Isotopic Compositions Of Individual N-alkanes Research Articles

Reconstruction of geochemical characteristics of original organic matter in drilling cuttings contaminated by oil-based mud

Oil-based drilling mud (OBM) contamination, commonly occurring in cuttings during petroleum exploration, has had been a problem seriously hindered source rock identification and evaluation. To eliminate the effect of contamination on “free” bitumen extracted directly from source rock, “enclosed” and “bound” bitumens were released from the mineral matrix by demineralization and from the kerogen structure using an improved catalytic hydropyrolysis technique, respectively. Geochemical and carbon isotopic compositions of the three types of bitumen were used to reconstruct the geochemical characteristics of original organic matter in their source rocks. Results indicate that “free” and “enclosed” bitumens have similar biomarker distributions and n-alkane δ13C values in non-contaminated cuttings, but the former is more susceptible to OBM contamination. “Enclosed” bitumen can thus be used to characterize the organic geochemistry of source rocks contaminated by OBM. The carbon isotopic compositions of individual n-alkanes in “free” and “enclosed” bitumens differ from those in “bound” bitumen, likely because of the effects of diagenesis and other secondary alteration. “Bound” bitumen is more likely to preserve the molecular carbon isotopic characteristics of original organic matter in source rocks, while the “free” and “enclosed” bitumens more represent the generation products. Based on biomarker characteristics and carbon isotopic compositions of “bound” hydrocarbons, Wenchang (WC) Formation (Fm.) source rocks in the Zhu I Depression of the Pearl River Mouth Basin (PRMB) can be divided into three types in normal medium–deep (WC–I), shore–shallow (WC-II), and special medium–deep (with different algal blooms growth rates; WC-III) lacustrine source rocks. The special medium–deep lacustrine source rocks of Wenchang Fm. are characterized by high ratio of C30 4-methylsteranes to C29 steranes (4-Me/C29 > 0.66) and heavy δ13C values (δ13Ckerogen < −26.1‰), which are obviously different from those of normal medium–deep lacustrine source rocks. The combination of “free”, “enclosed”, and “bound” hydrocarbons geochemical and isotopic compositions thus provides a comprehensive understanding of source rocks.

Diamondoid fractionation and implications for the Kekeya condensate field in the Southwestern Depression of the Tarim Basin, NW China

Oil and condensate reservoirs frequently experienced later gas charging and cap gas leakage. The influences of these two processes on diamondoid concentrations and compositions are not well documented. To investigate these issues, quantitative GC, GC-MS and GC-IRMS analyses were performed on 24 condensates from the Kekeya Field of gas condensate in the Southwestern Depression of the Tarim Basin, northwestern China. Only normal oils filled the reservoirs of this field prior to gas charging. Based on biomarker concentrations and carbon isotopic composition of individual n-alkanes, these oils were derived from multiple source rocks within the Permian and Middle–Lower Jurassic strata within the early to late window of oil generation. Diamondoid concentrations and compositions of condensates were mainly controlled by the gas that subsequently charged the reservoirs and the extents of cap gas leakage. Two deeper condensates KS101C2 and KS101C3 at 6651–6835 m have extremely high concentrations of 4- + 3-methyldiamantanes (4 + 3MD) in the range of 2523–4296 ppm and very low ratios of adamantane/diamantane (A/D) and methyladamantanes/methyldiamantanes (MAs/MDs) in the ranges of 0.10–0.11 and 0.28–0.33, respectively, demonstrating that these reservoirs were charged by the primary gas with high diamondoid concentration that was generated from very deeply buried source rocks at post mature stage in combination with extensive leakage of the cap gas (the secondary gas). The shallower condensate K2C2 at 3319–3326 m has a moderate 4 + 3MD concentration of 18 ppm and very low ratios of A/D and MAs/MDs of 0.00 and 0.19, respectively, demonstrating that this reservoir was charged by the secondary gas in combination with extensive leakage of the cap gas (the third-generation gas). The shallower condensate KX3C at 3765–3820 m has a lower 4 + 3MD concentration of 9 ppm but very high ratios of A/D and MAs/MDs of 8.60 and 16.73, respectively, demonstrating that this reservoir was charged by the third-generation gas with or without minor cap gas leakage. The other twenty condensates have 4 + 3MD concentrations in the range of 7–23 ppm and ratios of A/D and MAs/MDs in the ranges of 0.60–5.29 and 2.18–12.18, respectively, demonstrating that these reservoirs were charged by the secondary or/and third-generation gases with different extents of cap gas leakage. Furthermore, cap gas leakage influenced alkyladamantane ratios of MAI, DMAI-1, TMAI-1, TMAI-2 and EAI but not the alkyldiamantane ratios of MDI, DMDI-1 and DMDI-2 for maturity and source.

