Sample Preparation of Rock Samples
34 ditches cutting rock samples were washed and ground as described in Section 3.2 Soxhlet Extraction of Source Rock Cutting Samples
Cleaned and crushed shale samples were extracted using a Soxhlet extractor using pre-extracted cellulose extraction thimbles and cotton wool for 6 hours using an azeotropic solvent mixture of dichloromethane and methanol with the ratio DCM:MeOH of 93:7.
In order to remove any elemental sulphur from the extracts, activated copper turnings were added to each extraction beaker that contained 100 ml of the solvent mixture and a few anti-pumping granules. After the extraction processes were completed, the solvent was removed from the extracts by evaporation using a Heidolph rotary evaporator at a water bath temperature of 30-40ºC for several minutes. Then the extracts were collected and concentrated to 2-3 ml. An aliquot of the concentrated extract was transferred to a pre-weighed vial and the solvent was reduced to dryness in a stream of dry nitrogen in order to quantify the amount of total extractable material.
However, the process of removal of solvents may lead to loss of the lighter hydrocarbons which have similar evaporation rates to that of the solvent. In practice, only hydrocarbons heavier that about n-C15 are retained for further analysis.
Iatroscan Thin Layer Chromatography-Flame Ionisation Detection (TLC-FID)
Iatroscan analysis offers a relatively fast screening method for quantifying aliphatic and aromatic hydrocarbons and non-hydrocarbons in crude oil and rock extracts (Karlsen and Larter, 1991). Separation of the rock extracts and crude oil samples into their main components of saturated hydrocarbons, aromatic hydrocarbons, resin and asphaltenes (SARA) was performed using Chromarod-S III silica rods and an Iatroscan MK-5 analyser equipped with a flame ionisation detector (FID). The Iatroscan analysis
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was carried out for the geochemical screening of 34 ditches cutting shale samples and 51 crude oil samples from the oil fields and exploration wells of the Sirt Basin. This technique involved the application of a small volume of a sample of a solution of oil or rock extract to the chromatography rods and the development of the rods in a series of solvents to separate the various compound classes. Prior to the sample being applied to the rods, the rods were cleaned in the Iatroscan MK-5 FID analyser to remove any contamination by passing them through the hydrogen flame of the FID. A 3 µl aliquot of the crude oil or rock extract in a solvent solution (of about 5 mg/ml in DCM) was carefully applied dropwise onto the chromarods using a 10 µl syringe. A 3 µl aliquot of a standard (Crude oil from the Veslefrikk Field, Norwegian Sector of the North Sea) solution with a known SARA fraction percentage was always spotted onto the first two rods in each batch as shown in Table 3.2. The rods were then developed using three types of solvents, i.e. hexane, toluene, and finally a mixture of dichloromethane and methanol (93:7 v:v). The rods were first eluted in n-hexane up to 100% of rod length for about 25 minutes, and after that left to dry in the air for 3 minutes. The rods were then eluted in toluene to 60% of rod length for about 15 minutes, and left to dry in air for 6 minutes. Finally, the rods were eluted in a mixture of dichloromethane and methanol (93:7 v:v) up to 30% of the rod length before been dried in an oven at 60ºC for 90 seconds to remove any residual solvent. Then the rods were analysed using the Iatroscan MK-5 apparatus and the data were collected and processed using Lab Systems Atlas software. The Iatroscan chromatograms contained four peaks, which corresponded to the four separated fractions: saturated hydrocarbons, aromatic hydrocarbons, resins and asphaltenes (see Figure 3.2). To validate the results, each sample was run in duplicate and the average of the two results was recorded.
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Figure 3.2: Iatroscan TLC-FID chromatogram showing the distribution of saturated and aromatic hydrocarbon, resin and asphaltene fractions in crude oil from Sirt Basin.
The standard oil sample (VFO) was analysed along with every set of analysed samples to calculate the response factors for the distinct oil SARA fractions.
Equation 3.1
Response Factors = Area of standard
Standard conc. X volume standard solution applied onto rod (3 µl)
where:
Standard conc. = % of individual fractions of the standard oil (VFO) X concentration of oil (VFO) X vol. of oil loaded (3 µl).
