Molecular analysis of petroleum in ¯uid inclusions:
a practical methodology
D.M. Jones *, G. Macleod
1Fossil Fuels and Environmental Geochemistry (Postgraduate Institute): NRG, University of Newcastle upon Tyne, NE1 7RU, UK
Received 21 February 2000; accepted 2 August 2000 (returned to author for revision 12 April 2000)
Abstract
A method is described for extracting the petroleum from petroleum-bearing ¯uid inclusions hosted in the diagenetic cements of sedimentary rocks, whilst minimising contamination from petroleum in the pore space. A clean-up techni-que, involving the addition of extraction standards to the rock prior to analysis, has been developed that allows increased con®dence that the hydrocarbons extracted are only from included petroleum and are not mixtures of included petroleum and petroleum (or other organic material) adhering to the surface of the host minerals within the sample. Replicate quantitative analyses indicate that the technique produces highly reproducible results. The problems of analysing included petroleum and the clean-up method development are discussed and examples of analyses of included petroleums from carbonate reservoir and carrier units are given. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords:Petroleum; Fluid inclusions; Hydrocarbons; Biomarkers
1. Introduction
Fluid inclusions are tiny vacuoles in minerals, con-taining ¯uids that were present when the mineral cement formed. They thus can be regarded as time capsules of geo¯uids and as such are invaluable for understanding the evolution and migration of petroleum in sedimen-tary basins (cf. Roedder, 1984; Karlsen et al., 1993). An example of their use in the elucidation of the ®lling his-tory of a petroleum reservoir is given by Karlsen et al. (1993).
Petroleum-bearing ¯uid inclusions can be studied using microthermometric methods to obtain the mini-mum trapping temperature of single petroleum inclu-sions (Goldstein and Reynolds, 1994) and the gross
composition of included petroleum can be derived by confocal microscopy and pressure-volume-temperature (PVT) simulation (Macleod et al., 1996; Aplin et al., 1997, 1999). Assorted beam techniques can also be used to determine information on the general composition of the included petroleum, such as bond types present in the included petroleum, and some bulk compositional data from ultra violet (UV) spectroscopic techniques (e.g. Kihle, 1995). There have been a number of studies of the molecular compositions of petroleums in ¯uid inclusions by various techniques including thermal decrepitation or crushing (e.g. Murray, 1957; Hors®eld and McLimans, 1984; Etminan and Homan, 1989; Jesenius and Burruss, 1990; Karlsen et al., 1993; Macleod et al., 1994; Bigge et al., 1995; Jones et al., 1996; George et al., 1995, 1997, 1998a,b,c). To date, no beam technique has been reported that allows the analysis of biomarker compounds directly in the inclusions, nor does any technique exist to extract a single petroleum inclusion and manipulate the minute quantity of extracted petro-leum into gas chromatographic±mass spectrometric (GC±MS) or GC±MS±MS systems. There are two
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* Corresponding author. Tel.: 191-222-8628; fax: +44-191-222-5431.
E-mail address:[email protected] (D.M. Jones).
1 Present address: Shell Exploration and Production
probable reasons for this. Firstly, although extracting a single inclusion and removing and transferring the small quantities of petroleum onto GC±MS systems is not impossible, there are many potential technical dicul-ties. For example, such a small quantity of petroleum would adhere to syringe walls, or extraction lines as it was being transported to the inlet valve of the analytical unit, reducing the minute quantity of petroleum avail-able even further. Such problems are not insurmoun-table, but would require a major investment of time and scienti®c ingenuity. Secondly, it may be possible to analyse single unusually large petroleum bearing inclu-sions in the laboratory, but petroleum incluinclu-sions hosted in typical diagenetic cements from sedimentary basins are often sub micron to 10mm in length. If we assume that a single petroleum inclusion hosted in diagenetic cement, is spheroidal with a 10mm diameter, the volume of the inclusion will be 524mm3; if we assume that the
vapour phase is 6% by volume, the volume of the liquid phase will be just under 500mm3; if an average density
of the petroleum is taken to be 0.8 g/cm3, then the mass
of petroleum present in the inclusion will be approxi-mately 410ÿ10g. With an average biomarker
concentra-tion in the petroleum of, say, 100 ppm, it is unlikely that many of the useful biomarker compounds could be detected to a satisfactory level to allow interpretations based on their distributions.
