Article

An improved and rapid 5A molecular sieve method for gas chromatography isotope ratio mass spectrometry of n-alkanes (C8–C30+)

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Abstract

The accurate analysis of stable carbon isotope ratios of individual n-alkanes in petroleum by gas chromatography isotope ratio mass spectrometry (GC–irMS) requires that the n-alkanes be separated from branched/cyclic hydrocarbons. Conventional separation methods include urea adduction or separation using different types of molecular sieves (5A, ZSM5). Although the molecular sieves procedures are well established, they generally require the use of hydrofluoric (HF) acid, a potentially hazardous material, to digest and release compounds trapped within the sieve. In this study, a simplified 5A method that avoids the use of HF acid has been developed to isolate n-alkanes ranging from n-C8 to at least n-C30 from a saturated petroleum hydrocarbon fraction. 12% cyclohexane in n-pentane was found to be a sufficient solvent in extracting most of the n-alkanes (>90%) in the n-C8 to n-C30 range from the sieve. The new 5A sieve procedure is a rapid and safer method for extracting n-alkanes from petroleum to yield fractions amenable for routine GC–irMS.

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... [19] The individual fractions can then be further purified through the use of molecular sievecrystalline zeolite traps which concentrate the hydrocarbon compounds of interest while allowing the remaining unwanted compounds to pass through the porous structure. [20][21][22][23][24] The most common sieves used to separate nalkanes from crude oil saturate fractions are 5A and ZSM-5 sieves. [21][22][23] The collection of the n-alkanes via this process takes several hours (at a minimum), after which they must be released from the sieve, usually through dissolution in a mixture of hydrofluoric acid (HF) and organic solvents. ...
... [20][21][22][23][24] The most common sieves used to separate nalkanes from crude oil saturate fractions are 5A and ZSM-5 sieves. [21][22][23] The collection of the n-alkanes via this process takes several hours (at a minimum), after which they must be released from the sieve, usually through dissolution in a mixture of hydrofluoric acid (HF) and organic solvents. [5,25,26] More recent molecular sieve based methods have removed the requirement for HF; [23] however, in most cases, the use of solvents or the preparation procedure itself still results in the low molecular weight hydrocarbons either being lost through evaporation or contaminated by the presence of solvents. ...
... [21][22][23] The collection of the n-alkanes via this process takes several hours (at a minimum), after which they must be released from the sieve, usually through dissolution in a mixture of hydrofluoric acid (HF) and organic solvents. [5,25,26] More recent molecular sieve based methods have removed the requirement for HF; [23] however, in most cases, the use of solvents or the preparation procedure itself still results in the low molecular weight hydrocarbons either being lost through evaporation or contaminated by the presence of solvents. Although Grice et al. [23] report results for low molecular weight n-alkanes down to n-C 8 (using a cyclohexane/n-pentane dissolution procedure), this still means the potential loss of n-alkanes in the gasoline range (n-C4 to n-C 8 ) as well as all the potentially informative cycloalkane, aromatic and isoprenoid compounds. ...
Article
Rationale: Stable isotope analysis is a powerful tool in understanding the generation, history and correlation of hydrocarbons. Compound-specific δ(13) C measurements of oils allow detailed comparison of individual compound groupings; however, most studies of these sample materials separate and isolate individual fractions based on the chemistries of particular compound groups, potentially losing considerable valuable isotopic data. Even if all fractions are analyzed, this represents a large increase in the data-processing burden, effectively multiplying data evaluation time and effort by the number of fractions produced. Gas chromatography/isotope ratio mass spectrometry (GC/IRMS) of untreated, whole crude oils allows the immediate collection of a larger suite of valuable isotopic data for these studies. Methods: Untreated ('neat', undiluted), whole crude oils were directly injected and measured on a GC/IRMS system, using split (40:1) injections and a 50 m HP-PONA column. The GC method, 97 min in duration, was designed to maximize baseline separation of target analyte peaks, while an additional oxygen flow was admitted into the combustion reactor to maximize the lifetime of the combustion chemicals. Results: The method and setup utilized allow the measurement of a much greater range of the n-alkanes (n-C4 to n-C25+ ) than traditional methods, while also retaining important cycloalkane, aromatic and isoprenoid peaks within the same analysis. Carbon isotope (δ(13) C) evaluation of these additional compound classes reveals trends in maturity and origins which are not identifiable when exclusively assessing the traditional n-alkane package (>n-C12 ). Conclusions: The described setup and method open up new possibilities for assessing the origins and histories of crude oil samples. The data generated for the whole oil n-alkanes by this method is equivalent to that reported for isolated n-alkane studies, while also providing valuable additional data on many other important compounds. The end result of this method is a more complete assessment of the carbon isotopic composition of crude oils. Copyright © 2016 John Wiley & Sons, Ltd.
... 9,10 Since then, the focus has largely remained on applications and data acquisition rather than analytical improvements. While the development of successful separation and preconcentration techniques (e.g., molecular sieves [11][12][13] and urea/thiourea adduction [14][15][16] ) has increased the number and type of samples suitable for compound-specific isotope analyses (CSIA), [17][18][19] the analysis of many geological samples remains prohibitive due to their exceedingly low weight percentage of organic carbon, and consequently the trace quantities of biomarkers. Although low abundance frequently hinders the analysis of biomarkers such as steranes and hopanes, 20,21 even n-alkanes can evade analysis when extracted from organicpoor samples such as ancient soils. ...
... We evaluate the minimum peak widths possible with the pico-CSIA system and the maximum attainable level of precision for various amounts of CO2 reference gas. We then test the ability of the pico-CSIA system to achieve high precision at low sample sizes using a mixture of n-alkanes with independently verified δ 13 peaks of varying size were introduced to the ion source of the isotope ratio mass spectrometer by sampling the effluent of an exponential dilution flask (EDF). 7 The EDF was filled with CO2 and allowed to equilibrate for several hours before switching to the diluent gas, He. ...
Article
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Rationale: We report modifications to compound-specific isotope analyses (CSIA) to enable high-precision isotopic analysis of picomoles of carbon for intact organic molecules. This sample size is two orders of magnitude below the amounts required for commercial systems. The greatly enhanced sensitivity of this system expands molecular isotope studies and applications previously prohibited by low concentrations and small samples. Methods: We utilize the resolving power and low volumetric flow rates of narrow-bore capillary GC to improve sample transfer efficiency while maintaining narrow peak widths. Post-column peak broadening is minimized using a micro-fluidic valve for solvent diversion, capillary combustion reactor, narrow-bore capillary transfer lines, and cryogenic water trap. The mass spectrometer was fitted with collector amplifiers configured to 25 ms response times and a data logger board with firmware capable of rapid data acquisition. Carbon dioxide gas was introduced directly to the ion source to evaluate the dynamic range of the system and accuracy and precision of carbon isotope ratio (δ13C value) measurements. The accuracy and precision for combusted compounds were evaluated using a suite of n-alkanes. Results: For ≥30 pmol carbon introduced directly to the ion source, the mean difference between the measured and expected δ13C values is 0.03‰ (1 sigma, n=57) and the standard deviation of replicate measurements is 0.11‰ (1 s.d.). The CO2peak widths generated by the exponential dilution flask (EDF) were 250 ms and the peak widths produced by combusting n-alkanes were ~500 ms, <25% the width of conventional GC peaks. For a mixture of 15 n-alkanes (n-C16-n-C30), the accuracy is 0.3‰ (1 sigma) and precision is 0.9‰ (1 s.d.) for replicate δ13C measurements with 100 pmol carbon/compound on column. Conclusions: The pico-CSIA method described here offers improved chromatographic resolution and reduces sample size requirements by two orders of magnitude. These advances significantly broaden the available analytical window for CSIA in research areas frequently hindered by sample size limitations, such as forensics, paleoclimate, astrobiology, and biochemistry.
... Straight chain hydrocarbons were separated from branched and cyclic hydrocarbons by treating the saturated fractions with activated (250 o C, 8 hrs) 5A molecular sieves (Murphy, 1969;Dawson et al., 2005;Grice et al., 2008) in cyclohexane. In a typical 5A molecular sieving separation, a portion of the saturate fraction in cyclohexane was added to a 2 mL vial ¾ full of activated 5A molecular sieves. ...
... The resulting solution was then cooled and filtered through a small column of silica (pre-rinsed with cyclohexane), and the sieves were rinsed thoroughly with cyclohexane yielding the branched/cyclic fraction (5A excluded). The n-alkanes were obtained by dissolving the sieve with HF acid as described by Grice et al. (2008). ...
Chapter
We have been studying the effects of diagenesis on the paleomagnetic signal of lacustrine sediments by examining tephra layers found in unaltered and altered states in the same diagenetic environment. In this paper, we report rock magnetic data from seven such layers in six diagenetic environments ranging from saline-alkaline to mildly alkaline. The effects of diagenesis can be complicated and varied but certain patterns are evident. These patterns indicate that both the physical state of the magnetic grains and the geochemistry of the porewaters are important in determining the effects of diagenesis. In some cases, diagenesis simply reduces the intensity of the original magnetization, but in others it produces a new magnetization that completely overwhelms the original one. Measurements of various rock magnetic parameters have allowed us to relate the changes in intensity to changes in the particle-size distribution of the magnetic carriers. In particular, decreases in intensity appear to be associated with selective dissolution of the fine-grained magnetic carriers or with a general reduction in the quantity of magnetic carriers of all grain sizes; increases in intensity seem to result from a reduction in the numbers of coarse-grained magnetic carriers. Plots of the demagnetization behavior of natural and saturation isothermal remanent magnetizations may be useful in discriminating between unaltered and altered material, especially when used in conjunction with other rock magnetic information.
... For compound-specific carbon isotope analyses of pristane (Pr), phytane (Ph) and n-alkanes (Table S4) we undertook molecular sieving (Dawson et al., 2005;Grice et al., 2008;Aboglila et al., 2010) of free and desulfurized saturated fractions of samples from the Taylor White section. Saturated hydrocarbon fractions dissolved in cyclohexane were added to an activated 5 Å molecular sieve (Alltech) and heated for 8 h at 80 • C. Branched and cyclic compounds were recovered by extraction (five times) with cyclohexane. ...
Article
Full-text available
Late Paleocene deposition of an organic-rich sedimentary facies on the continental shelf and slope of New Zealand and eastern Australia has been linked to short-lived climatic cooling and terrestrial denudation following sea level fall. Recent studies confirm that the organic matter in this facies, termed “Waipawa organofacies”, is primarily of terrestrial origin, with a minor marine component. It is also unusually enriched in 13C. In this study we address the cause of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawke's Bay, paired palynofacies and carbon isotope analysis of heavy liquid-separated density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment may be partly due to lignin degradation. Compound-specific stable carbon isotope analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections display similar trends and further reveal a residual 13C enrichment of ∼ 2.5 ‰ in higher plant biomarkers (long chain n-alkanes and fatty acids) and a ∼ 2 ‰–5 ‰ change in subordinate marine biomarkers. Using the relationship between atmospheric CO2 and C3 plant tissue δ13C values, we determine that the 3 ‰ increase in terrestrial δ13C may represent a ∼ 35 % decrease in atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.5 Ma, which coincides with an interval of carbonate dissolution in the deep sea that is associated with a Paleocene oxygen isotope maximum (POIM, 59.7–58.1 Ma) and the onset of the Paleocene carbon isotope maximum (PCIM, 59.3–57.4 Ma). This association suggests that Waipawa deposition occurred during a time of cool climatic conditions and increased carbon burial. This relationship is further supported by published TEX86-based sea surface temperatures that indicate a pronounced regional cooling during deposition. We suggest that reduced greenhouse gas emissions from volcanism and accelerated carbon burial, due to tectonic factors, resulted in short-lived global cooling, growth of ephemeral ice sheets and a global fall in sea level. Accompanying erosion and carbonate dissolution in deep-sea sediment archives may have hidden the evidence of this “hypothermal” event until now.
... The n-alkanes were separated from branched/cyclic hydrocarbons by an improved 5 Å molecular sieve method (Grice et al. 2008) for the analysis of stable carbon isotope ratios on individual n-alkanes and isoprenoids. Compoundspecific carbon isotope measurements of the n-alkanes and isoprenoids were performed using a Trace GC-Ultra gas chromatograph attached to the ThermoFisher Delta-V isotope ratio mass spectrometer (IRMS) via a combustion and high temperature reduction interface (GC Isolink, Ther-moFisher). ...