The concentration of each fraction in the source rock extract and crude oil samples were quantified and calculated using the following equation:
Equation 3.2
F % (mg/g ) = Area F
Sum of areas of all fractions x RF Std
where: F = fraction normalised to the total identified area; RF Std = response factor of the fraction F
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Table 3-1: A composition based on the weight of fractions of crude oil from the Veslefrikk Field at Norwegian Sector of the North Sea used as standard for quantification of the Sirt Basin source rock extracts and crude oils.
Asphaltene Precipitation (Deasphaltening)
Crude oil or source rock extract samples, which have asphaltene contents of approximately 2% or more generally require deasphaltening prior to analysis using solid phase extraction (SPE) fractionation. This is to avoid the precipitation of asphaltenes, which can result in the blocking of the SPE column when hexane is used as either a diluent when diluting viscous oils or an eluent when eluting a hydrocarbon fraction from a C18 non-end caped SPE column. This may lead to restricted solvent flow, trapping of the aliphatic and aromatic hydrocarbon fractions on the column and thus leading to poor separation. For this reason, the 35 crude oil and 16 source rock samples from the Sirt Basin, which contained >2% asphaltenes were deasphaltene prior to their SPE separation. Aliquots (about 40–50 mg) of the crude oil or source rock extract samples were accurately weighed, and then 9ml cold n-hexane was added to each sample, which were then shaken and sonicate in a sonic bath for 3-4 minutes, then left overnight in a fridge at ~4°C. The next day the samples were centrifuged three times at 3000 rpm for about 5 minutes and the supernatant containing the fraction carefully transferred into a labelled flask using a Pasteur pipette. These steps were repeated 5-6 times until the supernatant was almost colourless. After that, the hexane solvent was evaporated using nitrogen to about 1ml and the fraction, and then transferred to a clean 3ml vial using only n-hexane solvent ready for the subsequent analysis.
Hydrocarbon Fraction Percentage (%)
Aliphatic 57
Aromatic 36
Resin 6
Asphaltene 2
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Solid Phase Extraction (SPE) Separation
In this study 3ml non-end caped C18 solid phase extraction (SPE) columns were used for the separation of hydrocarbon and non-hydrocarbon compounds from crude oil and source rock extracts according to the technique developed at Newcastle University and described by (Bennett and Larter, 2000).
Prior to starting solid phase extraction, mixtures of internal standards were prepared for the quantification of aliphatic and aromatic hydrocarbons, carbazoles and phenols.
Squalane and 1,1-binaphthyl were used as internal standards to quantify saturated and aromatic hydrocarbon fractions, respectively. The squalane standard was prepared at a concentration of 205.512mg/100 ml of n-hexane for the crude oil samples and 99.56mg/50 ml of n-hexane for source rock extracts. For the aromatic hydrocarbon fractions 1,1-binaphthyl (1,1 BN) standard was prepared at a concentration of 39.853 mg/100 ml of DCM for the crude oil samples and 9.96 mg/50 ml for organic extract samples.
The D8-carbazole internal standard was used to quantify carbazoles in crude oils and source rock extracts, and was prepared by adding 0.5mg and 0.73mg, respectively, of D8-carbazole to 25ml of a mixture of hexane and toluene (9:1 v:v). The standard was added to each sample before isolating non-hydrocarbon compounds, while the squalane and 1,1 BN internal standards were added to each sample before separating the aliphatic and aromatic hydrocarbon fractions.
For the isolation of hydrocarbon and non-hydrocarbon compounds from crude oil and source rock extracts, the 3ml non-end capped C18 columns were pre-cleaned with 6ml of DCM, and then the DCM was removed from each column using gentle air flush, after which the columns were left overnight to dry on top of an oven set at 60ºC.
Subsequently, 3ml of hexane was added prior to sample loading to each column for pre-conditioning. After that, the residual solvents were removed with a gentle air flush.
Approximately 30 to 70mg samples of crude oil or source rock extracts were carefully loaded on the top frit of the cartridges using a Pasteur pipette. In cases where the sample was not readily absorbed into the sorbent, a gentle, positive pressure was needed to force the sample into the frit. Then about 0.5ml of hexane was loaded to wash the
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column sides from any oil or organic extracts residues. After allowing the solvent height to fall to the level of the frit, another 0.5ml of hexane was carefully added to allow continued elution until the solvent reached the frit. The remaining 4ml of hexane was added to elute the hydrocarbon fractions. Finally, a very gentle air flush was applied to each column to displace residual hexane from the sorbent bed, and the lower tip of the cartridge was cleaned with 0.5ml of hexane. The hydrocarbon fractions were kept in 10ml vials in a fridge for further separation into aliphatic and aromatic hydrocarbon fractions.