The analysis of gasoline (and up to C21) range
com-pounds from the combined products of the decrepita-tion of between 10 and 100 petroleum inclusions has been shown using laser micropyrolysis GC±MS (Green-wood et al., 1998). However, to our knowledge, it has not been possible to analyse the isomer distributions of C21+biomarker compounds in petroleum-bearing ¯uid
inclusions, other than by crushing the host mineral cements below solvent and extracting small quantities of petroleum, which will have been yielded from hundreds or thousands of inclusions (e.g. Karlsen et al., 1993; Macleod et al., 1994; Aplin et al., 1997; George et al., 1997, 1998a,b,c.).
Such data are useful only if a careful petrographic and microthermometric study has been conducted beforehand. For example, it is possible that many dif-ferent generations of ¯uid inclusions are being analysed at the molecular level and thus any molecular data derived from such an analyses will not be representative of a single generation of petroleum ¯uid inclusions (i.e. a single period in geologic time). Even if a careful pet-rographic study of the samples has been conducted before the included petroleum is extracted, there are still several problems associated with the extraction process. Most of these problems pertain to a simple question: is the petroleum being analysed truly included petroleum, or is it petroleum strongly adhered to the surface of the mineral cements or stuck in micro-®ssures in the cements, or more likely petroleum adhered to the surface of, for
example, quartz grains ? Organic material such as kero-gen may also be present in the sample and could have adsorbed petroleum on its surfaces, or indeed, may degrade during work up to produce ``contaminant'' organic compounds. Hydrocarbons trapped/occluded in polar material strongly adsorbed onto mineral surfaces in reservoir sandstones have been detected using sequential extractions with increasing polar solvent mixtures by Wilhelms et al. (1996).
Previously published studies of the extraction and analysis of included petroleum have used assorted meth-odologies for the cleaning of the host mineral cements, before they are crushed below solvent and worked-up in the usual manner for biomarker analysis and GC ana-lysis. One clean-up process used to clean the mineral samples is solvent extraction, often with mixtures of di-chloromethane and methanol using a Soxhlet apparatus (Karlsen et al., 1993; Macleod et al., 1994). Other, more severe clean-up methods are also used such as washing samples in hydrogen peroxide (Aplin et al., 1997; George et al., 1997, 1998b,c) or chromic acid (Karlsen et al., 1993; Macleod et al., 1994; George et al., 1998b,c). Indications of the success of the clean-up procedures have been obtained by observing the cleaned-up miner-als under dark-®eld UV light microscopy for non-inclu-sion related ¯uorescence or by further Soxhlet extraction of the cleaned-up minerals and analysis of the extract by GC (Karlsen et al., 1993; George et al., 1997,1998a,b). Some procedures involve the disintegration of reservoir sandstones into individual grains which are then sepa-rated into mineral groups using sink-¯oat or magnetic separation techniques, before further clean-up (Karlsen et al., 1993; George et al., 1998b,c).
2. Methods
The samples required to yield the necessary informa-tion for the study can be selected after a petrographic and microthermometry study of the samples has been completed and a knowledge of the assorted petroleum ¯uid inclusion assemblages has been obtained (Gold-stein and Reynolds, 1994). In this work 10±20 g samples of host rock or minerals are routinely used, though useable quantities of included petroleum have been extracted from ca. 2±3 g of host material. The amount of material required to liberate the necessary quantity of petroleum for a reproducible biomarker analysis is directly proportional to the abundance of petroleum inclusions present in the sample; hence the importance of the initial petrographic and microthermometry observations for selecting appropriate samples.
In view of the very low amounts of hydrocarbons present in inclusions, strict precautions to avoid con-tamination are necessary; all solvents used are Distol grade (Fisher UK Ltd.) or redistilled on a 30 plate Old-ershaw column. Alumina (Brockmann Grade 1 for col-umn chromatography; BDH UK Ltd.) and silica gel (Merck Kieselgel, 60; BDH UK Ltd.) adsorbants and cotton wool plugs were pre-extracted before use and the adsorbants activated to 120C overnight prior to use.