Article
Full-text available
The Toarcian oceanic anoxic event (T-OAE) is associated with a prominent negative carbon isotope excursion (CIE; ~ 183 million years (Myr)). About 10-m-thick organic matter-rich sediments accumulated during the T-OAE in the Southwest German Basin (SWGB). Rock–Eval, maceral and biomarker analysis were used to determine variations of environmental conditions across the CIE interval. Carbon isotope records were determined for various n -alkanes, pristane and phytane to contribute to the reconstruction of the paleo-environment and to study the factors controlling molecular δ ¹³ C values. Geochemical redox indicators provide evidence for photic zone anoxia during the Toarcian CIE, which reached its maximum after deposition of the “Unterer Stein” marker horizon. The 2α-methylhopane index suggests enhanced activity of diazotrophic cyanobacteria, which is also supported by nitrogen isotope data. This distinguishes the SWGB from other basins with Toarcian black shale. Oxygen-depleted conditions, albeit with lower intensity continued after the CIE. All investigated compounds replicate the negative CIE, but the magnitudes vary considerably. The largest shift is observed for n -C 27 (9‰) and reflects the combined effect of the global CIE and a major change in organic matter input (termination of terrigenous organic matter input). The shift for short-chain n -alkanes, pristane, and phytane, interpreted to reflect marine biomass, varies between 4.5 and 5.0‰. This is the highest value observed so far for any Toarcian section. δ ¹³ C values of pristane and phytane reach a minimum near the base of the CIE interval and increase upsection. Thus, the maximum negative isotope shift predates the strongest basin restriction by about 450 thousand years (kyr).
... The δ 13 C values of n-alkanes (before ZSM-5 treatment), isoprenoids, tricyclic and tetracyclic terpanes and hopanoids (after ZSM-5 treatment) are shown in Table 2. Molecular sieving appears not to impart isotopic fractionation (Grice et al., 2008). Several high-abundance hopanoids (i. ...
Article
Extracts of a set of the Permian Lucaogou shales collected from the southern Junggar Basin in NW China were examined for their molecular compositions and compound-specific isotopic signatures. The distribution of n-alkanes, isoprenoids, steranes, hopanes (C27 to C34) and appearance of 25-norhopanes (C26 to C33) were found to vary between samples, indicating that different degrees of biodegradation occurred in the shale strata. The non-biodegraded sample showed a complete composition of n-alkanes and isoprenoids including pristane (Pr) and phytane (Ph). Moderately biodegraded shales were characterized by near-absence of n-alkanes, but Pr and Ph still showed noticeable contents; 25-norhopanes were not detected. Heavily biodegraded shale had imperceptible content of n-alkanes, isoprenoids and steranes, but a series of abundant 25-norhopanes appeared. Compound-specific isotopic data show negative δ13C values (–44.4‰ to –55.6‰) for the regular hopanes having the same carbon number in all the examined shales, indicating a substantial contribution of methanotrophs to the sediments. Moreover, the 25-norhopanes had consistent δ13C values to the corresponding regular hopanes, which are proposed to be their parent molecules. The distributional and isotopic signatures indicate the 25-norhopanes in the heavily biodegraded Permian Lucaogou shales were genetically formed during in situ biodegradation of the extractable bitumen when the strata were uplifted from depth and may have been linked to the incursion of meteoric water.
... For compound-specific carbon isotope analyses of pristane (Pr), phytane (Ph), and n-alkanes (Table S4) we undertook 165 molecular sieving (Dawson et al., 2005;Grice et al., 2008;Aboglila et al., 2010) of free and desulfurized saturated fractions of samples from the Taylor White section. Saturated hydrocarbon fractions dissolved in cyclohexane were added to activated 5Å molecular sieve (Alltech) and heated for 8 h at 80°C. ...
Preprint
Full-text available
Late Paleocene deposition of an organic-rich sedimentary facies on the continental shelf and slope of New Zealand and eastern Australia has been linked to short-lived climatic cooling and terrestrial denudation following sea-level fall. Recent studies have confirmed that the organic matter in this facies, termed Waipawa organofacies, is primarily of terrestrial origin, with a minor marine component. It is also unusually enriched in δ13C. In this study we aim to determine the cause or causes of this enrichment. For Waipawa organofacies and its bounding facies in the Taylor White section, Hawkes Bay, paired palynofacies and δ13C analysis of density fractions indicate that the heaviest δ13C values are associated with degraded phytoclasts (woody plant matter) and that the 13C enrichment is partly due to lignin degradation. Compound specific δ13C analyses of samples from the Taylor White and mid-Waipara (Canterbury) sections confirms this relationship but also reveal a residual 13C enrichment of ~ 2.5 ‰ in higher plant biomarkers (n-alkanes and n-alkanoic acids) and 3–4 ‰ in the subordinate marine component, which we interpret as indicating a significant drawdown of atmospheric CO2. Refined age control for Waipawa organofacies indicates that deposition occurred between 59.2 and 58.4 Ma, which coincides with a Paleocene oxygen isotope maximum (POIM) and the onset of the Paleocene carbon isotope maximum (PCIM). This timing suggests that this depositional event was related to global cooling and carbon burial. This relationship is further supported by published TEX86-based sea surface temperatures that indicate a pronounced regional cooling during deposition. We suggest that reduced greenhouse gas emissions from volcanism and accelerated carbon burial related to several tectonic factors and positive feedbacks resulted in short-lived global cooling, growth of ephemeral ice sheets, and a global fall in sea level. Accompanying erosion and carbonate dissolution in deep sea sediment archives may have hidden the evidence of this "hypothermal" event until now.
... Please refer to the following publications for expressing and reporting stable isotope ratios, 42,43 and in-depth protocols and discussions on extracting and analyzing plant wax biomarkers and measuring δ 13 C and δD from n-alkanes and n-alkanoic acids. [44][45][46][47][48] are often dominated by specific plant types due to differences among plant lifeforms and physiography. 69 Lipid biomarkers ablated from leaf surfaces can also be transported long distances before deposition, 70 and the size of a catchment area determines the amount of soilderived waxes that settle in marine or lacustrine sediments. ...
Article
Full-text available
Plant wax biomarkers are an innovative proxy for reconstructing vegetation composition and structure, rainfall intensity, temperature, and other climatic and environmental dynamics. Traditionally used in earth sciences and climate studies from “off-site” ocean and lake records, biomarker research is now incorporated in archeology and paleoanthropology to answer questions relating to past human-environment interactions and human evolution. Biomarker research is generating new and exciting information on the ecological context in which Homo and its closest relatives evolved, adapted, and invented stone tool technologies. In this review, we examine plant wax biomarkers and their use in reconstructing past plant landscapes and hydroclimates. We summarize the applications of plant wax molecular proxies in archeological research, assess challenges relating to taphonomy, consider the role of modern plant ecosystems in interpreting ancient habitats, and examine case studies conducted at key paleoanthropological locations in eastern and southern Africa and Europe.
... The n-alkanes were separated from branched/ cyclic hydrocarbons by an improved 5 Å molecular sieve method (Grice et al., 2008) for the analysis of stable carbon isotope ratios on individual n-alkanes and isoprenoids. Stable C isotope measurements were performed using a Trace GC-ultra gas chromatograph attached to the ThermoFisher Delta-V isotope ratio mass spectrometer (IRMS) via a combustion and high temperature reduction interface, respectively (GC Isolink, ThermoFisher). ...
Article
The Neogene Rioni and Kura foreland basins in Georgia are located between the converging Greater and Lesser Caucasus fold-and-thrust belts. The Rioni Basin continues westward into the Black Sea whereas the Kura Basin extends eastward into Azerbaijan and the Caspian Sea. “Pre-” and “post-salt” petroleum systems are distinguished in the Rioni Basin separated by an Upper Jurassic evaporite succession of regional extent. The pre-salt petroleum system in the northern Rioni Basin is still poorly understood. Bathonian shales have generated oil which has been recorded in Middle Jurassic sandstones. However, as the origin of the oil in Upper Jurassic sandstones (e.g. at the Okumi oil discovery) is still problematic, the pre-salt petroleum system remains poorly constrained. Gas-rich, high volatile bituminous coals of Bathonian age may represent a CBM play. The post-salt petroleum system in the Rioni Basin is charged by two prolific source rock units: the Middle Eocene Kuma Formation and the Oligo-Miocene Maikop Group. The petroleum potential of the Kuma Formation, which is about 40 m thick, is classified as good to very good. The Oligocene part of the Maikop Group is several hundred metres thick and contains source rocks with up to 5 wt.% TOC in its lower part. Additional source rocks are present in Cretaceous and lower Paleogene levels. Oil is produced from fractured Upper Cretaceous carbonates in anticlinal structures below the Neogene unconformity and from Mio-Pliocene siliciclastics in fault-related anticlines. Trap formation and hydrocarbon accumulation is interpreted to have occurred since Maeotian time. Proven oil reserves are very low (∼2 million tons) and suggest low charge efficiency. Several stratigraphic horizons containing potential source rocks are present in the Kura Basin of eastern Georgia. Although oil-source correlations have yielded unsatisfactory results, the Maikop Group is the most likely source rock, despite its relatively poor petroleum potential which is at best “fair” in the Tbilisi area in the west of the basin. Additional potential source rocks include Middle and Upper Eocene shales. Fractured Middle Eocene volcaniclastic rocks are the best producing reservoirs for hydrocarbons, but oil accumulations are also found in fractured Upper Cretaceous carbonates and in Lower and Upper Eocene, Oligocene and Neogene siliciclastics. Biomarker data suggest a Cenozoic (or Upper Cretaceous) source rock containing abundant terrigenous organic matter. Anticlines and positive flower structures related to compressional tectonics in front of the Greater and Lesser Caucasus fold-and-thrust belts form the main trap types. Samgori-Patardzeuli-Ninotsminda in the Tbilisi region is by far the largest oil field in Georgia and accounts for nearly 90% of the cumulative production of the country (28.5 million tons). The field was probably charged from a kitchen area located to the north. Strike-slip faults played a major role as migration pathways. Despite of the presence of many oil seeps, proven reserves in the Georgian part of the Kura Basin are very low (2.4 million tons). This may reflects the presence of traps with poor seal integrity.
... The n-alkanes were separated from branched/ cyclic hydrocarbons by an improved 5 Å molecular sieve method (Grice et al., 2008) for the analysis of stable carbon isotope ratios on individual n-alkanes and isoprenoids. Stable C isotope measurements were performed using a Trace GC-ultra gas chromatograph attached to the ThermoFisher Delta-V isotope ratio mass spectrometer (IRMS) via a combustion and high temperature reduction interface, respectively (GC Isolink, ThermoFisher). ...
Article
Hydrocarbon reserves of the order of 1140 MM brl oe have been identified in the northern foreland of the eastern Greater Caucasus, principally in the Terek-Caspian fold-and-thrust belt and the Prikumsk Swell in the north of the Terek-Caspian foredeep. Despite the great economic significance of these areas and their long exploration history, the origin of the hydrocarbons is still poorly understood. In the present paper, geochemical data from 73 oil samples representing 28 fields are used to investigate the presence of oil families and to correlate the oils with potential source rocks. Biomarker composition of oils in Cretaceous and Miocene reservoirs in the Terek-Caspian fold-and-thrust belt is mainly controlled by reservoir depth (100-5700 m) and maturity (0.70-1.15 %Ro), and it is therefore difficult to separate maturity and facies effects. For example, a downward increase in diasterane/sterane ratios may indicate a change in source rock facies or may be attributed to increasing maturity. Some shallow oils are biodegraded. The presence of short-chain n-alkanes in biodegraded oils indicates recent hydrocarbon migration. Biomarker data (e.g. the presence of oleanane) and compound-specific isotope data suggest that the Khadum Formation in the lower part of the Maikop Group is the main source rock. However data from Cretaceous and Paleogene organic-rich rocks, which may also have contributed to the accumulated oils, are urgently needed in order to quantify their possible input. In the Prikumsk Swell, at least two oil families, characterized by low and high C28/C29 sterane ratios respectively, can be distinguished in reservoir rocks of Triassic to Cretaceous age. Most oils are characterized by low C28/C29 sterane ratios and the absence of oleanane (“Group B oils”). These characteristics suggest a pre-Upper Cretaceous source for the oils, which is also supported by the geological setting. Hierarchical cluster analysis suggests the presence of four sub-groups (Sub-Groups B1 to B4). Typically, biomarker ratios in oils in Cretaceous reservoirs are more uniform than those in Triassic and Jurassic reservoirs. Potential source rocks include Lower Triassic deep-water clayey limestones and shales as well as Middle Jurassic and Aptian-Albian marine shales. Three oil samples from Triassic and Cretaceous reservoirs form a separate oil family (“Group A”), which is genetically related to oils from the Terek-Caspian fold-and-thrust belt. Group A oils have high C28/C29 sterane ratios and in general contain at least some oleanane. A contribution by Cenozoic source rocks to Group A oils is likely.
... 28 Molecular sieves have been applied in the separation and enrichment of specific components in 29 geological samples for a long period of time. The 5 Å zeolite, which has a pore size such that only 30 linear components can be incorporated into the pores, enables the isolation of linear saturated 31 classes from branched/cyclic compounds (Tolosa and Ogrinc, 2007;Grice et al., 2008;Águeda et 32 al., 2017). Bissada et al. (2016) separated the saturate fraction into normal paraffins, iso-paraffins 33 and naphthenes, respectively using 5.2 Å and 6.2 Å molecular sieve. ...
Article
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In this study, alkali/acid treatment and ion exchange post-synthesis modification methods to modify the pore diameter and structure of molecular sieve 13X (MS13X) were applied to obtain products, which could then be implemented to improve the enrichment recovery of hopanoids in petroleum. The modification products were analyzed by SEM with EDS, XRD and nitrogen physisorption. The results indicated that the product which had been prepared in the oxalic acid ethanol solution formed some mesopores, and also retained the initial framework of crystal. It was revealed that this product was able to remarkably increase the recovery rate of hopanoids in petroleum without leading to a significant isotope fractionation phenomenon. It is thus shown that this modification method of non-aqueous oxalic acid solution for MS13X can be used as an effective pretreatment tool in the application of compound-specific isotope analysis of hopanes with low concentration.