The polar compounds were eluted from the SPE column with 5ml of DCM.
Approximately 0.5ml of DCM was used to wash the column sides and lower walls of the cartridge and the remaining DCM was added to elute the entire polar fraction. A very gentle air flush was applied to each column to displace residual DCM from the sorbent bed, and also the lower tip of the cartridge was cleaned with about 0.5ml of DCM. The DCM elutes containing polar compounds, including carbazoles and phenols, were kept in 10ml vials. The polar fractions were concentrated to 0.5ml by evaporating under a stream of nitrogen before further geochemical analysis were conducted. However, the samples were not evaporated to dryness in order to avoid loss of more volatile compounds such as phenol. Approximately 0.25ml of the polar fractions containing the carbazoles was further reduced to 100µl and transferred to 150µl tapered inserts in auto-sample vials and sealed before carbazole analysis by GC-MS.
Separation of Saturated and Aromatic Hydrocarbons using Silver Nitrate Impregnated Silica (SPE Ag+ silica)
3.9.6.1 Column Preparation
The saturated and aromatic hydrocarbons were separated using the silver nitrate Ag+
silica method developed at Newcastle University by Bennett and Larter (2000). The packed columns are not available commercially and therefore they needed to be prepared in the laboratory. Exactly 30g of Kiesel 60G silica gel (Merck) was weighed into a 500ml conical flask containing 60ml of distilled water plus 3g of silver nitrate in order to make about 40 solid phase extraction columns. The mixture was carefully
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shaken until a homogenous creamy liquid is formed, with no lumps of silica visible.
The sides and bottom of the flask was covered completely with aluminium foil to prevent photochemical degradation of the silver nitrate. To allow water vapour to escape from the mixture, small holes were made in the aluminium foil cap on the top of the flask, and then the flask placed in an oven at 80ºC to dry for up to 1 week. After the mixture was completely dry, the columns were prepared by adding approximately 550mg of the silica to each empty SPE cartridge column, which was then carefully compacted using a glass rod. The prepared columns were used immediately for optimum efficiency; otherwise light may affect the silver nitrate within the silica.
3.9.6.2 Sample Addition
The prepared columns were carefully cleaned with 5ml of hexane, using a positive air pressure to flush the solvent through the columns. This was performed using a plastic syringe installed in each individual column via a PTFE adaptor and manual pressure.
The hydrocarbon fractions obtained from the crude oil and source rock extracts using the C18 non-end capped SPE method, were evaporated down close to 1ml (1000 µl).
10µl of an internal standard with a mixture of 822.048µg squalane and 1.19559µg 1,1-binaphtyl were added to the hydrocarbons fraction to quantify the n-alkane, aliphatic, and aromatic hydrocarbons, respectively. For source rock extract samples 10µl aliquots of the internal standard containing a mixture of 59.736µg squalane and 0.996µg 1,1-binaphthyl were also added to each hydrocarbon sample. A 100µl aliquot of the solution containing the internal standards and hydrocarbon fraction was then loaded into the Ag+ impregnated silica column. The aliquot was loaded directly into the top frit of the column. The frit and column wall were cleaned and washed with 0.5ml taken from 2ml of hexane. A slightly positive air pressure was carefully forced into the aliquot to pass the sample into the Ag+ impregnated silica. After that, the remaining 1.5ml hexane was slowly added to the column to elute the saturated hydrocarbon fraction. Then gentle air pressure was applied to displace the residual solvent and sample retained in the silica, and also the bottom tip of the column was then cleaned with 0.5ml of hexane. At the end of the procedure, the saturated hydrocarbon fractions for each sample were collected in 10ml vials. Subsequently, and by applying a similar process that is used for isolation the saturated hydrocarbon fractions, the aromatic
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fractions were eluted from the Ag+ impregnated silica column using 4ml of DCM and were collected in separate 10ml vials.
The saturated and aromatic hydrocarbon fractions were reduced in volume to 1ml and transferred to auto sampler vials for analysis using GC and GC-MS.
3.10 Gas Chromatography (GC) of Aliphatic Hydrocarbon Fractions of the Crude