Organic geochemical characteristics of Eocene crude oils from Zhanhua Depression, Bohai Bay Basin, China

Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression. In this study, thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression, Bohai Bay Basin, NE China have been geochemically analyzed and their organic geochemical characteristics have been applied to differentiate groups of oils. These oil samples can be classified into two families based on multiple biomarker proxies and stable carbon isotopic values. Family I is characterized by a low ratio of pristane over phytane (Pr/Ph < 0.7), a relatively high ratio of phytane over n-C18 (Ph/n-C18), varying ratios of gammacerane over C30 hopane (Ga/C30H) and C22/C21 tricyclic terpane, and a low ratio of C19/C23 tricyclic terpane. Family II is marked by a relatively high Pr/Ph ratio (0.7–1.6), relative low ratios of Ph/n-C18 and C22/C21 tricyclic terpane, and avarying ratio of C19/C23 tricyclic terpane. Both families I and II within these crude oils can be subdivided into two families based on different values of stable carbon isotopic composition of individual n-alkanes. Moreover, the potential source rocks of oil samples in Family I and Family II were likely derived from the upper Es4 member and Es3 member, respectively, based on the correlation of organic geochemical characteristics of the oils and source rocks. The results of oil–source rock correlation provide insight into the process from oil generation to migration and to final accumulation, providing a better understanding of factors controlling oil–gas distribution for prediction of sweet spots.

Experimental simulation study on the influence of diagenetic water medium on sedimentary n-alkanes and their respective hydrogen isotopes

In order to understand the influence of the diagenetic water medium on the hydrogen isotope of sedimentary n-alkanes, thermal simulation experiments were carried out involving the initial peat under the addition of alkaline saltwater, acidic freshwater and peat alone condition. The generated n-alkanes were analyzed in terms of molecule composition and hydrogen isotopic composition of individual n-alkanes. The results show that the addition of water had a significant effect on n-alkane nC21−/nC22+ ratio, ACL value and Pr/Ph ratio. The n-alkanes display significant heavier hydrogen isotopic composition from the alkaline saltwater added experiments, but obvious lighter hydrogen isotopic composition from the acidic freshwater added experiments. The difference of the average δD value of n-alkanes between the saltwater and freshwater experiments ranged from 6‰ to 73‰, and maximum difference occurred from the experiments of 350 °C. The exchange of inorganic hydrogen in water and organic hydrogen was more intensive in the acidic freshwater experiments than that in the alkaline saltwater experiments. The average δD value of n-alkanes generated under the participation of saltwater and freshwater media appear to be higher heavier along with the increase in experimental temperature. Also, the average δD value of n-alkanes generated from the saltwater added experiments was always higher than that from the freshwater added experiments. The correlations among the δD composition of n-alkanes generated under different condition were established. These results provide information to understand of the genesis of sedimentary n-alkanes formed under the participation of diverse diagenetic water media at various stages of maturity.