... The saturated and aromatic compounds were concentrated and internal standards were added (deuterated n-tetracosane and 1, 1 ′ -binaphthyl, respectively). The normal and branched-cyclic alkanes were further separated for compound-specific isotope analyses using an improved 5 Å molecular sieve method (Grice et al., 2008). The isotope fractionation of compounds seems unlikely based on the low-temperature-controlled chemical reactions during sample preparation. ...
Article
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Eocene and Lower Oligocene rocks are potential source rocks for crude oil accumulations in the Hungarian Palaeogene Basin. To enhance the understanding of the hydrocarbon system, this study (i) assesses the petroleum potential of Palaeogene formations, (ii) characterises the source rock facies of the accumulated oils, and (iii) provides an oil-to-source correlation. Rock-Eval data of samples from three boreholes (W–1, W–9 and W–12) show that most Palaeogene formations are mature at depths exceeding 2.1–2.5 km. The coal-bearing Kosd Formation includes good to excellent gas- (and oil-) prone source rocks. The overlying Buda Marl Formation is typically organic matter-lean but contains oil-prone rocks with up to 2.3 wt% TOC and a fair petroleum potential in borehole W–9. The Tard Clay Formation in W–12 reaches up to 1.9 wt% TOC and shows HI values up to 440 mg HC/g TOC, characterising the deposits as good petroleum source rocks. Based on low TOC contents, the Kiscell Clay Formation is not considered a source rock. Molecular parameters of 12 crude oil samples indicate a shaly source rock deposited in a marine/brackish environment. Salinity stratification, causing the development of oxygen-depleted conditions, is likely. The organic matter is dominated by aquatic biomass, including algae, dinoflagellates and chemoautotrophic bacteria. Minor angiosperm-dominated organic matter was transported into the basin from the shoreline. Specific V-shaped compound-specific carbon isotope patterns of n-alkanes observed in crude oils and extracts from the Tard Clay prove the dominant source rock. Minor differences between biomarker ratios are related to vertical and lateral facies variations with the Tard Clay Formation. The accumulated oils are slightly more mature than the Tard Clay in borehole W–12.
... The B&C fraction was evaporated to dryness and weighed. It is important to note that the molecular sieve technique does not introduce isotope bias to the compound specific C isotope analysis, as has been shown previously Grice et al., 2001Grice et al., , 2008Maslen et al., 2011). ...
Article
Thiophenic compounds can provide significant geochemical information and may be used as paleoenvironmental indicators as long as their biological origins and formation pathways are understood. To this end, we investigated the structures, distribution, δ¹³C and δ³⁴S values of specific thiophenic compounds extracted from immature sedimentary rocks from the Upper Cretaceous (Ghareb Formation, Shefela Basin, Israel). Isoprenoidal alkylthiophenes (ATs) showed a general trend of depletion in their ¹³C and ³⁴S relative to the normal (linear) ATs extracted from the studied samples. In addition, a consistent enrichment of up to 2‰ in ¹³C and 8‰ in ³⁴S of methylated ATs (m/z= 111), relative to non-methylated ones (m/z= 97), was recorded. This suggests that AT precursors derived from different organisms and diagenetic pathways that later affected their sulfurization mechanisms. The large variation of ∼15 ‰, in the δ³⁴S values of individual ATs, along with the general trend of ¹³C depletion with increasing Tmax, suggests that ATs formed by a set of diagenetic processes of mild thermochemical alteration. The δ¹³C values of benzothiophenes (BTs) were enriched in ¹³C relative to the rest of the organic sulfur compounds, by ∼3‰ on average. This in turn suggests that BTs were generated mainly by sulfurization and subsequent ring closure, or aromatization of ¹³C-enriched aromatic/ alkyl cyclohexane compounds with unsaturation and/or functional groups in their alkyl chain. Such ¹³C-enriched precursors can be generated from carotenoids and terrestrial compounds (e.g., by lignin degradation). The large variation in δ³⁴S values of individual BTs of ∼15 ‰, suggests that the BTs in our samples were formed during diagenesis and did not experience advanced thermal alteration processes. Therefore, their occurrence in immature sedimentary rocks might be used as a proxy for early thermochemical alteration that cannot be detected using conventional geochemical indicators, such as Tmax. The combination of structural, ¹³C and ³⁴S investigations of individual organic sulfur compounds enhanced an understanding of their sulfurization pathways and origins, which in turn may expand their utilization as paleoenvironmental indicators.
... The enhanced framework stability under pressure with fluid PTM strongly suggests the penetration of the PTM into the MIL-101 pores, [17][18][19]25 which is not possible with the solid NaCl. Indeed, the inclusion of Nujol, made of long-chain alkanes, was expected since the average molecular size is smaller than MIL-101 pore windows (12−16 Å). 25,44 We observed a similar behavior when using silicone oil and Nujol as PTM, which suggests that even silicone oil, made of long PDMS chains, can enter into the pores despite its high viscosity. ...
Article
The chromium terephthalate MIL-101 is a mesoporous metal-organic framework (MOF) with unprecedented adsorption capacities due to the presence of giant pores. The application of an external pressure can effectively modify the open structure of MOFs and its interaction with guest molecules. In this work, we study MIL-101 under pressure by synchrotron X-ray diffraction and infrared (IR) spectroscopy with several pressure transmitting media (PTM). Our experimental results clearly show that when a solid medium as NaCl is employed, an irreversible amorphization of the empty structure occurs at about 0.4 GPa. Using a fluid PTM, as Nujol or high-viscosity silicone oil, results in a slight lattice expansion and a strong modification of the peak frequency and shape of the MOF hydroxyl vibration below 0.1 GPa. Moreover, the framework stability is enhanced under pressure with the amorphization onset shifted to about 7 GPa. This coherent set of results points out the insertion of the fluid inside the MIL-101 pores. Above 7 GPa, concomitantly to the nucleation of the amorphous phase, we observe a peculiar medium-dependent lattice expansion. The behavior of the OH stretching vibrations under pressure is profoundly affected by the presence of the guest fluid, showing that OH bonds are sensitive vibrational probes of the host-guest interactions. The present study demonstrates that even a polydimethylsiloxane silicone oil, although highly viscous, can be effectively inserted into the MIL-101 pores at a pressure below 0.2 GPa. High pressure can thus promote the incorporation of large polymers in mesoporous MOFs.
... After GC-MS analysis, n-alkanes were separated from branched/cyclic fractions in a saturated fraction using 5 Å molecular sieve (Grice et al., 2008). Carbon isotopic analyses of individual n-alkanes were performed using an Isoprime IRMS interfaced to a Table 3 Aromatic parameters for pyrolysate and hydropyrolysate. ...
Article
The pyrobitumen in conventional petroleum reservoirs is a thermally altered product of ancient oil pools, which may provide valuable source information using its molecular and isotopic signatures. Pyrobitumen formation is often associated with thermal alteration of asphaltene or polar-rich components rather than that of oil as a whole. Although asphaltenes are useful in source correlation of altered oils, the geochemical characteristics of asphaltene-derived pyrobitumen are poorly understood. In this study, artificial pyrobitumen formation through oil asphaltene cracking was performed at different thermal simulation (i.e., pyrolysis) temperatures. Systematic variations in the amounts and distributions of extractable and bound hydrocarbons, released by catalytic hydropyrolysis of the artificially produced pyrobitumen were studied. Pyrobitumen production yield was high at pyrolysis temperatures corresponding to post oil peak maturities, with a maximum yield of ∼70 wt% of asphaltene being reached in the early condensate–wet-gas stage. The molecular structure of pyrobitumen varied only slightly over the high maturity range (EasyRo 1.64–2.51%). Compared with the parent asphaltene, the pyrobitumen had a low biomarker (e.g., regular steranes and terpanes) content, and even the carbon isotopic values of the more stable bound n-alkanes were strongly altered. Thermal cracking of asphaltene alone, rather than whole oil, accelerates cross-linking of aromatic units, and cleavage or condensation of molecules bound in the pyrobitumen. These molecular changes suggest that care is needed when using the geochemical characteristics of bound hydrocarbons in natural pyrobitumen for source determinations. Nevertheless, the carbon isotopic ratios of bulk pyrobitumen hydropyrolysate were similar to those of bulk oil or pyrobitumen, even at very high maturities, suggesting these may be reliable indices for source tracing. A comparison of carbon isotopic compositions between pyrobitumen-bound and solvent-extractable n-alkanes could be useful in determining whether they have the same origin. Furthermore, the compositional and isotopic characteristics of bound hydrocarbons in pyrobitumen may provide information on the stage of maturity under geological conditions. The investigation of free and bound molecules may thus elucidate pyrobitumen genesis as related to its source, thermal maturity, and possible later hydrocarbon charging.
... Each fraction was concentrated on rotary evaporator before subjecting to elemental analysis-isotope ratio mass spectrometry (EA-IRMS). In preparation for the gas chromatography-isotope ratio mass spectrometry analysis, nalkanes were isolated from saturated hydrocarbons by treating the saturated fractions with activated (250°C, 8 h) 5A molecular sieves (Dawson D et al., 2005;Grice K et al., 2008a) in cyclohexane. Half of a 2 mL vial was packed with activated 5A molecular sieves in a typical 5A molecular sieve separation. ...
Article
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Thirty-nine crude oils and twenty-one rock samples from Niger Delta basin, Nigeria have been characterized based on their isotope compositions by elemental analysis-isotope ratio mass spectrometry and gas chromatography-isotope ratio mass spectrometry. The bulk carbon isotopic values of the whole rock extracts, saturate and aromatic fractions range from -28.7‰ to -26.8‰, -29.2‰ to -27.2 ‰ and -28.5‰ to -26.7‰, respectively while the bulk carbon isotopic values of the whole oils, saturate and aromatic fractions range from -25.4‰ to -27.8‰, -25.9‰ to -28.4‰ and -23.5‰ to -26.9‰, respectively. The average carbon isotopic compositions of individual alkanes (nC12-nC33) in the rock samples range from -34.9‰ to -28.2‰ whereas the average isotopic values of individual n-alkanes in the oils range from -31.1‰ to -23.8‰. The δ13C isotope ratios of pristane and phytane in the rock samples range from -29.2‰ to -28.2‰ and -30.2‰ to -27.4‰ respectively while the pristane and Phytane isotopic values range from -32.1‰ to 21.9‰ and -30.5‰ to -26.9‰, respectively. The isotopic values recorded for the samples indicated that the crude oils were formed from the mixed input of terrigenous and marine organic matter and deposited under oxic to sub-oxic condition in lacustrine-fluvial/deltaic environments. The stable carbon isotopic compositions were found to be effective in assessing the origin and depositional environments of crude oils in the Niger Delta Basin.
... After GC-MS-MS analysis, n-alkanes in the saturated fractions were separated from branched/cyclic fractions by 5 Å molecular sieving (Grice et al., 2008). Carbon isotopic measurements of individual n-alkanes were performed on an Isoprime isotope ratio mass spectrometer (IRMS) interfaced to a HP6890 GC via a combustion interface. ...
Article
Oil asphaltenes have been used to characterize the source(s) and timing of expulsion of early charged oils. However, the application on asphaltene in a complex petroleum basin have been limited. Marine oils from the Tarim Basin, NW China, were selected to release asphaltene-bound molecules by the catalytic hydropyrolysis technique (Hypy). The analyzed samples were 10 heavy oils from two major oil-production regions (the Tabei and Tazhong uplifts) in the basin. The asphaltenes may be mixed products from multiple hydrocarbon charging stages, if we consider the main oil filling events and the regional geothermal histories of corresponding reservoirs. However, the similar molecular and isotopic characteristics of the hydropyrolyzates indicate contributions from one major source rock (possibly lower Cambrian mudstone) during relatively early periods, based on the presently known source characteristics. The preservation of asphaltenes probably reflects a low reservoir temperature after charging. The asphaltenes were derived from marine algae deposited in a strongly reducing environment, given the biomarker data (e.g., relatively abundant aryl isoprenoids and low Pr/Ph) and carbon isotopic data (−35.5‰ to −34.4‰). Asphaltenes were expelled around the oil generation peak. Moreover, molecular ratios indicative of maturity (e.g., tricyclic terpanes/hopanes), derived by Hypy of asphaltenes, suggest that the asphaltenes of the Tazhong Uplift are generally more mature than those of the Tabei Uplift. Moreover, this is mainly related to different maturities when the oil was expelled. The free hydrocarbons in the Tazhong Uplift show greater molecular and isotopic variations than those in the Tabei Uplift. This might be attributed to more complex accumulation processes in the former. Late oils with a range of maturities, mixing proportions with early biodegraded oils, and degrees of alteration by thermochemical sulfate reduction could all have played a role in generating this variability. The results indicate that the hydrocarbons released from oil asphaltenes by Hypy can provide insights into multiple charging processes in an old composite petroleum basin.