All solvents and adsorbants were tested before use to ensure purity. All glassware used was cleaned using fresh chromic acid and rinsed in distilled water before use. Procedural blanks were run with each batch to monitor any contamination problems.
2.1. Sample preparation and crushing
The sample clean-up methodology is shown schema-tically in Fig. 1. Firstly, the amount of petroleum adhering to the surface of the samples was determined, which also provided an aliquot of the so called ``free hydrocarbons'' for comparison with the included petro-leum. For this, a sub-sample of the rock was extracted using dichloromethane (DCM) as follows. An aliquot (3 g) of the rock chips supplied was extracted (ultra-sonic bath, 10 min) into DCM (5 ml). The total extract was cleaned up by elution through an activated alumina (1 cm bed) and silica gel (2 cm bed) Pasteur pipette mini-column with hexane (10 ml) and DCM (10 ml) in order to remove heavy polars, particulates etc. The elu-ate was then concentrelu-ated by rotary evaporation and analysed by gas chromatography (GC) in order to assess the amount of free hydrocarbons present in the sample and hence the amount of recovery (surrogate) standards to add to achieve comparable concentrations. The surrogate standard used was a 2 mg/ml solution of squalane (BDH Chemicals Ltd), 1,10-binaphthyl (Kodak
Ltd.) and n-phenylcarbazole (Aldrich UK Ltd.) in DCM.
The rock sample for analysis of included petroleum was ®rst disaggregated as gently as possible using a pestle and mortar so as not to cause excessive losses of ¯uid inclusions in mineral grains. The surrogate stan-dard (diluted if necessary to achieve a volume of1 ml) was added to an aliquot of the disaggregated rock sam-ple (10±20 g) in a 30 ml vial, which was then allowed to dry at room temperature for at least 2 h. The sample was then ultrasonically extracted ®ve times with succes-sive 20 ml amounts of DCM. The extracts were com-bined and saved. The sample was then Soxhlet extracted for 24 h using 250 ml of an azeotropic DCM/methanol
(93:7) mixture. After this time the extract ¯ask was removed, the solvent rotary evaporated and the extract saved and combined with the ultrasonic extracts. Another ¯ask containing fresh solvent was then used to Soxhlet extract the sample for a further 24 h.
After thorough drying (40C, 48 h), the thoroughly
extracted rock sample was transferred to a 200 ml bea-ker to which about 50 ml of hydrogen peroxide (100 volumes, i.e. about 30%; BDH Ltd. UK) was added. Some samples generated many small bubbles on addi-tion of the hydrogen peroxide. After 24 h the hydrogen peroxide was decanted and a fresh aliquot of hydrogen peroxide added. If initial bubbling was minor then the samples were placed in an ultrasonic bath for 5 min. This procedure was repeated twice more giving a total of 4 hydrogen peroxide treatments over 5 days. Some samples partially decompose to a ®ne clayey sludge with sandy grains during the hydrogen peroxide treatment. This sludge was removed after the ®nal hydrogen per-oxide treatment when the samples were washed (6) with distilled water. The remaining rock particles were then air dried at 40C for 48 h.
The solvent extracted and hydrogen peroxide treated dry rock particles were then further Soxhlet extracted twice. The ®nal Soxhlet extract was cleaned-up on a mini-column as described above, concentrated to100
ml and analysed by GC and GC±MS.