Petroleum sources in the Xihu Depression, East China Sea: Evidence from stable carbon isotopic compositions of individual n-alkanes and isoprenoids

In order to better understand the relative contributions of various source rocks for petroleum in the Xihu Depression, East China Sea Basin, gas chromatography-mass spectrometry analyses of the saturated and aromatic fractions and carbon isotopic analyses of the purified n-alkanes and isoprenoids were performed. The molecular composition and individual n-alkane and isoprenoid carbon isotope compositions were determined for oils and extracts of Paleogene mudstones (TOC: 0.45–2.3%), carbonaceous mudstones (12.7–19.6%), and coals (48–60%) in the Xihu Depression, so as to discriminate the hydrocarbon sources of coals and associated mudstones to the petroleum generated in the basin. Abundant diterpanes (e.g., isopimarane, phyllocladane) are present in all the samples. The differences in the δ13C values of pristane and phytane indicate separate origins for the isoprenoids. A general negative sloping profile with increasing n-alkane carbon chain length was observed in coals, mudstones and oils from the Huangyan oilfield, with individual n-alkane δ13C values between −25.6‰ and −34.4‰. These values are characteristic of the isotopic compositions of C3 plants, and are also typical of fluvial/lacustrine depositional settings. The isotope compositions of individual n-alkanes from the extracts of coals and mudstones correlate with the relative concentration of terrigenous higher plant-derived diterpenoids, which suggests that plant phyla/classes (for example gymnosperm-derived diterpanes versus angiosperm-derived diterpanes) may be a control on the carbon isotope profiles of n-alkanes. For samples dominated by terrigenous organic matter, long chain n-alkanes with a large proportion of gymnosperm input are approximately 2–3‰ enriched in 13C relative to those with angiosperm input, regardless of lithology. The connections between the oils, coals, and mudstones allow determination of the patterns and variations for different hydrocarbon contributions, and provide a way of differentiating specific source contributions to petroleum in the Xihu Depression. Oils generated from the coals have heavier δ13C values with increasing chain length within a narrow range (approx. 2–3‰) and have more heterotrophic origins, whereas oils from the mudstones are more depleted in 13C with increasing carbon number (n-C20-31) and have a greater derivation from land plants. The 825B oils in the Huangyan oilfield have a greater angiosperm-sourced hydrocarbon contribution than those of the 825AD oils, which have a greater gymnosperm-sourced hydrocarbon contribution. The tricyclic and tetracyclic diterpanes and the isotopic compositions of individual n-alkanes and isoprenoids may be useful in differentiating contributions of coals and associated mudstones to petroleum.

Hydrogen Isotope Composition of n-Alkanes Generated during Anhydrous Pyrolysis of Peats from Different Environments

The variation and its level of hydrogen isotopic composition of individual n-alkanes in sediments from different latitudes and climatic environments from immature to post-mature are still unclear. ...

Sources, distributions, and burial efficiency of terrigenous organic matter in surface sediments from the Yellow River mouth, northeast China

Organic geochemical studies of surface sediments from Laizhou Bay, the Yellow River mouth, northeast China, were conducted to reveal the sources, distributions, and burial efficiency of terrigenous OC exported by the Yellow River. Bulk OC contents and δ13Corg values from 204 samples show that terrigenous OC mainly accumulates in two muddy patches, basically controlled by hydrodynamic conditions. Distributions and stable carbon isotopic compositions of individual n-alkanes from 45 samples suggest that short-chain n-alkanes, with even carbon-number predominance, are most likely microbial products from degradation of riverine fossil OC, whereas long-chain odd-numbered n-alkanes (n-C27, n-C29 and n-C31) and long-chain even-numbered n-alkanes (n-C26,n-C28 and n-C30) are representative of riverine soil OC and fossil OC, respectively. Integrated with previous reports of ∼100% burial efficiency of soil OC in the Yellow River mouth, concentration ratios of n-C27+29+31 to OC and n-C26+28+30 to n-C27+29+31 from the riverine particles to surface sediments in Laizhou Bay were constructed to indicate relative terrigenous OC contributions and fossil OC burial efficiency, respectively. The results suggest that the terrigenous OC contribution ranges from 14% to 57% in Laizhou Bay, with the highest values in the two muddy patches as well as in areas near the river mouth. The fossil OC burial efficiency ranges from 36% to 76%, with an TOC-weighted mean value of 42%. The 58% fossil OC lost during the transport and burial processes in the Yellow River mouth may provide a significant CO2 source.