... Saturated hydrocarbon fractions were subjected to 5 A molecular sieving as illustrated by [36]. Abit of the aliphatic fraction in cyclohexane was put into a 2 ml vial, filled with an activated sieve (7 g). ...
Article
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This present paper includes a detailed evaluation of specific biomarkers together with stable carbon isotope (δ13C) by gas chromatography-mass spectrometry (GC–MS) and Gas Chromatograph– Isotope Ratio Mass Spectrometry (GC–IR–MS). Eight crude oil samples were collected from the A, B, H (east) and H (west) Fields, located in the Murzuq Basin, Libya. Stable Carbon isotope data (δ13C) together with biomarker ratios data of individual hydrocarbons, n-alkanes, isoprenoids, terpenes, hopanes, steranes and aromatic have been determined in crude oils to delineate their bacterial degradation, source facies, organic matter precursors, depositional conditions and a variation of maturation. Based on source-specific parameters including n-C19 alkane, % C27S, %C28S, %C29S, %C23TT, %C30αβ, %rC28, DBT/P, CPI, Pr/Ph, Ts/Tm, dh 30/h 30, 1 MN, 2 MN, 26-27 DMN, 15 DMN, 236 TMN, 146-135 TMN, 125 TMN, 136 TMN ratios and δ13C‰ of saturates and aromatics fractions. Such oils showed non-biodegradation, n-C19 peak proved oils generated from a Lower Palaeozoic source rocks as emphasizedvian-C19 peak, the dominance of over C27 and C28 with light Carbon isotope ratio (δ13C‰) values. The abundances of isosterane C29, C27, C28.Tricyclohexaprenol and bacteriohopane polyols and aminopolyols, recommended as mixture bioprecursors of tricyclic terpenes and hopanes, furthermore regular sterane ratio gives values characteristic of Lower Palaeozoic marine source rocks and holding green algae and most likely a quantity of contribution from acritarchs. Carbon preference indices (CPIs)>0.9 pointed to an anoxic deposition, dibenzothiophene to phenanthrene (DBT/Prange 0.49 - 0.58) recommend a siliciclastic source rather than carbonate and/or evaporate saline deposition. The ratios of CPIs, pristane/n-C17 and phytane/n-C18, n-alkanes (C16 to C22) against (C23 to C33), Ts/Tm, C30diahopane/C30hopane, methylnaphthalene, dimethyl naphthalene and trimethyl naphthalene indicated that the oils analysed are mature except the B Field oil being slightly less mature than the A, H (east) and H (west) Fields oils.
... Saturated hydrocarbon fractions were subjected to 5 A molecular sieving as illustrated by [36]. Abit of the aliphatic fraction in cyclohexane was put into a 2 ml vial, filled with an activated sieve (7 g). ...
Article
Full-text available
This present paper includes a detailed evaluation of specific biomarkers together with stable carbon isotope (δ13C) by gas chromatography-mass spectrometry (GC–MS) and Gas Chromatograph–Isotope Ratio Mass Spectrometry (GC–IR–MS). Eight crude oil samples were collected from the A, B, H (east) and H (west) Fields, located in the Murzuq Basin, Libya. Stable Carbon isotope data (δ13C) together with biomarker ratios data of individual hydrocarbons, n-alkanes, isoprenoids, terpenes, hopanes, steranes and aromatic have been determined in crude oils to delineate their bacterial degradation, source facies, organic matter precursors, depositional conditions and a variation of maturation. Based on source-specific parameters including n-C19 alkane, % C27S, %C28S, %C29S, %C23TT, %C30αβ, %rC28, DBT/P, CPI, Pr/Ph, Ts/Tm, dh 30/h 30, 1 MN, 2 MN, 26-27 DMN, 15 DMN, 236 TMN, 146-135 TMN, 125 TMN, 136 TMN ratios and δ13C‰ of saturates and aromatics fractions. Such oils showed non-biodegradation, n-C19 peak proved oils generated from a Lower Palaeozoic source rocks as emphasizedvian-C19 peak, the dominance of C29steranes over C27 and C28 with light Carbon isotope ratio (δ13C‰) values. The abundances of isosterane C29, C27, C28.Tricyclohexaprenol and bacteriohopane polyols and aminopolyols, recommended as mixture bioprecursors of tricyclic terpenes and hopanes, furthermore regular sterane ratio gives values characteristic of Lower Palaeozoic marine source rocks and holding green algae and most likely a quantity of contribution from acritarchs. Carbon preference indices (CPIs)>0.9 pointed to an anoxic deposition, dibenzothiophene to phenanthrene (DBT/Prange 0.49 - 0.58) recommend a siliciclastic source rather than carbonate and/or evaporate saline deposition. The ratios of CPIs, pristane/n-C17 and phytane/n-C18, n-alkanes (C16 to C22) against (C23 to C33), Ts/Tm, C30diahopane/C30hopane, methylnaphthalene, dimethyl naphthalene and trimethyl naphthalene indicated that the oils analysed are mature except the B Field oil being slightly less mature than the A, H (east) and H (west) Fields oils.
... The elutant was analyzed by GC before being separated further by 5A molecular sieving. Following Grice et al. (2008), the n-alkanes were separated from branched/cyclic hydrocarbons using 5A molecular sieves. The n-alkanes were trapped in a 5A molecular sieve that was washed with cyclohexane to remove branched/cyclic hydrocarbons and then sealed in an ASE vial (60 ml) with a mixture of cyclohexane/pentane (9:91, v/v). ...
... Prior to gas chromatography-mass spectrometry (GC-MS) analyses, the acid fraction was methylated with BF 3 -methanol and heated at 70 C for 1 h, while the neutral fraction was silanized with 40 mL N,O-bis (trimethylsilyl) trifluoroacetamide (BSTFA) and heated at 70 C for 1 h. The n-alkanes were separated from branched/cyclic hydrocarbons before compound-specific carbon isotope analysis by 5Å molecular sieve method (Grice et al., 2008). The blank samples were simultaneously measured following the same procedures as sediment samples to ascertain the exclusion of experimental contaminants. ...
Article
Organic matter, as an electron donor, plays a vital role in As mobilization mediated by microorganisms during reductive dissolution of Fe/Mn oxides in shallow aquifers. However, the specific types and sources of organic matter involved in biogeochemical processes accelerating As mobilization are still controversial. Both sediment and groundwater samples were collected at different depths from aquifers of the Hetao Basin, a typical inland basin hosting high As groundwater. Sedimentary lipids and their compound-specific carbon isotope ratios were analyzed to evaluate characteristics and sources of organic matter. Results show that sedimentary As were well correlated with Fe and Mn oxides, suggesting that As exist as Fe/Mn oxide bound forms. Groundwater As far exceeded the drinking water guide value of 10 μg/L. Moreover, As concentrations in shallow groundwater were relatively higher. Lipids in clay were mainly originated from terrestrial higher plants, while that in fine sand samples were derived from terrestrial higher plants, microorganism and petroleum. Shallow fine sand samples were also characterized by evident in-situ biodegradation. Compound-specific carbon isotope compositions of sedimentary lipids showed that short-chain n-alkanes and n-alkanoic acids had more positive δ13C values compared to long-chain compounds, especially in shallow fine sand samples. δ13CTOC were also low in shallow fine sand samples. These results jointly indicate that these lipids in shallow fine sand samples acted as carbon source for indigenous microorganism and the short-chain components were particularly more vulnerable to biodegradation, which may contribute to high As concentrations in shallow groundwater. The new findings provide the first evidence that short chain length n-alkyl compounds afforded a source of potential electron donors for microbially mediated As mobilization process in the shallow aquifers.
... saturates, aromatics). Generally, especially for δ 2 H measurements, an additional step to separate the branched from straight-chain compounds is recommended using urea adduction or molecular sieves (Grice et al. 2008). ...
Article
Full-text available
Compound-specific isotope analysis (CSIA) of individual organic compounds is a powerful but underutilized tool in petroleum exploration. When integrated with other organic geochemical methodologies it can provide evidence of fluid histories including source, maturity, charge history and reservoir processes that can support field development planning and exploration efforts. The purpose of this chapter is to provide a review of the methodologies used for generating carbon and hydrogen isotope data for mid- and high-molecular-weight n -alkanes. We discuss the factors that control stable carbon and hydrogen isotope compositions of n -alkanes and related compounds in sedimentary and petroleum systems and review current and future applications of this methodology for petroleum exploration. We discuss basin-specific case studies that demonstrate the usefulness of CSIA either when addressing particular aspects of petroleum exploration (e.g. charge evaluation, source rock–oil correlation, and investigation of maturity and in-reservoir processes) or when this technique is used to corroborate interpretations from integrated petroleum systems analysis, providing unique insights which may not be revealed when using other methods. CSIA of n -alkanes and related n -alkyl structures can provide independent data to strengthen petroleum systems concepts from generation and expulsion of fluids from source rock, to charge history, connectivity, and in-reservoir processes.
... saturates, aromatics). Generally, especially for δ 2 H measurements, an additional step to separate the branched from straight-chain compounds is recommended using urea adduction or molecular sieves (Grice et al. 2008). ...
... The saturated and aromatic hydrocarbons were eluted with n-hexane and n-hexane/dichloromethane (3:2, v/v), respectively. After gas chromatography-mass spectrometry (GC-MS) analysis, saturated hydrocarbons were further purified to obtain linear alkanes by 5 Å molecular sieving for isotope analysis [23]. ...
Article
When oil has been greatly altered by severe biodegradation, asphaltene pyrolysis is commonly conducted to obtain intact n-alkanes for carbon isotope analysis, through which oil source characterization and oil–oil correlations can be performed. The ratio of hydrogen isotopes deuterium and hydrogen (D/H ratio) in the individual n-alkanes of oil maltenes has been shown to indicate differences between the depositional environments of the oil sources. The utility of the D/H ratio analysis of n-alkanes from the asphaltene pyrolysis for oil correlation need to be demonstrated. In this study, 18 asphaltene samples from different sources were collected and isothermally pyrolyzed in sealed gold tubes. Effects of the pyrolysis temperature were first illustrated for molecular ratios of alkanes and aromatic compounds and for D/H ratios of pyrolysis-yielded n-alkanes. In the case of n-alkanes in oil maltenes being unavailable for analysis after severe biodegradation, the D/H ratio range of the n-alkanes from the asphaltene pyrolysis was found to be consistent both in normal and biodegraded oils from the same source, indicating that biodegradation has had little effect on their hydrogen isotope signatures. In addition, n-alkanes yielded by asphaltenes from different sources possess clearly different D/H ratios, enabling n-alkane correlation between maltenes that are otherwise very similar. However, relatively large differences of D/H ratios can exist between n-alkanes in the asphaltene pyrolyzates and in maltenes, once the oil was altered by oil mixing formed at different maturity levels. The proposed method is thus helpful in oil-oil correlation for significantly biodegraded oil and oil from a mixed origin.
... 124 In addition, if any contaminants in the sample are similar to the compounds of interest, i.e., branched-chain alkanes similar to the n-alkanes, then detection can be compromised. Cleaning the GCMS samples, e.g., urea adduction (Mazeas, et al., 2002;Wakeham & Pease, 2004) or molecular sieve column (Grice, et al., 2008) before GC-IRMS analysis to obtain only n-alkanes can be performed to improve detection of the compound-specific isotope peak values. Cleaning of the GCMS samples, preferably using a molecular sieve column, should be done for future studies. ...
Thesis
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The purpose of this thesis was to identify the different patterns of food consumption across space and time at Liangchengzhen, a Longshan (ca. 2600-1900 B.C.) site located in Shandong Province, China. The primary hypothesis of the research contended that evidence of increasing social inequality with respect to food consumption would be found from early to late phases at Liangchengzhen. In addition, rice and meat from mammals, especially pigs, were hypothesized as the most likely types of prestigious foods for daily and ritual activities. Fish and marine foods in general were hypothesized to be foods that average households could obtain since Liangchengzhen was close to the sea and would not have as high a value as mammal meat. Pottery was sampled from Early Phase storage/trash and ritual pits as well as Late Phase storage/trash and ritual pits located in Excavation Area One. Pottery types included ding and guan, hypothesized for cooking meat, and yan, hypothesized for steaming vegetables and grains. Lipid residue analysis was performed using gas chromatography/mass spectrometry (GCMS) to quantify the amount of C15 and C17 alkane peaks in the pottery and compare these quantities to the amount of C15 and C17 alkane peaks in terrestrial and marine food reference sources. Results indicated that socially valued food consumption transitioned from marine food sources in the early phase ritual pits to rice and pig in the late phase ritual pits. Millet and plant residues were consistently present in storage/trash pits from both early and late phases. Findings also indicated that the use of pottery types for cooking were not limited to one source, i.e., marine, rice, millet and plant residues were found in all pottery types while pig residues were found in ding and yan pottery. Results of the lipid residue analysis provide partial support of increasing social inequality with respect to food consumption from early to late phases at Liangchengzhen, The findings from the lipid residue analysis in this thesis more closely resemble the distribution of integrative, communal consumption pattern in the early phase and a hierarchical consumption pattern during the late phase. Fish, more abundant in the early phase, was almost non-existent by the late phase. Pig and rice, hypothesized as preferred foods, were found only during the late phase, primarily in the ritual pit, H31. Millet and plant were conspicuously present during both phases, but had greater separation from ritual pits during the late phase. However, these findings are surprising since it does not match the material remains of rice and pig found in early phase pits or late phase storage/trash pits from Excavation Area One. It can be concluded that patterns of consumption at Liangchengzhen changed substantially from the early phase to the late phase with regards to food residues found in hypothesized ritual pits. Considering these data with the understanding that food in China has historically been used as a tool to wield influence and power, it can be hypothesized that a social hierarchy may have developed by the late phase that was not present during the early phase. However, participation in the activities held in late phase ritual pits may have been inclusive for all Liangchangzhen residents rather than exclusive for higher status individuals. The current research provides a starting point for further investigation into the foodways at Liangchengzhen. This thesis is the first systematic study of food residues from the interior of Neolithic vessels from ancient China that relates the results of the residue analysis to patterns of food consumption and social change.