If the chromatograms still showed the presence of surrogate standards, then the hydrogen peroxide and subsequent Soxhlet extraction procedure was repeated. If the chromatogram showed no or negligible amounts of surrogate standards and other hydrocarbons, the rock particles were then crushed under 30 ml of DCM:methanol (9: 1) using a chromic acid cleaned pestle and mortar to release the hydrocarbons from any petroleum ¯uid inclusions in the rock particles. This was termed the crush-leach extraction. The resulting slurry was transferred to a glass centrifuge tube and cen-trifuged at 3000 rpm for 5 min. The supernatant was transferred to a round bottomed ¯ask and combined with the supernatants from two further DCM washes of the sludge in the centrifuge tube. Since the supernatants often still contained suspended clayey material they were carefully rotary evaporated just to dryness so that the suspended material remained stuck to the walls of the ¯ask. The ¯ask was then carefully rinsed (3) with
2 ml aliquots of DCM. This DCM extract was then
reduced in volume to0.5 ml and cleaned-up on a mini-column as described above, concentrated to100ml and analysed by GC and GC±MS. For quantitative analysis, internal standards of n-heptadecylcyclohexane (ICN Biochemicals Ltd), p-terphenyl (Fluka Ltd), n -dode-cylperhydroanthracene (a gift from BP) and D4 choles-tane (synthesised in this laboratory) for quanti®cation of the saturated hydrocarbons, aromatic hydrocarbons, triterpanes and steranes, respectively, were added to the
vial containing the included hydrocarbons, just prior to GC and GC±MS analysis. These internal standards were chosen on the same basis as the surrogate stan-dards i.e. that they do not generally coelute with other abundant oil components and they are unlikely to occur naturally in petroleums in signi®cant quantities (though caution is required since, for example, some oils can contain squalane).
2.2. GC and GC±MS
Gas chromatographic analyses on the free hydro-carbon fractions were carried out on a Hewlett Packard 5890A-II instrument ®tted with a Hewlett Packard 7673 autosampler. A Hewlett Packard HP-1 poly-methylsilicone coated (0.25 mm ®lm thickness) fused silica capillary column (25 m0.25 mm i.d.) was employed, using hydrogen as carrier gas and a ¯ame ionisation detector. Splitless injection was used and the oven was programmed from 50C (2 min) to 300C at
4C minÿ1. The gas chromatographic analyses of the
included oils were carried out using cold on-column injection onto a Carlo Erba Mega series 5360 gas chro-matograph, ®tted with a Hewlett Packard HP-5 phe-nylmethylsilicone-coated (0.25mm ®lm thickness) fused silica capillary column (25 m0.25 mm i.d.). Hydrogen was used as carrier gas and ¯ame ionisation detection was used. The GC oven program was as follows: 50C
for 2 min; 50±300C at 6C minÿ1; 300C for 20 min.
Data were acquired and processed using a VG Multi-chrom Multi-chromatography data system. Gas Multi- chromato-graphy±mass spectrometry (GC±MS) analyses were carried out on a Hewlett Packard 5890-5972 MSD quadrupole instrument ®tted with a HP-1 coated (0.25
mm ®lm thickness) fused silica column (25 m0.25 mm i.d.). Splitless (1 min) injection was used and the GC oven temperature programmed from 40C (held for 5
min) to 300C at 4C minÿ1 where it was held for 20
min. Data were mainly acquired in the selected ion monitoring (SIM) mode, though some full scan data were also acquired. Some analyses were also carried out, under similar analytical conditions, on a VG/Fisons Trio 1000 GC±MS system.
3. Results and discussion
¯uid inclusions. These carbonate rocks samples con-tained stylolytes with abundant insoluble organic matter which was dicult to remove. However, since these samples were carbonates, the hydrogen peroxide oxida-tion procedure was necessary for the sample clean-up since an aggressive acidic oxidising agent (such as chro-mic acid) could not be used, though the latter would clearly be a very eective agent for the clean-up of quartz grains (e.g. Karlsen et al., 1993; George et al., 1998b). Furthermore, in samples where rock dis-aggregation and separation into single grains of indivi-dual minerals is possible (e.g. Karlsen et al., 1993; George et al., 1998b), then in addition to increasing the selectivity of the inclusions analysed this can also reduce the number of clean-up steps required and improve the clean-up eciency, since it is possible that standards added to composite grains may not easily penetrate microfractures or voids in mineral cements.