Correlation of crude oils and oil components from reservoirs and source rocks using carbon isotopic compositions of individual n-alkanes in the Tazhong and Tabei Uplift of the Tarim Basin, China

Carbon isotopic compositions were determined by GC–IRMS for individual n-alkanes in crude oils and the free, adsorbed and inclusion oils recovered by sequential extraction from reservoir rocks in the Tazhong Uplift and Tahe oilfield in the Tabei Uplift of Tarim Basin as well as extracts of the Cambrian–Ordovician source rocks in the basin. The variations of the δ13C values of individual n-alkanes among the 15 oils from the Tazhong Uplift and among the 15 oils from the Triassic and Carboniferous sandstone reservoirs and the 21 oils from the Ordovician carbonate reservoirs in the Tahe oilfield demonstrate that these marine oils are derived from two end member source rocks. The major proportion of these marine oils is derived from the type A source rocks with low δ13C values while a minor proportion is derived from the type B source rocks with high δ13C values. Type A source rocks are within either the Cambrian–Lower Ordovician or the Middle–Upper Ordovician strata (not drilled so far) while type B source rocks are within the Cambrian–Lower Ordovician strata, as found in boreholes TD2 and Fang 1. In addition, the three oils from the Cretaceous sandstone reservoirs in the Tahe oilfield with exceptionally high Pr/Ph ratio and δ13C values of individual n-alkanes are derived, or mainly derived, from the Triassic–Jurassic terrigenous source rocks located in Quka Depression.The difference of the δ13C values of individual n-alkanes among the free, adsorbed and inclusion oils in the reservoir rocks and corresponding crude oils reflects source variation during the reservoir filling process. In general, the initial oil charge is derived from the type B source rocks with high δ13C values while the later oil charge is derived from the type A source rocks with low δ13C values.The δ13C values of individual n-alkanes do not simply correlate with the biomarker parameters for the marine oils in the Tazhong Uplift and Tahe oilfield, suggesting that molecular parameters alone are not adequate for reliable oil-source correlation for high maturity oils with complex mixing.

Carbon isotopes of organic matter from the upper Jurassic oil shales from the Volgian-Pechora shale province and its accumulation mechanisms

The isotopic composition of carbon from the organic matter of late Jurassic oil shales from the Volgian-Pechora shale province is studied. The existence of a dependence between Corg content in the rock and the isotopic composition of kerogen carbon is ascertained. The content of the heavy carbon isotope increases with increasing Corg. This dependence is accounted for by the progressive accumulation of isotopically heavy hydrocarbons of the initial organic matter due to sulfurization. The data on the isotopic composition of individual n-alkanes of bitumen in the rocks and the data on the absence of isotopic fractionation between thermobitumen and the residual kerogen from oil shales from the Volgian-Pechora shale province obtained by treating shale in an autoclave in the presence of water are presented first in this paper.

Application of hollow fiber liquid-phase microextraction in identification of oil spill sources

In this study, hollow fiber based liquid-phase microextraction (HF-LPME), coupled with GC, GC–MS and GC–IRMS detections, was employed to determine petroleum hydrocarbons in spilled oils. According to the results, the HF-LPME method collected more low-molecular weight components, such as C 7–C 11 n-alkanes, naphthalene, and phenanthrene, than those collected in conventional liquid–liquid extraction (LLE). The results also showed that this method had no remarkable effect on the distributions of high-molecular weight compounds such as >C 18 n-alkanes, C 1–C 3 phenanthrene, and hopanes. Also, the carbon isotopic compositions of individual n-alkanes in the two preparation processes were identical. Accordingly, HF-LPME, as a simple, fast, and inexpensive sample preparation technique, could become a promising method for the identification of oil spill sources.

