onto the coated surface of the cover slips, and then left to dry in a closed fume cupboard to eliminate airborne contamination.

4. A crystal pen was used to mark the edges on the unfrosted surfaces of the frosted microscope slides.

Material and Methods

Chapter II

5. One drop of resin was placed in the centre of the frosted side of the slides. Then the slide was held in contact over the cover slip, the resin allowed to spread. The slides were then quickly placed in a horizontal position to allow the resin to spread uniformly beneath the residue. The slides were left for 2-3 days to set.

6. After the complete setting of the resin, the cover slips were prised off using a razor blade, the organic matter remaining embedded in the thin layer of resin on the slide.

Vitrinite reflectivity measurements were carried out using a Zeiss Universal Microspectrophotometer (UMSP 50) in the School of Ocean and Earth Science, University of Southampton. This equipment is composed of three main units: the Zeiss Universal Microspectrophotometer 50 itself, a microscope photometer control unit (MPC 64), and a desktop computer. The vitrinite reflectance measurements were carried out using halogen light (Hal 100, 12V 100W), with a Zeiss epi-condenser II P epi-condenser, and a standard H-Pl-Pol Zeiss reflected light prism. An ANTIFLEX Zeiss EPIPLAN 40/0.85 Pol oil objective was used with immersion oil of ne= 1.518 ± 0.0004 at 23 ºC. The technique depends on measuring the intensity of light reflected from the surface of the investigated sample, which is compared to a standard where the reflected light intensity is measured by a photomultiplier (HTVR 928) which converts the reflected light rays into electrical signals and processes

them. The data from the photometer is then processed using a reflectance measuring programme, SLAP, written by S. J. Hillier (Ecolé Normal, Paris).

Calibration of the photometer using an artificial 3G garnet (gadolinium gallium garnet) with a known standard reflectance (RI = 0.919) was carried out before taking any measurements. Five reflectance measurements of the standard before and after measurements of each investigated sample were also made, in order to estimate the relative errors for each investigated sample. During the measurement process several measurements for each unknown sample were taken (48-87) from different parts of the vitrinite particles. Where particles contained inclusions or pyrite crystals, these were avoided. Scratches on the surface of the polished vitrinite particles were also avoided during the measurement process.

Finally, correction for error of average reflectance measurements of each sample was made as follows:

corr Rv = (avg Rv * act Std)/avg Stds (2) Where;

corr Rv = corrected average measurement of each sample avg Rv = average reflectance measurements of each sample avg Stds = average reflectance measurements of the standard act Std = actual reflectance known of the standard

Material and Methods

Chapter II

2.2.3 Total organic carbon (TOC) analysis

Samples selected to undergo total organic carbon analysis were prepared for elemental analysis as follows:

1. 2 grams of each sample was crushed into fine powder using an agate mortar, and split into two halves.

2. The first half of each sample was labelled for total carbon determination as `Cb` , with its corresponding borehole name and depth, and oven dried at 160 ºC for 3 days, before being placed in desiccator before elemental analysis.

3. The other split of each sample was labelled `Ca`, for acid treatment, where dilute 36% HCl analar was added to samples to dissolve mineral carbon present in the form of calcium carbonate.

4. Each Ca sample then washed several times with deionised water to remove any remaining acid traces, and the organic residue then concentrated using a 10 µm nylon mesh screen.

5. The concentrate of each acid treated-sample was dried in an oven at 160 ºC for 3 days, and then placed in a desiccator before elemental analysis.

6. Approximately 3 mg of both the acid-treated and untreated splits of each sample were weighed into tin capsules. These capsules were sealed to force out any trapped air, and then placed in the analyser sample chamber.

Elemental analyses were carried out using a Carlo Erba CHNS-O, EA1108 elemental analyser, running with Carlo Erba software EAGER 100. The technique depends on the oxidation of samples at about 1200 ºC under a mixture of oxygen and helium gases to convert C, H, and N into oxides. These oxides then undergo reduction and gas chromatographic separation (GC). Each organic element induces electric charges detected by a thermal conductivity detector, and produces a

spectrum (time sec versus mv) characteristic for each compound (CO2, H2O, and NO2). The EAGER 100 software then processes these data and produces the calculated wt% of each element. The elemental analyser was calibrated before running any analysis using the standard compound sulphanilamide (C6H8N2O2S) of known carbon content (41.84%). Stability of the analyser was assessed by analyzing the sulphanilamide standard after the analysis of five unknowns, and to estimate the relative errors.

As this elemental analyser cannot determine the organic carbon apart from mineral carbon within calcium carbonate, each sample (Cb and Ca) was analysed separately in order to calculate total organic carbon (TOC). The equation of Wilkinson (1991) was used to calculate the TOC as follows:

C = 100 Ca (1-0.0833 Cb)/ (100-8.33 Ca) (3) Where;

C = total organic carbon (TOC) wt%

Ca =C wt% in acid treated sample Cb = C wt% in untreated sample

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