3.1. Saturated hydrocarbons
Examples of the gas chromatograms of the free hydrocarbons from the ®rst solvent extraction (S1) of the samples GA1 and GA2 are shown in Fig. 2, while Fig. 3 shows those of the included hydrocarbons released by the crush-leach procedure after exhaustive clean-up of these samples. Comparison of Figs. 2 and 3 clearly shows the major dierences in the saturated hydrocarbon distributions between the free (extractable) and the included fractions released by the crush-leach procedure. While the free saturated hydrocarbons are dominated by narrow ranges of n-alkanes (centered around C19and C15in samples GA1 and GA2,
respec-tively) and contain signi®cant unresolved complex mix-tures (UCMs) in their chromatograms, similar to those often seen in oil-based-mud contaminated samples, the distributions seen in the crush-leach fractions are much more like those found in crude oils. Although the chro-matograms from the crush-leach hydrocarbon fractions also tend to show a ``humpy'' baseline, this is exaggerated
Fig. 2. Gas chromatograms of GA1 (a) and GA2 (b) S1 free total hydrocarbons.n-Alkanes are numbered, Pr is pristane, Ph is phytane, SPCZ, SBN and SSQ are the surrogate standard spikes n-phenylcarbazole, 1,10-binaphthyl and squalane,
respectively.
by the very low signal intensities and the increased visi-bility of the baseline hump due to column bleed that occurs after about 55 min in the chromatograms. It is also clear from the chromatograms of the crush-leach hydrocarbons that the saturated distribution from the GA2 sample was signi®cantly dierent from the others that were analysed from this region, being much more waxy and containing much lower relative abundances of pristane and phytane.
Quanti®cation of the individual hydrocarbons released by the crush-leach procedure (see Table 1) showed that they were generally in extremely low abundance in these samples, with many being found in the low ng/g (ppb) levels. In these particular samples, the individual inclu-ded n-alkane concentrations ranged from below the detection limit of around 1 ng/g in some samples to over 100 ng/g for certain n-alkanes in the GA2 sample. In this latter sample, the ¯uid inclusion C15±C35
hydro-carbons were about an order of magnitude more abun-dant than some of the other samples analysed from this region. Assuming that the n-alkanes comprise about 10% of a typical petroleum, then from the summed
n-C15-ton-C35concentrations in Table 1, the amounts of
petroleum released by the crush-leach procedure was
calculated to be approximately 3000 and 14,000 ng/g rock for the GA1 and GA2 samples, respectively. Since approximately 10 g of each rock sample was crushed and we previously estimated that a typical petroleum ¯uid inclusion may contain 0.4 ng of petroleum, then it would appear that the number of inclusions analysed from the GA1 and GA2 samples were approximately 75,000 and 350,000, respectively. Clearly, with these numbers of inclusions being crushed, the presence of more than a single generation of inclusions would result in mixtures of hydrocarbons being analysed.
3.2. Aromatic hydrocarbons
Alkylated naphthalenes, phenanthrenes and diben-zothiophenes were detected by GC±MS in the included hydrocarbon crush-leach fractions and ratios (see Table 2) commonly used for maturity assessment, such as those based on methylphenanthrenes and methyldibenzothio-phenes were measured (e.g. Radke, 1987). Such aromatic hydrocarbon parameters have previously been reported from petroleum ¯uid inclusions (e.g. George et al., 1997, 1998a). Interestingly, anthracene and methylanthracenes were observed in many, but not all, of the included
Table 1
Included petroleum individual alkane concentrations (ng/g rock) from crush-leach total hydrocarbon fractions
Analyte GA1(ng/g) GA2Ab(ng/g) GA2Bb(ng/g) GA3(ng/g) GA4(ng/g) GA5(ng/g) GA6(ng/g)
nC15 10.8 110.2 120.2 11.6 2.4 13.5 12.1
nC16 14.9 106.2 108.9 8.1 1.7 21.4 14.4
nC17 21.8 103.0 102.6 14.3 2.1 25.0 15.5
nC18 26.7 91.3 88.2 21.5 3.7 26.9 15.6
nC19 25.4 82.5 79.5 25.3 4.8 23.9 16.1
nC20 28.0 87.0 81.9 29.1 7.8 24.1 16.1
nC21 25.5 82.9 78.1 27.3 7.8 22.8 14.8
nC22 21.7 78.4 73.1 26.8 7.0 23.2 13.6
nC23 18.4 69.9 69.5 22.2 6.0 20.1 11.7
nC24 17.4 69.2 64.4 19.9 5.3 21.1 10.7
nC25 15.0 65.8 61.0 17.3 5.1 20.0 9.8
nC26 13.6 67.3 63.3 16.4 4.1 21.4 9.1
nC27 11.4 58.5 54.2 15.3 4.2 18.5 10.8
nC28 10.1 57.0 53.4 11.7 3.2 19.0 11.1
nC29 10.1 48.0 44.1 15.3 3.5 14.5 12.4
nC30 9.0 55.9 52.0 12.0 2.2 18.3 10.4
nC31 8.0 46.6 43.8 10.1 2.5 12.9 9.1
nC32 7.4 47.8 40.9 8.1 1.7 15.0 6.6
nC33 5.4 31.0 28.2 6.5 1.3 9.7 4.7
nC34 5.4 25.4 23.6 5.6 nma 8.3 3.2
nC35 5.8 25.6 19.3 5.0 nm 7.7 3.0
Pristane 8.5 9.5 10.8 5.7 0.8 2.7 9.3
Phytane 9.9 20.2 20.6 10.9 1.4 6.0 5.8
C29abhopane 0.97 24.59 25.75 1.18 nm 11.94 0.16
C30abhopane 1.29 11.96 11.61 2.03 nm 10.52 0.21
a nm : Not measurable.