The application of stable carbon and hydrogen isotopic compositions of individual n-alkanes to Paleozoic oil/source rock correlation enigmas in the Huanghua depression, China

The carbon and hydrogen isotopic compositions of individual n-alkanes from the Paleozoic source rocks, oil and oil sand in the Huanghua depression, China, were determined for oil/source correlation purposes. The sedimentary facies of the upper Paleozoic (terrigenous-marine inter-bedded) is in this paper concluded to be different from that of the lower Paleozoic (marine) in the Huanghua depression. As shown by the results of the isotope analyse, the δ 13C and δD (δ 2H) values of the individual n-alkanes of the lower Paleozoic samples range from − 29‰ to − 33‰ and − 110‰ to − 140‰, respectively, while they range from − 27‰ to − 29‰ and − 140‰ to − 170‰, respectively, for the upper Paleozoic samples. The latter is relatively enriched 13C and depleted in D ( 2H). For well Konggu 7 crude oil, the δ 13C and δD values of the individual n-alkanes are − 29‰ to − 30‰ and − 126‰ to − 140‰. However, for well Konggu 4 oil sandstone, the δ 13C and δD values of the individual n-alkanes are − 27‰ to − 29‰ and − 144‰ to − 162‰. This is irrefutable proof that the Konggu 7 oil was sourced from the lower Paleozoic source rocks, while the Konggu 4 oil originated from the upper Paleozoic source rocks. To conclude, data of δ 13C values and δD values of the individual n-alkanes may be used to assess the organic facies of source rocks and depositional environments in addition to being of inverse help for correlating oil/oil and oil/source.

05/01456 Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks : Xiong, Y. et al. Applied Geochemistry, 2005, 20, (3), 455–464

05/01456 Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks : Xiong, Y. et al. Applied Geochemistry, 2005, 20, (3), 455–464

05/01338 The influence of hydrocarbon expulsion on carbon isotopic compositions of individual n-alkanes in pyrolysates of selected terrestrial kerogens

05/01338 The influence of hydrocarbon expulsion on carbon isotopic compositions of individual n-alkanes in pyrolysates of selected terrestrial kerogens

Characterization of the hydrogen isotopic composition of individual n-alkanes in terrestrial source rocks

In order to characterize the H isotopic compositions of individual lipid compounds from different terrestrial depositional environments, the δD values of C-bound H in individual n-alkanes from typical terrestrial source rocks of the Liaohe Basin and the Turpan Basin, China, were measured using gas chromatography–thermal conversion–isotope ratio mass spectrometry (GC–TC–IRMS). The analytical results indicate that the δD values of individual n-alkanes in the extracts of terrestrial source rocks have a large variation, ranging from −140‰ to −250‰, and are obviously lighter than the δD of marine-sourced n-alkanes. Moreover, a trend of depletion in 2H(D) was observed for individual n-alkanes from different terrestrial depositional environments, from saline lacustrine to freshwater paralic lacustrine, and to swamp. For example, the δD values of n-alkanes from a stratified saline lacustrine environment vary from −140‰ to −200‰, δD for n-alkanes from swamp facies range from −200‰ to −250‰, while those from freshwater paralic lacustrine–lacustrine environments fall between the δD values of the end members. The shift toward lighter δD from saltwater to freshwater environments indicates that the source water δD is the major controlling factor for the H isotopic composition of individual compounds. In addition, H exchange between formation water and sedimentary organic matter may possibly be important in regard to the δD of individual n-alkanes. Therefore, other lines of geochemical evidence must be considered when depositional paleoenvironments of source rocks are reconstructed based on the H isotopic composition of individual n-alkanes.