... The saturated fraction was purified by a 5 Å molecular sieve as described in the previously reported procedure to remove all branched alkanes and most alkenes. [19] The n-alkane fraction obtained in this way was further purified using an AgNO 3 -silica gel column to remove the few residual alkenes. ...
Article
RationaleCarbon isotope analysis of n-alkanes produced by the pyrolysis of oil asphaltenes is a useful tool for characterizing and correlating oil sources. Low-temperature (320-350 degrees C) pyrolysis lasting 2-3days is usually employed in such studies. Establishing a rapid pyrolysis method is necessary to reduce the time taken for the pretreatment process in isotope analyses. Methods One asphaltene sample was pyrolyzed in sealed ampoules for different durations (60-120s) at 610 degrees C. The C-13 values of the pyrolysates were determined by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The molecular characteristics and isotopic signatures of the pyrolysates were investigated for the different pyrolysis durations and compared with results obtained using the normal pyrolysis method, to determine the optimum time interval. Several asphaltene samples derived from various sources were analyzed using this method. ResultsThe asphaltene pyrolysates of each sample were similar to those obtained by the flash pyrolysis method on similar samples. However, the molecular characteristics of the pyrolysates obtained over durations longer than 90s showed intensified secondary reactions. The carbon isotopic signatures of individual compounds obtained at pyrolysis durations less than 90s were consistent with those obtained from typical low-temperature pyrolysis. Several asphaltene samples from various sources released n-alkanes with distinct carbon isotopic signatures. Conclusions This easy-to-use pyrolysis method, combined with a subsequent purification procedure, can be used to rapidly obtain clean n-alkanes from oil asphaltenes. Carbon isotopic signatures of n-alkanes released from oil asphaltenes from different sources demonstrate the potential application of this method in oil-oil' and oil-source' correlations. Copyright (c) 2016 John Wiley & Sons, Ltd.
... The total extracts were rotary evaporated under near vacuum and separated by solid phase extraction (SPE) with silica gel solid phase. The alkanes were eluted with n-pentane, and analyzed by gas chromatograph (GC) before being separated further by 5 Ǻ molecular sieving (Grice et al. 2008). The 5 Ǻ molecular sieve was washed with cyclohexane to get rid of branched/cyclic hydrocarbons, and then sealed in an ASE vial (60 ml) with a mixture of cyclohexane/pentane 9: 91 (v/v). ...
Article
Full-text available
The last deglaciation is of great interest because Northern Hemisphere climate has gone through several abrupt changes. We present an n-alkane and compound-specific carbon isotope record of the last deglaciation from the annually laminated sedimentary sequence of Lake Xiaolongwan, northeastern China. The n-alkane distribution suggests that the sparsely distributed vegetation prior to 14.7 ka BP, changed to wood plants and aquatic macrophytes in the early Bølling-Allerød, and was followed by a broadleaved deciduous forest after 11.4 ka BP. In this forest region, where the vegetation is dominated by C3 plants, the compound-specific δ13C value of the long-chain n-alkanes (nC27, nC29 and nC31) is interpreted as a proxy of effective precipitation, while temperature might play a minor secondary role. The positive δ13C27–31 values during the Bølling-Allerød suggest a dry-warm climatic condition, while the negative δ13C27–31 at 13.0–11.4 ka BP indicates a cold-wet climate during the Younger Dryas in this region. Two periods with lower δ13C27–31 occurred at 16.1–16.5 and 17.3–17.7 ka BP and could be associated with the Heinrich event-1a and the Heinrich event-1b, respectively. The synchroneity between the biomarker time series in Lake Xiaolongwan and the δ18O record from the Greenland ice core suggest the dominance of high latitude processes on regional monsoon evolution from the last glacial, the Bølling-Allerød, the Younger Dryas to early Holocene. Regionally, the Okhotsk High might have played an important role for abrupt changes during the glacial/interglacial transition.
... En la fracción saturada, aislada del petróleo o del extracto obtenido de una roca, arena o esquisto bituminoso, junto con las iso-parafinas, llamados también iso-alcanos o alcanos ramificados, y cicloparafinas (o cicloalcanos), están presentes los hidrocarburos parafínicos no ramificados o n-alcanos, que -a menudo-, predominan en la mezcla. Una de las estrategias, muy empleada para separar los alcanos lineales (n-parafinas) de los iso-alcanos (e.g., biomarcadores fitano y pristano) y cicloalcanos (la mayoría de biomarcadores saturados pertenecen a este grupo), consiste en emplear diferentes tamices moleculares [47][48][49] o adicionar urea [50] . Varias zeolitas (e.g., 5A, ZSM-5, zeolita Y, otras) se han usado para el aislamiento de n-alcanos y para el enriquecimiento de la fracción saturada con iso-alcanos o cicloalcanos (biomarcadores saturados). ...
Article
Full-text available
Resumo La determinación de biomarcadores del petróleo en mezclas complejas es un reto analítico formidable. Se presentan los resultados favorables de combinar el incremento en la selectividad en el proceso extractivo de los biomarcadores, con la alta especificidad en su análisis por cromatografía de gases acoplada a espectrometría de masas con triple cuadrupolo. En la técnica de dispersión de la matriz en fase sólida se utilizaron tamices moleculares para retirar del extracto los alcanos lineales. En el análisis cromatográfico, se emplearon la inyección splitless pulsada y el monitoreo de reacciones múltiples, para detectar diferentes familias de biomarcadores con especificidad muy superior a la que se logra con el procedimiento tradicional de extracción Soxhlet, seguida del análisis GC-MS en modo de monitoreo de iones seleccionados. Palavras-chave: Cromatografía de gases; Cromatografía de gases acoplada a espectrometría de masas; Espectrometría de masas tándem con triple cuadrupolo; Monitoreo de reacciones múltiples; Biomarcadores; Roca bituminosa; Dispersión de la matriz en fase sólida. Abstract The determination of petroleum biomarkers in complex mixtures is a formidable analytical challenge. This work presents the advantageous results of the combination of the high selectivity in the extractive process, with the highly specific analysis by gas chromatography coupled to mass spectrometry with triple quadrupole. Molecular sieves employed in the matrix solid-phase dispersion technique, removed linear alkanes from the extract. In the chromatographic analysis, pulsed splitless injection and multiple reaction monitoring permitted the detection of different biomarker families with a specificity that was much higher than that achieved with the traditional procedure of Soxhlet extraction followed by GC-MS analysis with selected ion monitoring.
... For compound specific isotope analysis (CSIA), aromatic hydrocarbon fractions (containing the vascular plant biomarkers of specific interest) were separated by alumina thin layer chromatography (TLC) into monoaromatic, diaromatic, triaromatic and tetraaromatic hydrocarbon fractions as described by Ellis et al. (1994). Aliquots of aliphatic hydrocarbon fractions were separated by 5 Å molecular sieving as outlined in Grice et al. (2008) using hydrofluoric acid to digest the 5 Å sieve. ...
... For compound specific isotope analysis (CSIA), aromatic hydrocarbon fractions (containing the vascular plant biomarkers of specific interest) were separated by alumina thin layer chromatography (TLC) into monoaromatic, diaromatic, triaromatic and tetraaromatic hydrocarbon fractions as described by Ellis et al. (1994). Aliquots of aliphatic hydrocarbon fractions were separated by 5 Å molecular sieving as outlined in Grice et al. (2008) using hydrofluoric acid to digest the 5 Å sieve. ...
Article
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Coastal acid sulfate soils (CASS) constitute a serious and global environmental problem. Oxidation of iron sulfide minerals exposed to air generates sulfuric acid with consequently negative impacts on coastal and estuarine ecosystems. Tidal inundation represents one current treatment strategy for CASS, with the aim of neutralizing acidity by triggering microbial iron- and sulfate-reduction and inducing the precipitation of iron-sulfides. Although well-known functional guilds of bacteria drive these processes, their distributions within CASS environments, as well as their relationships to tidal cycling and the availability of nutrients and electron acceptors, are poorly understood. These factors will determine the long-term efficacy of "passive" CASS remediation strategies. Here we studied microbial community structure and functional guild distribution in sediment cores obtained from 10 depths ranging from 0 to 20 cm in three sites located in the supra-, inter- and sub-tidal segments, respectively, of a CASS-affected salt marsh (East Trinity, Cairns, Australia). Whole community 16S rRNA gene diversity within each site was assessed by 454 pyrotag sequencing and bioinformatic analyses in the context of local hydrological, geochemical, and lithological factors. The results illustrate spatial overlap, or close association, of iron-, and sulfate-reducing bacteria (SRB) in an environment rich in organic matter and controlled by parameters such as acidity, redox potential, degree of water saturation, and mineralization. The observed spatial distribution implies the need for empirical understanding of the timing, relative to tidal cycling, of various terminal electron-accepting processes that control acid generation and biogeochemical iron and sulfur cycling.
... Rutherford [3] also used 5A molecular sieve to capture CO2 from mixture. Furthermore, in 2008, Grice et al [4] applied 5A to separate N-alkanes from crude oil. Production of pure hydrogen from a gas mixture containing 60-90 mol% hydrogen is an interesting subject in the petrochemical and oil refining industries. ...
Conference Paper
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5A molecular sieve is used in petroleum and gas industries as an adsorbent, especially for hydrogen purification. In this study, 5A molecular sieve for hydrogen purification in pressure swing adsorber (PSA) is synthesized, and its physical properties are reported. At the first stage, the 4A zeolite powder produced as the precursor of 5A zeolite is prepared using hydrothermal method with the molar ratio of SiO2/Al2O3≈1.5, and then kaolin powder as binding agent is used for preparing extrudates. Then, these pellets were converted to 5A type through ion exchange process using calcium chloride solution. After determining physical properties, adsorption tests for the separation of CH4 and CO from a feed stream (H2=89.9 mol%, CH4= 10 mol%, and CO=0.1 mol%) under severe operating conditions (P=40atm, T=40°C, and GHSV from 1200 to 1750 h-1) are carried out in a laboratory scale fixed-bed adsorber. Results show that the synthesized 5A molecular sieve has acceptable physical properties in comparison to data reported in literature. Moreover, a breakthrough time about 27 sec can be expected.
... The fractions of the hexane soluble organic matter were separated into polar compounds, saturated hydrocarbons and aromatic hydrocarbons by medium pressure liquid chromatography using a Köhnen-Willsch MPLC instrument (Radke et al., 1980). The n-alkanes were separated from branched/cyclic hydrocarbons by an improved 5 Å molecular sieve method (Grice et al., 2008) for the analysis of stable carbon and hydrogen isotope ratios on individual n-alkanes and isoprenoids. Stable C and H isotope measurements were made using a Trace GC-ultra gas chromatograph attached to the ThermoFisher Delta-V isotope ratio mass spectrometer (irMS) via a combustion and high temperature reduction interface, respectively (GC Isolink, ThermoFisher). ...