petroleum crush-leach extracts that were analysed in this and other ¯uid inclusion studies in this laboratory (e.g. see Fig. 4). They have also been reported in petro-leum ¯uid inclusion studies by others (e.g. George et al., 1995, 1997, 1998a). Anthracenes are rarely associated with conventional crude oils but they have recently been noted to occur in unusual crude oils from the Canadian Williston Basin, where they were thought to have been generated by short term, high temperature pyrolysis reactions (Li et al., 1998). Anthracene and methylan-thracenes have previously been detected in coaly sedi-ments, especially those with vitrinite re¯ectance values below about 1.0% (Radke et al., 1982; Garrigues et al., 1988) and methylanthracenes were apparent (D. Karl-sen, 2000, personal communication) in the aromatic hydrocarbon fraction of some of the more terrestrially-in¯uenced crude oils from the Haltenbanken region of the Norwegian continental shelf shown by Karlsen et al., (1993). The presence of anthracenes requires caution when using methylphenanthrene maturity parameters on included petroleum released by crush-leach techni-ques because of the coelution of 1-methylanthracene with the 9- or 1-methylphenanthrene, depending on the gas chromatographic stationary phase. Although pre-sent in a minority of included oils, typically less than 20% (S. George, 2000, personal communication), the relatively common occurrence of anthracenes in inclu-ded petroleums compared to crude oils also raises a question of whether they could be analytical artefacts. For example, are they formed by some kind of very localised heating which occurs when the mineral grains are crushed to release the ¯uids from the inclusions, even though this was done under solvent and, in our case, by relatively gentle hand crushing using a pestle and mortar? It is possible, for example, that local temperatures near
Table 2
Geochemical parameters from included petroleums
Sample Pr/n-C17 Pr/Phc Ts/(Tm+Ts)d C29H/C30Hd C3122S/(S+R)
homohopane
C29Sterane
(bb/bb+aa)e
MPI-1f MPI-3f R
c(%)f 4-/1-MDBTg
GA1 0.39 0.85 0.40 0.75 0.56 nm 0.51 0.78 0.71 2.89
GA2A*b 0.09 0.47 0.28 2.06 0.51 0.56 0.60 0.65 0.76 1.83
GA2B*b 0.11 0.52 0.27 2.22 0.51 0.59 0.62 0.66 0.77 1.96
GA3 0.39 0.52 0.36 0.58 0.50 nm 0.64 0.75 0.78 2.44
GA4 0.37 0.55 nma nm nm nm 0.84 0.90 0.90 2.24
GA5 0.11 0.46 0.13 1.13 0.54 nm 0.68 1.01 0.81 1.05
GA6 0.60 1.60 0.55 0.75 nm nm 0.92 1.18 0.95 3.00
a m=Not measurable.
b GA2A* and GA2B* are two aliquots of the same sample analysed separately. c Pr and Ph are pristane and phytane, respectively.
d H denotes hopane, Ts and Tm are de®ned in Table 3.
e bbandaarefer to the isomeric positions at C-14 and C-17 of the C
29sterane. f MPI-1, MPI-3 andR
c(%) are methylphenanthrene based maturity parameters de®ned in Radke (1987). g 4- and 1-MDBT are methyldibenzothiophenes.
shearing grains may be very high. An alternative expla-nation may be that these aromatic hydrocarbons could have been generated from terrigenous organic matter, and were adsorbed onto mineral surfaces then preserved in inclusions.