The influence of hydrocarbon expulsion on carbon isotopic compositions of individual n-alkanes in pyrolysates of selected terrestrial kerogens

Expulsion of petroleum from source rock is a complex part of the entire migration process. There exist fractional effects on chemical compositions in hydrocarbon expulsion. Does the carbon isotopic fractionation occur during expulsion and to what extent? Here the influence of hydrocarbon expulsion on carbon isotopic compositions of individual n-alkanes from pyrolysates of selected terrestrial kerogens from Tuha basin and Fushun, Liaoning Province of China has been experimentally studied. The pyrogeneration-expulsion experiments were carried out under semi-closed system. The carbon isotopic compositions of individual n-alkanes were measured by GC-IRMS. The main conclusions are as follows. First, there is carbon isotopic fractionation associated with hydrocarbon expulsion from Type III kerogens in Tuha Basin. There exist differences of carbon isotopic compositions between the unexpelled n-alkanes and expelled n-alkanes from Tuha desmocollinite and Tuha mudstone. Second, there is almost no carbon isotopic fractionation associated with hydrocarbon expulsion from Type II kerogens in Fushun and Liaohe Basin. Third, carbon isotopic fractionation in hydrocarbon expulsion should be considered in making oil-source correlation of Type III kerogens at least in the Tuha Basin. Further studies need to be carried out to determine whether this is true in other basins. Fourth, oil and source at different maturity levels cannot be correlated directly for Type III kerogens since the carbon isotopic compositions of expelled hydrocarbons at different temperatures are different. The expelled hydrocarbons are usually lighter (depleted in 13C) than the hydrocarbons remaining in the source rock at the same maturity.

Compound-specific δD– δ13C analyses of n-alkanes extracted from terrestrial and aquatic plants

Stable hydrogen and carbon isotopic compositions of individual n-alkanes were determined for various terrestrial plants (33 samples including 27 species) and aquatic plants (six species) in natural environments from Japan and Thailand. In C3 plants, n-alkanes extracted from angiosperms have a δD value of −152±26‰ (relative to Standard Mean Ocean Water [SMOW]) and δ 13C value of −36.1±2.7‰ (relative to Peedde Belemnite [PDB]), and those from gymnosperms have a δD value of −149±16‰ and δ 13C value of −31.6±1.7‰. Angiosperms have n-alkanes depleted in 13C relative to gymnosperms. n-Alkanes from C4 plants have a δD value of −171±12‰ and δ 13C value of −20.5±2.1‰, being a little depleted in D and much enriched in 13C compared to C3 plants. n-Alkanes of CAM plants are a little depleted in D and vary widely in δ 13C relative to those of C3 and C4 plants. In aquatic plants, n-alkanes from freshwater plants have a δD value of −187±16‰ and δ 13C value of −25.3±1.9‰, and those from seaweeds have a δD value of −155±34‰ and δ 13C value of −22.8±1.0‰. All n-alkanes from various plant classes are more depleted in D and 13C relative to environmental water and bulk tissue, respectively. In addition, the hydrogen and carbon isotopic fractionations during n-alkane synthesis are distinctive for these various plant classes. While C3 plants have smaller isotopic fractionations in both D and 13C, seaweed has larger isotopic fractionations.

Evidence from carbon isotope measurements for biological origins of individual longchainn-alkanes in sediments from the Nansha Sea, China

Carbon isotopes are measured for individual long-chainn-alkanes in sediments from the Nansha Sea. The features of carbon isotopic compositions of individualn-alkanes and their origins are studied. The results show that the long-chainn-alkanes have a light carbon isotopic composition and a genetic feature of mixing sources, and low-latitude higher plants and microbes are considered to be their main end member sources. Based on the abundances and carbon isotopic compositions of individualn-alkanes, the fractional contributions of the two end member sources to individualn-alkanes are quantitatively calculated by using a mixing model. The obtained data indicate that the fractional contributions of the two biological sources are different in the three samples. A trend is that the contribution of microbes increases with the depth. These results provide the theory basis and quantitatively studied method for carbon isotopic applied research of individualn-alkanes.