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The Austrian part of the Alpine Foreland Basin has been studied previously by organic geochemical methods to establish oil-oil and oil-source rock correlations. The lateral distance of oil fields to the position of mature source rocks (Lower Oligocene Schöneck and Eggerding formations) beneath the Alpine nappes in the south suggests long distance migration up to 50km. Minor compositional trends in W-E direction were obtained reflecting differences in depositional environment (facies) and maturity of potential source rocks. Therefore, the oil and source rock samples are thought to provide valuable insights into carbazole distributions in relation to migration distance, maturity and facies. Compound specific carbon isotope analyses on n-alkanes and isoprenoids reflect the W-E trend, already seen within the total hydrocarbon fractions. The observed trend toward lighter δ13C values of alkanes from oil fields in a W-E direction are consistent with lower δ13C values of organic matter in unit c and similar differences obtained from sediments of the Schöneck Formation at wells Oberhofen 1 (W) and Molln 1 (E). The δ13C patterns of n-alkanes and isoprenoids are in agreement with data obtained from the potential marine source rocks (Schöneck Formation) of the oils. The hydrogen isotopic composition of short chain n-alkanes (C15-19) of most oil samples are lower than the data obtained from the extracts of the marine Permian Kupferschiefer of the Polish Zechstein Basin at similar maturation stages. Only the oils from the western oil fields yield δD values of n-alkanes comparable to those obtained from marine source rocks. The data imply brackish water conditions during deposition of 'unit c' of the Schöneck Formation. Differences in the position of δD of isoprenoids in relation to the respective maturation trend line are most probably related to calibration errors. Alkylated carbazoles and benzocarbazoles are only present in sufficient amounts for quantification in oils sampled from the eastern fields. Rock extracts of the Schöneck Formation from drill cores Molln 1 (5200m depth, 0.92% Rr) and from the marginally mature (ca. 0.6% Rr) samples from well Oberhofen 1 (4300m depth) obtained sufficient amounts of carbazoles. The significantly different concentrations of benzocarbazoles in the extracts (around 10μg/g at Oberhofen 1; > 50μg/g at Molln 1) are related to maturity differences, whereas the predominance of 1-methylcarbazole in the Oberhofen samples is inferred to reflect differences in source rock facies. In the oils from the eastern deposits, neither the concentration ratios of methylcarbazoles, nor the benzocarbazole ratio show indications for the influence of facies differences. Maturity of oil expulsion is estimated on the basis of MPI 1 (Rc=0.76-0.88%), confirmed by sterane isomerization parameters. The results argue for a minor influence of maturity differences on the benzocarbazole ratios of the oil samples. The obtained ratios and the reservoir source fractionation parameters, calculated from the data of Molln 1 source rocks and the respective oils, show a systematic decrease with the estimated migration distance. The results argue for the applicability of benzocarbazoles for the assessment of relative differences in migration distances in oil plays characterized by minor differences in source rock facies and maturity during oil expulsion.
... For the analysis of carbon isotopic compositions of individual compounds in the saturated hydrocarbon fractions, n-alkanes were separated from the rest of the hydrocarbons (i.e. branched/cyclic compounds) by the 5 Å molecular sieving method modified according to Grice et al. (2008). Carbon isotope determination on the n-alkanes, the branched/cyclic and the aromatic hydrocarbons was performed using a Trace GC ultra attached to the ir-MS via a combustion interface (GC Isolink, ThermoFisher). ...
Article
Hydrocarbon source rocks (i.e. Tard Clay Formation), containing type-II organic matter, were deposited in the Hungarian Paleogene Basin during Lower Oligocene. A major contribution of aquatic organisms (green algae, dinoflagellates, Chrysophyte algae) and minor inputs from macrophytes and land plants to organic matter accumulation is indicated by n-alkane distribution patterns, composition of steroids, and δ¹³C of hydrocarbon biomarkers. Microbial communities included heterotrophic bacteria, cyanobacteria, chemoautotrophic bacteria, as well as green sulphur bacteria. The presence of methanotrophic bacteria is indicated by ¹³C-depleted hop-17(21)-ene. Higher inputs of terrestrial organic matter occurred during deposition of the lower and uppermost units of the Tard Clay Formation. The terpenoid hydrocarbon composition argues for angiosperm-dominated vegetation in the area of the Hungarian Paleogene Basin. Diterpenoid hydrocarbons, derived from the resins of conifers, are about 2–3‰ enriched in ¹³C compared to the angiosperm biomarkers.
... 5A molecular sieving and recovery of n-alkanes from molecular sieves Saturated fractions obtained from liquid chromatography were further subjected for separation of n-alkanes from branched/cyclic fractions using a 5A molecular sieve. The procedure and conditions of molecular sieving and recovery of n-alkanes were similar to those described by Grice et al. (2008). Briefly, solution of saturated fraction (up to 15 mg/1-2 ml of cyclohexane) was added into a 2 mL vial quarter filled (2 g) with an activated molecular sieve. ...
... The elutant was analyzed by GC before being separated further by 5A molecular sieving. Following Grice et al. (2008), the n-alkanes were separated from branched/cyclic hydrocarbons using 5A molecular sieves. The n-alkanes were trapped in a 5A molecular sieve that was washed with cyclohexane to remove branched/cyclic hydrocarbons and then sealed in an ASE vial (60 ml) with a mixture of cyclohexane/pentane (9:91, v/v). ...
... The saturated hydrocarbon fractions were distilled with n-hexane, and then analyzed by GC-MS. Straight chain hydrocarbons were separated from branched and cyclic hydrocarbons by treating the saturated fractions with 5A molecular sieve using the procedure described by Duan et al. (2003) and Grice et al. (2008). The purified n-alkane fractions were determined by Gas chromatography-isotope ratio mass spectrometry (GC-IRMS). ...
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Composition and distribution of triaromatic steroid hydrocarbons in oil residue after biodegradation and photo-oxidation process were detected, and the diagnostic ratios for oil spill identification were developed and evaluated based on relative standard deviation (RSD) and the repeatability limit. The preferential loss of C27 methyl triaromatic sterane (MTAS) relative to C28 MTAS and C29 MTAS was shown during the photo-oxidation process. In contrast with the photochemical degradation, the MTAS with the original 20R biological configuration was preferentially degraded during the biodegradation process. The triaromatic steroid hydrocarbons retained their molecular compositions after biodegradation and photo-oxidation and most diagnostic ratios derived from them could be efficiently used in oil spill identification. With the exception of the ratio of MTAS-4, the relative standard deviations of other diagnostic ratios of MTAS ranged from 9 to 84% during the photo-oxidation process. However, the RSD of such ratios derived from MTAS were all < 5% even in highly biodegradation, and such parameters may also provide new methods on oil spill identification.
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Intermediate volatility organic compounds (IVOCs) are important precursors of secondary organic aerosols, and their sources remain poorly defined. N-alkanes represent a considerable portion of IVOCs in atmosphere, which can be well identified and quantified out of the complex IVOC pool. To investigate the potential source diversity of intermediate volatility n-alkanes (IVnAs, nC12-nC20), we apportioned the sources of IVnAs in the atmosphere of four North China cities, based on their compound-specific δ13C-δD isotope compositions and Bayesian model analysis. The concentration level of IVnAs reached 1195 ± 594 ng/m3. The δ13C values of IVnAs ranged -32.3 to -27.6‰ and δD values -161 to -90‰. The δD values showed a general increasing trend toward higher carbon number alkanes, albeit a zigzag odd-even prevalence. Bayesian MixSIAR model using δ13C and δD compositions revealed that the source patterns of individual IVnAs were inconsistent; the relative contributions of liquid fossil combustion were higher for lighter IVnAs (e.g., nC12-nC13), while those of coal combustion were higher for heavier IVnAs (e.g., nC17-nC20). This result agrees with principal component analysis of the dual isotope data. Overall, coal combustion, liquid fossil fuel combustion, and biomass burning contributed about 47.8 ± 0.1, 35.7 ± 4.0, and 16.3 ± 4.2% to the total IVnAs, respectively, highlighting the importance of coal combustion as an IVnA source in North China. Our study demonstrates that the dual-isotope approach is a powerful tool for source apportionment of atmospheric IVOCs.
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Stable carbon and hydrogen isotope ratios of individual n‐alkanes and acyclic isoprenoids are important tools in petroleum geochemistry. However, the analysis requires baseline separation and peak profiles using gas chromatography–isotope ratio mass spectrometry to obtain accurate compound‐specific isotope data. Time‐consuming isolation or purification is typically conducted to separate the compounds to avoid co‐elution with other compounds or matrices in crude oils. We developed a simple gas chromatography separation method to simultaneously measure the compound‐specific carbon or hydrogen isotope compositions of n‐alkanes and acyclic isoprenoids. It was achieved by direct injection of the whole crude condensate and light oil or the saturated fractions of different types of crude oils using a 60 m DB‐17ms column. This method simplifies the pre‐treatment of compound‐specific isotope analysis, saves manpower and time, and reduces the use of organic solvents to be more environmentally friendly.
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Crocetane and phytane are two isoprenoids isomers with similar molecular structures and often present together in methane-seep sediments and some Palaeozoic crude oils. Their commonly co-elution on gas chromatography is challenging for quality and quantity analysis, making it impossible to determine their compound-specific isotopic composition, and thus, insight their geological and geochemical significance. A new gas chromatography method is reported here using a DB-17MS column (50%-phenyl-methyl polysiloxane as the stationary phase) that successfully achieved baseline separation of crocetane and phytane and can be used to accurately identify and quantify them on gas chromatography and gas chromatography-mass spectrometry. Routine steroids and terpenoids biomarkers can also be analysed simultaneously. Additionally, their compound-specific carbon isotopic compositions were also measured without matrix influence using this method. This is the first time that a simple chromatographic method for direct determination of compound-specific carbon isotopic composition of crocetane has been reported publicly.
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In this study, biomarkers, together with stable carbon (δ¹³C) and hydrogen (δD) isotopic compositions of n-alkanes have been examined in a suite of condensates collected from the East China Sea Shelf Basin (ECSSB) in order to delineate their source organic matter input, depositional conditions and evaluate their thermal maturity. Previously, GC-MS analyses have shown that all the condensates are formed in oxidizing environment with terrestrial plants as their main source input. No significant differences were apparent for biomarker parameters, likely due to the low biomarker content and high maturity of these condensates. Conventional GC-MS analysis however, may provides limited information on the sources and thermal maturity of complex mixtures due to insufficient component resolution. In the current study, we used comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC×GC-TOFMS) to increase the chromatographic resolution. Compounds such as alkyl cyclohexanes, alkyl cyclopentanes and diamondoids, which can be difficult to identify using conventional GC-MS analysis, were successfully identified using GC×GC-TOFMS. From our analyses we propose two possibly unreported indicators, including one maturity indicator (C5⁻-cyclohexane/5⁺-cyclohexane) and one oxidation-reduction environment indicator (alkyl-cyclohexane/alkyl-cyclopentane). Multiple petroleum charging events were proposed as an explanation for the maturity indicators indexes discrepancy between methyl-phenanthrene index (MPI) and methyl-adamantane index (MDI). In addition, the stable isotopic results show that condensates from the Paleogene have significantly higher positive δ¹³C values of individual n-alkanes than the Neogene samples. Based on δD values, the samples can be divided into two groups, the differences between which are likely to be attributed to different biosynthetic precursors. Variation within each group can likely be attributed to vaporization.
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A complete analysis of oils and their fractions allows correlations to be defined between their composition and derivatives or related geological materials. This work focused on optimization and implementation of a method for separation and quantification of n-alkanes in Brazilian oil samples by urea adduction and GC-FID techniques. Ten samples with different (o)API were analyzed in triplicate to quantify individual n-alkanes and cyclic/branched alkane fraction. For individual quantification of n-alkanes, internal standardization with deuterated n-tetracosane was used. The use of urea adduction for the separation and quantification of n-alkanes was highly effective, with recovery values of above 80%.
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Thesis
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The microbial ecology and, hence, related aspects of the paleoenvironment, of the Green River Lake system (Eocene, Wyoming, Utah, and Colorado) have been reconstructed through geochemical analyses. Specifically, concentrations and isotopic compositions of bulk carbon, sulfur, and nitrogen, as well as carbon-isotopic compositions and abundances of numerous individual hydrocarbons ("biomarkers") have been examined. The Green River Formation was deposited in two large alkaline, saline lakes occupying hydrologically closed basins. Utilizing core material from the Piceance Creek Basin, this study has focused on (1) the sources and cycling of nutrients for the producer community and (2) the pathways of carbon flow in the ecosystem. Estimates of annual accumulation of organic carbon in the Green River lakes are low compared to measurements of productivity in modem saline lakes, suggesting that a great deal of the organic material was reworked prior to preservation. Carbon sulfur relationships indicate sulfate reduction was limited by available sulfate. Organic material enriched in 15N indicates a complex and active nitrogen cycle in which nitrogenous intermediates were cycled through microbial redox reactions. Enrichment of 13C in carbonates suggests extensive methanogenic activity in the sediments. In order to examine effects of environmental changes, samples for isotopic analysis were taken at intervals of 60 m through three different "sulfur cycles", intervals in which concentrations of total sedimentary S varied between -0.2 and -3.5%. Stratigraphic variations of 13C abundance in individual n-alkanes indicated derivation from at least five distinct biological sources. Isotopic variations in numerous materials derived from polyisoprenoids and from di-, tri- and tetraterpenoids indicate mixing of three components: products of primary photoautotrophs and their immediate consumers, products of methanotrophic bacteria, and products of unknown bacteria. Independent variations in 13C contents of a variety of photosynthetic products indicated the presence of a diverse producer community. Hopanoids produced at times of low sulfur concentration were particularly depleted in 13C. indicating significant inputs from methanotrophic bacteria. and suggesting competition between methanogenic bacteria and sulfate-reducing bacteria for metabolic intemediates. This study has been the first to combine detailed stratigraphic sampling with compound-specific isotopic analyses. and indicates that this approach can significantly enhance process-oriented detail in the reconstruction of ancient environments.