3.3. Biomarkers
Examples of the m/z 191 and 218 mass chromato-grams of the included hydrocarbons released by the crush-leach procedure from samples GA1 and GA2 are shown in Figs. 5 and 6. The concentrations of the hopanes in many of these samples were very low (see Table 1), and were close to the detection limits of the GC±MS instrument used, resulting in poor signal to noise ratios in the mass chromatograms from some samples (e.g. GA1). Sterane concentrations in the crush-leach fractions were even lower, with individual C29
sterane isomers being between 4 and about 70 times lower in abundance than the C3017a(H),21b(H)-hopane
peak, and generally were too low to accurately quantify. They were thus of limited use for correlation purposes, though a higher sensitivity mass spectrometer may widen the useful range of concentrations of these compounds. However, even with the instrumentation used, them/z
191 mass chromatogram showing the hopane distribu-tion of the included petroleum from the GA2 sample
displayed a clear and distinctive distribution which was useful for correlation purposes. The C2917a(H),21b
(H)-norhopane to C3017a(H),21b(H)-hopane ratio in this
sample was also signi®cantly dierent from those of the other samples in this region and was consistent with the clear dierences seen in the gas chromatograms of these crush-leach extracts.
3.4. Free hydrocarbon input to crush-leach extracts
An assessment of potential contamination by surface hydrocarbons was made by using mass chromatograms
Fig. 5. m/z 191 and m/z 218 mass chromatograms of GA1 crush-leach (included petroleum) total hydrocarbon fraction showing the presence of hopanes and steranes in abundances close to the detection limits of the GC±MS system used. Hopane peak assignments are given in Table 3, the retention time positions of the C27, C28and C2914b(H),17b(H) steranes
in them/z218 mass chromatograms are marked.
Fig. 6. m/z191 andm/z 218 mass chromatograms of GA2A crush-leach (included petroleum) total hydrocarbon fraction showing the presence of hopane and sterane distributions. Peak assignments are as for Fig. 5.
Table 3
Hopane peak assignments inm/z191 mass chromatograms
Peak Assignment
H1 C2718a(H)-trisnorneohopane (Ts)
H2 C2717a(H)-trisnorhopane (Tm)
H3 C2917a(H),21b(H)-norhopane
H4 C3017a(H),21b(H)-hopane
H5 C3117a(H),21b(H)-homohopane [22S]
to monitor for the presence of surrogate standards which were used to spike the rock before analysis. The abundant peaks due to then-phenylcarbazole (100mg), 1,10-binaphthyl (200 mg) and the squalane (500 mg)
which were added to a 10 g aliquots of the sample GA2 prior to clean-up are clearly seen in the respectivem/z
243, 253 and 57 mass chromatograms of the ®rst solvent extract of the free hydrocarbons of this sample which are shown in Fig. 7a. These peaks are no longer seen in the corresponding crush-leach hydrocarbon fraction mass chromatograms (Fig. 7b). A peak eluting around 55 min in them/z253 mass chromatogram of the crush-leach hydrocarbon fraction has a similar retention time as that of 1,10-binaphthyl, but is probably an unknown
coelutant. A quantitative assessment of the likely amount of cross contamination of the included hydro-carbons from the crush-leach extraction by remaining free hydrocarbons in the samples was made by analysing the hydrocarbons in the ®nal Soxhlet extraction after the ®nal hydrogen peroxide treatment. The results of these are given in Table 4. They show that although none of the samples in this set were rich in petroleum ¯uid inclusions, and some were very lean, the con-centrations of individual alkanes in the crush leach extracts in these samples were between about 100 (in the richest) and 3 (in the leanest) times greater than those in their corresponding ®nal Soxhlet cleanup extract. This
Fig. 7. Comparison of mass chromatograms used to show the presence of the surrogate standard spike compounds in the GA2A free (a) and corresponding (b) crush-leach (included petroleum) total hydrocarbon fractions. Peak assignments are as given in Fig. 2.