Hydrogen isotopic composition of individual n-alkanes as an intrinsic tracer for bioremediation and source identification of petroleum contamination.

The isotopic signatures of crude oil hydrocarbons are potentially powerful intrinsic tracers to their origins and the processes by which the oils are modified in the environment. Stable carbon isotopic data are of limited use for studying petroleum contaminants because of the relatively small amount of isotopic fractionation that occurs during natural processes. Hydrogen isotopes, in contrast, are commonly fractionated to a much greater extent and as a result display larger variations in delta values. We studied the effect of in vitro aerobic biodegradation on the hydrogen isotopic composition of individual n-alkanes from crude oil. The isotopic analysis was conducted using gas chromatography-thermal conversion-isotope ratio mass spectrometry. In general, biodegradation rates decreased with increasing hydrocarbon chain length, consistent with previous studies. More importantly the n-alkanes that were degraded at the fastest rates (n-C15 to n-C18) also showed the largest overall isotopic fractionation (approximately 12-25 per thousand deuterium enrichment), suggesting that the lower molecular weight n-alkanes can be used to monitor in-situ bioremediation of crude oil contamination. The hydrogen isotopic compositions of the longer chain alkanes (n-C19 to n-C27) were relatively stable during biodegradation (<5%o overall deuterium enrichment), indicating that these compounds are effective tracers for oil-source identification studies.

Normal alkanes in meteorites: molecular δ 13C values indicate an origin by terrestrial contamination

In the time period since 1961, n-alkanes (straight-chain hydrocarbons) have been detected in varying amounts in many meteorites. Proposed origins for these compounds have included extraterrestrial biotic, extraterrestrial abiotic and terrestrial contamination processes. To help establish the source of these compounds, we determined the carbon isotopic compositions of individual n-alkanes in meteorites of several different classes and terrestrial histories: Orgueil (CI1), Cold Bokkeveld (CM2), Murchison (CM2), Vigarano (CV3), Allende (CV3), Ornans (CO3), and Bishunpur (LL3). The efficiency of supercritical fluid extraction (SFE) was exploited to provide extractable non-polar organic matter from the meteorites. The n-alkanes in these extracts were then identified with gas chromatography–mass spectrometry (GC–MS) and the carbon isotopic composition of individual molecules was determined using isotope ratio monitoring–gas chromatography–mass spectrometry (irm–GC–MS). n-Alkanes were found in all but one of the meteorites analysed (Allende), but carbon number distributions varied between samples. Pristane and Phytane were also detected in five of the seven meteorites. δ 13C values for the individual n-alkanes occupied a range from −25.3 to −38.7‰. The δ 13C values for the meteoritic n-alkanes have a similar range as those for n-alkanes measured from petroleum in the literature. Therefore, the n-alkanes in meteorites appear to be terrestrial contaminants which may have originated from fossil hydrocarbons or petroleum products. This type of contamination is analogous to that which will be threatening future meteorite falls and the samples returned from spaceflight missions over the next two decades and beyond. For falls already contaminated, irm–GC–MS appears useful in discriminating between indigenous compounds and those introduced by terrestrial contamination.

Organic Geochemistry of Hydrothermal-petroleum Seepage at the Geothermal Region of Waiotapu, North Island, New Zealand

Hydrothermal-petroleum and hydrothermally-altered mud in Mud Volcano were collected from the geothermal region of Waiotapu, North Island, New Zealand. Biomarker composition and carbon isotopic composition of individual n-alkanes were examined for these samples, and a characterization was carried for a comparison with hydrothermal-petroleum, hydrothermally-altered mud, and reservoir petroleum.The hydrothermal-petroleum, and hydrothermally-altered mud are characterized by low CPI, low Ts/ (Ts + Tm) ratio, a significant amount of anteiso-alkane and 17 β (H) -hopane, a small amount of isoprenoid alkane and UCM, and lack of hopene and sterene.