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We measured the stable hydrogen isotopic composition (δD) of selected aliphatic hydrocarbons in nine sediments of various maturity and two crude oils, all from the northern Perth Basin (on-shore, Western Australia). The sediments and crude oils are from the Lower Triassic Hovea Member (Sapropelic Interval) of the Kockatea Shale. The n-alkanes, pristane and phytane from two immature sediments have δD values that represent the expected isotopic compositions of their precursors. Phytane is enriched in deuterium (D) relative to pristane in all sediment extracts and crude oils; this is attributed to either different sources, or to different isotopic effects during their derivation from phytol. With increasing maturity, pristane and phytane become enriched in D while the n-alkanes generally remain at a constant isotopic composition. A more rapid enrichment of D in isoprenoids relative to n-alkanes with increasing maturity suggests that hydrogen isotopic exchange is occurring via a carbocation mechanism. The phytane diastereomer ratio for each of the sediments correlates linearly with the progressive enrichment of D in phytane and therefore maturity, suggesting that significant hydrogen exchange occurs at the chiral carbons of phytane. The δD values of pristane and phytane from the crude oils are similar or more positive relative to the δD values of the n-alkanes. The average δD values of pristane and phytane for the sediments correlate well with equivalent vitrinite reflectance values, as does the conventional biomarker maturity parameter Ts/Tm, indicating that δD values may be useful for establishing maturity over a wider range than conventional biomarker parameters. The results provide further evidence that care must be taken not to over-interpret δD values of sedimentary hydrocarbons in samples of high thermal maturity.
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Early research into the stable hydrogen isotopic compositions (δD) of petroleum involved bulk deuterium/hydrogen (D/H) measurements which, while providing some useful information, had to contend with the analysis of complex mixtures of hydrocarbons, and alteration resulting from the rapid exchange of nitrogen-, oxygen- and sulphur-bound hydrogen. The use of gas chromatography-isotope ratio mass spectrometry (GC-irMS) overcomes these problems by allowing the analysis of individual compounds containing only the most isotopically conservative aliphatic carbon-bound (C-bound) hydrogen. This project investigates the geochemical utility and reliability of compound-specific δD values, with the aim to better understand and exploit this analytical capability. To demonstrate the source diagnostic potential of compound-specific δD values, normal and branched alkanes extracted from series of immature bog-head coals (torbanites) were analysed. The torbanites contain immature organic matter predominantly from a single, freshwater algal source, i.e. Botryococcus braunii (B. braunii). The δD values of n-alkanes reflect the climate regime at the time of deposition of the torbanites, and vary mainly in response to the δD values of the source meteoric waters in their depositional environments. n-Alkanes from torbanites deposited at high latitude in a glacial climate are depleted in D by up to 70% relative to those from a torbanite deposited at low latitude under a tropical climate regime. Torbanites deposited in a mid-latitude region under cool-temperate conditions contain n-alkanes with δD values falling in between those of n-alkanes from tropical and glacial torbanites. The δD values of the n-alkanes also reflect their multiple source inputs. For example, a saw-toothed profile of n-alkane δD values in Australian torbanites is attributed to a dual-source system: a predominant B. braunii input, with a minor terrestrial plant input to odd-carbon-numbered n-alkanes in the range n-C20 [subscript] to n-C29 [subscript]. The δD values of n-alkanes and isoprenoids (pristane and phytane) differ significantly in two Permian torbanites from Australia, thought to be reflective of the offset between the δD values of their precursors in extant organisms. The torbanite data indicate that a biological δD signal has been preserved for at least 260–280 million years, extending the utility of δD values for palaeoclimate studies. To elucidate the effect of sedimentary processes on the δD values of petroleum hydrocarbons, three sedimentary sequences have been studied. These comprise one from the Perth Basin (Western Australia) and two from the Vulcan Sub-basin (northern Australia) covering a wide range of maturities, i.e. 0.53–1.6% vitrinite reflectance (Ro). The δD values of n-alkanes extracted from immature-early mature sediments (marine shales/siltstones and mudstones) are consistent with that expected of marine-derived n-alkyl lipids. The hydrocarbons become enriched in D with increasing maturity. The large (ca. 115%) biologically-derived offset between the δD values of n-alkanes and acyclic isoprenoids from immature sediments gradually decreases with increasing maturity, as the isoprenoids become enriched in D more rapidly than the n-alkanes. The D-enrichment in isoprenoids correlates strongly with Ro and traditional molecular maturity parameters. This suggests that H/D exchange during maturation occurs via a mechanism involving carbocation-like intermediates, which proceeds more rapidly with compounds containing tertiary carbon centres. Significant epimerisation of pristane and phytane coincides with their D-enrichment, suggesting that hydrogen exchange occurs at their tertiary carbons. A mechanism is proposed which can account for both H/D exchange and the epimerisation of pristane and phytane in the sedimentary environment. Pristane and phytane extracted from a post-mature sediment from the Paqualin-1 sequence are significantly enriched in D (ca. 40%) relative to the n-alkanes, indicating that D-enrichment persists at very high maturity, and is more pronounced for the regular isoprenoids than the n-alkanes. This supports the notion that H/D exchange causes the observed shift in δD values, rather than free-radical hydrogen transfer. The differences between the δD values of pristane and phytane show opposite trends in the Perth Basin and Vulcan Sub-basin sediments. In the Perth Basin, phytane is enriched in D relative to pristane, likely due to a dominant algal source. In the Vulcan Sub-basin, pristane is enriched in D relative to phytane, and thus is attributed to a lower relative input of algal organic matter. The variance of the δD values of pristane and phytane is generally consistent throughout the maturity range and provides evidence that pristane and phytane exchange hydrogen at similar rates. δD analysis of crude oils and condensates reservoired in the Perth Basin and Vulcan Sub-basin has been carried out to evaluate potential applications in oil-source correlation. The n-alkanes from crude oils and condensates are often more enriched in D than n-alkanes extracted from their supposed source rocks, and the oils also show relatively small differences between the δD values of n-alkanes and isoprenoids. These results suggest significant H/D exchange has occurred, implying that the liquids were generated from mature source rocks. A Perth Basin crude oil (Gage Roads-1) thought to be derived from a lacustrine/terrestrial source contains hydrocarbons that are significantly depleted in D relative to Perth Basin oils derived from a marine source, attributed to variability in the isotopic composition of marine and terrestrial source waters. δD values of n-alkanes from Vulcan Sub-basin crude oils and condensates are largely consistent with their prior classification into two groups: Group A, having a marine source affinity; and Group B, having a terrigenous source affinity. Some oils and condensates are suggested to be mixtures of Group A and Group B hydrocarbons, or Group A hydrocarbons and other as yet unknown sources. An exception is a former Group A oil (Tenacious-1) containing n-alkanes that are enriched in D relative to those from other Group A oils and condensates, attributed to mixing with another source of more mature hydrocarbons. The n-alkane δD profile appears to be indicative of source and sedimentary processes. One Perth Basin crude oil (Dongara-14) contains lower-molecular-weight n-alkanes that are depleted in D relative to higher-molecular-weight n-alkanes, attributed to a mixed marine/terrestrial source. Group A crude oils and condensates from the Vulcan Sub-basin display a ‘bowl-shaped’ profile of n-alkane δD values. An upward inflection in the n-alkane δD profile from n-C11 [subscript] to n-C15 [subscript] is suggested to represent the addition of D-enriched lower-molecular-weight n-alkanes from a more mature wet gas/condensate to an initial charge of lower maturity oil. Ultimately, this project has demonstrated that the δD values of individual petroleum hydrocarbons can be used to elucidate the nature of source organic matter and depositional environments. The preservation potential of lipid δD values is greater than previously thought, although it is clear that H/D exchange accompanying maturation can have a significant effect on the δD values of certain hydrocarbons. Thus, great care must be taken when interpreting δD values of individual hydrocarbons, particularly those derived from sediments of high thermal maturity.
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Thirty-five different samples from three different sulfur cycles were examined in this stratigraphically oriented study of the Shell 22x-l well (U.S.G.S. C177 core) in the Piceance Basin, Colorado. Carbon isotopic compositions of constituents of Green River bitumens indicate mixing of three main components: products of primary photoautotrophs and their immediate consumers (delta approximately -30% vs PDB), products of methanotrophic bacteria (delta approximately -85%), and products of unknown bacteria (delta approximately -40%). For individual compounds synthesized by primary producers, delta-values ranged from -28 to -32%. 13C contents of individual primary products (beta-carotane, steranes, acyclic isoprenoids, tricyclic triterpenoids) were not closely correlated, suggesting diverse origins for these materials. 13C contents of numerous hopanoids were inversely related to sulfur abundance, indicating that they derived both from methanotrophs and from other bacteria, with abundances of methanotrophs depressed when sulfur was plentiful in the paleoenvironment. gamma-Cerane coeluted with 3 beta(CH3),17 alpha(H),21 beta(H)-hopane, but delta-values could be determined after deconvolution. gamma-Cerane (delta approximately -25%) probably derives from a eukaryotic heterotroph grazing on primary materials, the latter compound (delta approximately -90%) must derive from methanotrophic organisms. 13C contents of n-alkanes in bitumen differed markedly from those of paraffins generated pyrolytically. Isotopic and quantitative relationships suggest that alkanes released by pyrolysis derived from a resistant biopolymer of eukaryotic origin and that this was a dominant constituent of total organic carbon.
Chapter
Organic geochemists from varying backgrounds focus several specialities on the analysis of geological samples in order to reach conclusions on the nature of the components present in a rock or sediment, their fate since burial, and the original form of the material in a living organism. The complexity of compounds extracted from rock and sediment specimens requires the discipline of analytical chemistry with the assistance of modern instrumentation for good results and meaningful conclusions. The kinds and relative weights of various component classes in a geological sample provide the specific data on which the geochemist infers the history of a sample. Any valid conclusion depends upon the accuracy and reliability of the methods employed in the analysis. The geochemist attempts to discover how much is present before stating how come it is present in the sample. Often it is important to know the quantity of a component before any interpretation is made on how it is possible for the compound to be present in the specimen.
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A crude oil monoaromatic fraction was applied to a Pasteur pipette packed with dealuminated mordenite molecular sieve and eluted with pentane solvent. n-Alkylbenzenes, n-alkyltoluenes and some n-alkylxylene isomers were sorbed into the mordenite molecular sieve and recovered by acid digestion of the sieve and solvent extraction of the liberated hydrocarbons. The excluded fraction which was eluted in the pentane solvent was enriched in the 3,5- and 2,6-n-alkylxylene isomers and other compounds possessing a higher degree of structural complexity. The mordenite molecular sieve provides a rapid and convenient method for the fractionation of monoaromatic hydrocarbons allowing the more subtle characteristics of these fractions to be studied in greater detail.
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Preparative high pressure liquid chromatography on US-Y zeolite shape-selective molecular sieve was studied for carbon isotopic fractionation effects. We tested a standard mixture [17β, 21β(H)-hopane, 5α-cholestane] and complex natural hydrocarbon mixtures dominated by tetra- and pentacyclic triterpenoids extracted from Oxford Clay shales. We confirmed that steroids and hopanoids were separated on the basis of stereochemical configuration while isotopic analysis of eluents indicated that shape-selective chromatography did not result in isotopic fractionation. US-Y zeolite chromatography can be used to simplify hydrocarbon mixtures and prepare well resolved mixtures of molecular fossils for compound-specific isotopic analyses (CSIA).
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The relative distributions and stable carbon isotopic compositions of certain aliphatic and aromatic biomarkers from 11 Botryococcus braunii rich torbanites from Scotland, South Africa and Australia covering the Late Carboniferous to Late Permian were investigated. The data was scrutinised for any evidence of molecular features which may be characteristic of palaeogeography. The torbanites studied were selected to cover a range of age and palaeoclimatic conditions (e.g. Permian and Late Carboniferous when Gondwana was covered by an extensive ice sheet, the Late Permian when the climate warmed from glacial to cool to temperate and the Carboniferous when Laurasia was located within the equatorial zone). All torbanites are composed of abundant n-alkanes and novel macrocyclic alkanes which, based on similar δ13C values, are ascribed a common origin. All samples are also characterised by a high cyanobacterial input as indicated by abundant hopanoids. Other biomarker distributions and stable carbon isotopic compositional differences separated the torbanites into three groups, which also correlate to three different palaeogeographical/palaeoclimatic precincts: (i) Group A—southern Africa and eastern Australia (Temi) torbanites which are characterised by a high relative abundance of cyanobacterial and methylotrophic hopanoids, abundant branched hydrocarbons and 13C enriched homologous series of monomethylakanes; (ii) Group B—eastern Australia (Newnes and Glen Davis) torbanites contain relatively high amounts of drimanes and abundant 13C enriched homologous series of monomethylakanes; and (iii) Group C—Scottish torbanites (Torbane Hill and Westfield) contain high relative amounts of cyanobacterial hopanoids, methylotrophic hopanoids and abundant branched aliphatics (but no 13C enriched homologous series of monomethylakanes).