Table 4
Individual alkane concentrations (ng/g rock) of free total hydro-carbons in the ®nal soxhlet extract after peroxide treatment
Analyte GA1 GA2A*b GA2B*b GA3 GA4 GA5 GA6
(ng/g) (ng/g) (ng/g) (ng/g) (ng/g) (ng/g) (ng/g)
nC15 2.4 2.5 0.9 0.8 0.0 0.0 0.6
nC16 2.2 2.2 1.8 2.7 0.2 0.5 0.7
nC17 4.6 0.7 1.4 2.2 0.2 0.4 1.1
nC18 3.1 1.0 2.0 2.2 0.8 0.3 0.5
nC19 1.4 1.0 1.2 1.4 2.2 0.5 0.6
nC20 2.1 1.0 1.0 nma 1.4 0.4 0.4
nC21 2.3 1.0 1.4 0.8 1.5 0.3 0.4
nC22 2.6 1.2 1.1 1.4 1.4 0.3 0.4
nC23 2.3 1.2 1.0 1.2 1.3 0.2 0.4
nC24 2.3 1.3 1.2 1.4 1.1 0.0 0.4
nC25 1.6 1.3 1.4 1.3 0.8 0.3 0.5
nC26 2.3 1.6 1.5 1.1 1.1 0.2 0.2
nC27 1.3 1.1 2.5 1.2 1.5 0.2 0.3
nC28 1.4 1.0 3.5 1.5 1.0 0.2 0.2
nC29 1.5 0.9 4.2 1.1 2.7 0.2 0.3
nC30 1.1 1.0 4.3 0.0 0.9 0.2 0.2
nC31 nm nm 4.0 nm nm nm nm
nC32 nm nm 3.2 nm nm nm nm
nC33 nm nm 2.5 nm nm nm nm
nC34 nm nm 1.7 nm nm nm nm
nC35 nm nm 1.1 nm nm nm nm
Pristane 3.0 0.5 1.4 2.2 nm nm nm
Phytane 1.9 0.4 1.0 1.2 nm nm nm
a nm: Not measurable.
b*GA2A and *GA2B are two aliquots of the same sample
shows that the reliability of the included petroleum analyses is vastly improved in samples that contain more abundant ¯uid inclusions.
3.5. Reproducibility
After initial disaggregation, sample GA2 was divided into two aliquots A and B, which were then cleaned-up and analysed separately as though they were two dier-ent samples. The quantitative analysis results given in Tables 1 and 2 show excellent reproducibility for this duplicate analysis both in terms of the quantities of hydrocarbons released and also the composition of them as shown by the various geochemical parameters and ratios measured.
4. Conclusions
A method, based on spiking samples with a mixture of standard hydrocarbons, has been developed allowing the analysis of biomarker and other hydrocarbons in petroleum ¯uid inclusions in reservoir rocks and carrier systems to be made with more con®dence that the hydrocarbons extracted are free from extraneous hydro-carbons derived from the surface of the mineral. The method, which ultimately involves crushing the cleaned-up mineral grains under solvent in order to extract the hydrocarbons released from the ¯uid inclusions (termed crush-leach), is very time-consuming but necessary to ensure a satisfactory clean-up. However, meaningful results from this method are also critically dependent on there being a sucient abundance of petroleum ¯uid inclusions in the sample and also that these inclusions are representative of a single period of geological time (i.e. a single generation of inclusions). This requires that suitable samples can only be selected after a careful petrographic and microthermometric study has been conducted. In some circumstances, the method therefore generates data which can be combined with micro-thermometry and paleo-PVT data in order to give a detailed picture of the source, maturity and physical properties of palaeo-petroleum.
Acknowledgements
We are grateful to M. Chen for her technical assistance in the method development and quantitation work. We also thank Steve Larter and Andy Aplin for their useful discussions and P. Donohoe, K. Noke, R. Hunter and I. Harrison for their technical support. We acknowledge the reviewers D. Karlsen and T. Ruble and associate editor S. George for their constructive criticism and helpful com-ments which considerably improved this paper.
Associate EditorÐS.C. George
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