Article
Carbon isotopic compositions of saturated hydrocarbon, aromatic hydrocarbon, NSO (resin), and asphaltene fractions of vein-forming gilsonites (Cowboy, Bonanza, Independence, Wagonhound and Harrison veins) in the NE Uinta Basin, Utah, are all similar and isotopically resemble Mahogany Zone shale extracts of the Green River Formation (). Individual molecular fossils in the gilsonites show a wider variability in compositions indicative of their paleoecological origins within the Eocene Uinta/Greater Green River paleolake system. Carbon isotopic compositions of C28 and C29 steranes (), pristance and phytane (−33 to —34%) and perhydroβ-carotene (carotane) () suggest that organisms producing these compounds grew near the lake surface. The C33 carotenoid, lexane, is isotopically identical to the C40 carotenoid, carotane, supporting a common C40 carotenoid precursor. Two groups of hopanes can be differentiated in the gilsonites based on carbon isotopic compositions: C29, C31 and C32 hopanes and moretanes (−40.9 to −44.3%), consistent with a source from midwater bacteria and C30 hopanes and moretanes (−51.0 and −60.5%) for which the strong 13C depletion suggests at least partial derivation from methylotrophs. The isotopic signatures of these molecular fossils suggest the source beds of the gilsonites were deposited in a stratified paleolake system with a depositional environment similar to that which formed the Mahogany Zone shales of the Piceance Creek Basin. Algal biomarkers in the gilsonites are isotopically uniform throughout the area of the gilsonite vein occurrence (∼1500 km3) and resemble those in the Mahogany Zone shale in the Piceance Creek Basin over 50 km to the East. This remarkable lateral uniformity suggests that very similar photic zone water conditions existed across the entire Uinta/Piceance Creek paleolake system at the time of deposition of the gilsonite source bed (Mahogany Zone shale). Sterane isotopic compositions are effective for the genetic correlation of the solid bitumens/immature oils. In contrast, the carbon isotopic compositions of the bacterial biomarkers in the gilsonites vary significantly, suggesting that conditions in deeper waters of the paleolake system were more variable. Carbon isotopic signatures of individual hopane biomarkers, in the Green River formation, appear to be valuable for identifying different source facies characteristics within the same paleolake depositional system.
Article
Peak resolution is an essential requirement for achieving accurate irm-GC/MS compound-specific isotope measurements. Obtaining accurate isotope data from petroleum extracts, in particular, has proven especially difficult due to co-elutions that typically comprise crude oil mixtures. New enhancements in peak resolution in irm-GC/MS analyses have been achieved by substituting a polyimide-coated glass capillary as the furnace reactor. The advantages of the design are significant, and include: (1) incorporation of a zero dead-volume combustion reactor, resulting in almost no loss in gas chromatographic peak resolution, equating to less compound interferences and more accurately measured isotope values and (2) use of direct capillary-to-capillary leak tight, zero dead-volume connections that reduce the incidence of leak problems generally encountered as a result of using fittings of substantially different sizes. In addition, improvements in analytical chromatography using size-exclusion techniques such as urea adduction and molecular sieving reduces the number of components in a sample, and have resulted in the acquisition of more accurate isotope data. Experiments comparing paraffin isotope measurements for whole oil samples to those of urea adducted samples of the same oil have shown that differences of 2‰ are not uncommon. Data from experiments with isotope standards indicate that urea adduction and molecular sieving techniques have no observable fractionation effect on the measurement of isotope values for paraffin components in these mixtures. Considering the intensive effort and expense in generating irm-GC/MS data, the use of relatively inexpensive chromatography techniques such as urea adduction and molecular sieving in isolating components of interest for accurate isotope analysis is justified.
Article
While isotope analysis of bulk fractions is a well-tried and useful tool in oil correlation, use of compound specific isotope ratio mass spectrometry (CS-IRMS) of individual hydrocarbons is still in its infancy. This technique has been developed over the past few years as a useful research tool, but its potential in petroleum exploration has yet to be demonstrated. In this paper both bulk fraction and individual hydrocarbon isotopic data were acquired. Their use was evaluated and compared with traditional geochemical measurements such as gas chromatography and GC-MS. Data on 72 oils from the Norwegian sector of the Central Graben were acquired. Data include δ 13C isotope composition of whole oil and topped oil, as well as saturated and aromatic hydrocarbon, NSO and asphaltene fractions, and δ 13C isotopic composition of individual n-alkanes in the range nC4 to nC35. The same samples were also analysed by capillary GC and GC-MS. GC, GC-MS and bulk fraction isotope data suggest that many of the analysed Greater Ekofisk oils (e.g. Valhall, Edlfisk), i.e. within the Lindesnes Ridge Inversion zone have more than one source facies. Outside of this area, particularly towards the northwest, oils are probably mainly from a single source facies. The isotopic compositions of individual hydrocarbon compounds also reveal the same divisions. In addition, several further features were observed. The normal alkanes of many of the Greater Ekofisk oils are more depleted in 12C between nC20–nC25 than at higher or lower molecular weight (possibly representative of the original microbial precursors). Other oils have relatively uniform n-alkane isotope profiles, with n-alkanes which are enriched in 12C relative to the n-alkanes of oils with the other type of profile. The two different isotope profiles are interpreted as being separate source facies. The least mature Greater Ekofisk oils also become more enriched in 12C with decreasing molecular weight. Mature oils from this area tend to have very similar n-alkane isotope values throughout the measured n-alkane range. The higher maturity oils and condensates have n-alkanes < nC8, which are more depleted in 12C than the n-alkanes >nC8. The tentative interpretation of this is that the highly mature oils/condensates are sourced only from highly mature source rocks, i.e. of a condensate window maturity and are not products of thermal alteration in the reservoir.
Article
A method for separating petroleum hapanoid classes based on the shape and size of their carbon skeletons is demonstrated. The technique is based on the molecular sieving properties of ultrastable Y zeolite (US-Y). This sieve provides a shape-selective window which distinguishes between the various hopanoids. Column chromatography with US-Y zeolite separates the 17α(H)-diahopanes, 18α(H)-norneohopane, 22S 17α(H)-hopanes, 22R 17α(H)-hopanes and the moretanes.
Article
The complete isolation of monomethylalkanes from complex hydrocarbon mixtures is demonstrated. The technique is based on the molecular sieve properties of silicalite, a recently synthesized zeolite. Silicalite provides a direct isolation, ca. 40% pure, of the monomethylalkanes, which are sorbed by silicalite but, unlike the n-alkanes, are excluded from Linde 5A molecular sieve. The use of these two sieves thus provides a shape-selective window, approximately 1 Å wide. The final purification to ca. 98% or more is obtained using bidimensional preparative gas chromatography (GC). Based on the findings, a new and more rapid method for n-alkane determination is now possible. The high selectivity, reversibility and high thermal stability of silicalite are major advantages. Column chromatography with silicalite separates the 2-, 3- and 4-methylalkane isomers with sample loads 100,000-fold higher than capillary GC.
Article
A method for the isolation of hopane and tricyclic terpane concentrates from petroleum saturates is presented. These important biomarkers are selectively adsorbed onto a large-pore-size molecular sieve (zeolite NaX) from pentane solution. The procedure is optimized for quantitative recovery of the biomarkers and is a simple method suitable for routine analysis. Subsequent gas chromatographic analysis allows the quantitative determination of these biomarkers.
Article
A study of the distributions of free and S-bound biomarkers and their δ13C values in the extract of a bulk sample of the Permian Kupferschiefer from the Lower Rhine Basin has been carried out to provide a more detailed characterization of the biological inputs. The distributions and isotopic compositions of selected components released from nickel boride desulphurization have been compared with those of components in the free fraction. The free biomarkers occur in greater abundance than their S-bound counterparts. The distribution and carbon isotopic composition of both free and bound components in the extract indicate a major cyanobacterial and algal input. Likewise, the free and bound aromatic hydrocarbons contain components which reveal an origin from the carotenoids of Chlorobiaceae (green sulphur bacteria), providing molecular evidence for the occurrence of photic zone anoxia in the water column. Most of the components in the complex mixture of mono-, di-, tri- and tetra-aromatics in the free aromatic hydrocarbon fraction result from cyclization and aromatization reactions.
Chapter
The objective of this chapter is to review the open literature on molecular sieve zeolite synthesis, highlighting information regarding the fundamental mechanisms of zeolite crystallization in hydrothermal systems. The text, therefore, focuses on the three primary mechanistic steps in the crystalliza- tion process: nucleation of new populations of zeolite crystals, growth of existing populations of crystals, and the role played by existing zeolite crystal mass in the subsequent nucleation of new crystals or the growth of zeolite crystals in the system.
Article
Branched and cyclic alkane components of crude oil and sediments have been subjected to a liquid chromatography procedure in which a molecular sieve (ultrastable Y) was used as the stationary phase. n-Pentane was used as the mobile phase and the fractions eluted from the column were analysed using GC-MS techniques. The compositions of the first two fractions were markedly different to that of the initial branched and cyclic alkane mixture, especially when crude oils derived from higher-plant sources were used. In particular, the pentacyclic terpanes including bicadinanes, a spirotriterpane, oleananes and taraxastanes were enriched. Other compounds enriched in these fractions included the cadinanes and homocadinanes. Isolation of crude oil fractions enriched in pentacyclic alkanes using this procedure enabled lower concentrations of bicadinanes to be analysed than was previously possible by applying selective ion detection GC-MS techniques to branched and cyclic alkane fractions. Analysis of the fractions enriched in pentacyclic terpanes from an Indonesian crude oil revealed the presence of previously unreported C29 and C30 triterpanes. A comparison of mass spectral data, GC retention behaviours and molecular sieve sorption characteristics of these compounds with those of triterpanes of known molecular structure was used to suggest structures for the unknown compounds.
Article
In this note we describe a procedure using silicalite sorption for rapidly and conveniently isolating a branched and cyclic fraction of a petroleum suitable for analysis of saturated biomarker components by GC-MS. By examination of m/z 191, 217, 218 and 259 mass chromatograms of silicalite-included fractions, and comparison of 5 Å excluded fractions with silicalite-excluded fractions, we have rigorously established that this method is valid for analysis of sterane and hopane biomarker components of petroleum. The method makes possible a one-day turnaround for routine biomarker analysis.
Article
A molecular sieve method using NaX (13X) has been used to separate macrocyclic alkanes (in the carbon number range C14–C34), markers of the freshwater alga Botryococcus braunii, from the branched/cyclic hydrocarbon fractions of sediments and crude oils. The macrocyclic alkanes were included in the 13X sieve and were recovered by HF digestion.
Article
We report a convenient paper layer chromatographic method involving urea inclusion for the separation of straight chain and branched/cyclic alkanes in saturated fractions from sedimentary organic matter. Compared with the well-known 5 A molecular sieving and traditional urea adduction methods, urea paper layer chromatography is more effective, simple and economic.
Article
A rapid small-scale method for the separation and analysis of saturate, aromatic and polar components from petroleum is described using silica gel liquid chromatography and GC–MS techniques. Crude oil samples rich in long chain alkylbenzenes and mono-aromatic steranes were used to demonstrate that complete separation can be obtained using disposable glassware such as Pasteur pipettes and minimal amounts of solvents and silica gel, with sample sizes up to 20 mg. This method affords rapid sample processing suitable for routine use in petroleum geochemistry and environmental forensics.
Article
Forty-two oils/condensates from the Western Canada Sedimentary Basin (WCSB) ranging in reservoir age from Devonian to Cretaceous are compared using the stable carbon isotopic ratio of individual hydrocarbons in the gasoline-range fraction (C5–C8) of these oils. Continuous Flow-Isotope Ratio Mass Spectrometry (CF-IRMS), using a modified solid dilution, purge and trap inletting system can provide reproducible isotopic ratios for the ca. 26 dominant n-, iso-, and cyclo-alkanes and aromatics in the gasoline range. The resulting isotopic signatures, or gasoline “isotopograms”, for oils/condensates provide a diagnostic geochemical technique for oil–oil and oil–source correlations. In many instances, the non-straight-chained hydrocarbons are more diagnostic than n-alkanes. Hierarchical cluster analysis provides a non-subjective approach to the treatment of individual isotopic information. This study confirms that oils from similar or the same sources produce isotopic signatures that are highly correlated.Clear carbon isotopic relationships between individual hydrocarbon compounds in the oils suggest a steady-state kinetic relationship for the generation of oil and gas, as opposed to a thermodynamic equilibrium relationship. This mechanism explains the consistent shift in isotopic ratios between similar compounds, such as the methylalkanes and dimethylalkanes.
Article
A procedure using 5A zeolite sorption to separate cyclic/branched organic compounds from the linear ones was developed and carbon isotopic fractionation effects were investigated in different families of compounds, e.g. within the hydrocarbon and alcohol compounds. The 5A sieve has a pore size such that only linear components can be incorporated into the pores whereas the cyclic/branched compounds are remaining free in the organic solution. The sorbed compounds were released from the molecular sieve with HF and solvent extracted with hexane. The method enables the isolation of linear saturated classes, such as n-alkanes and n-fatty alcohols from branched/cyclic compounds without isotopic fractionation for compound-specific isotope analysis (CSIA) of delta(13)C. However, alkene hydrocarbons, sterols and some aromatics were completely or partly degraded with the molecular sieve.
Distributions and stable carbon isotopic com-positions of individual biological markers from the Permian Kupferschiefer (Lower Rhine Basin, N.W. Germany)
  • K Grice
Grice, K., 1995. Distributions and stable carbon isotopic com-positions of individual biological markers from the Permian Kupferschiefer (Lower Rhine Basin, N.W. Germany). PhD Thesis, University of Bristol.
Distributions and stable carbon isotopic compositions of individual biological markers from the Permian Kupferschiefer
  • K Grice