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1.
A laboratory study has been conducted to determine the best methods for the detection of C10–C40 hydrocarbons at naturally occurring oil seeps in marine sediments. The results indicate that a commercially available method using n-C6 to extract sediments and gas chromatography–flame ionization detection (GC–FID) to screen the resulting extract is effective at recognizing the presence of migrated hydrocarbons at concentrations from 50 to 5000 ppm. When non-biodegraded, the amount of oil charge is effectively tracked by the sum of n-alkanes in the gas chromatogram. However, once the charge oil becomes biodegraded, with the loss of n-alkanes and isoprenoids, the amount of oil is tracked by the quantification of the unresolved complex mixture (UCM). Gas chromatography–mass spectrometry (GC–MS) was also found to be very effective for the recognition of petroleum related hydrocarbons and results indicate that GC–MS would be a very effective tool for screening samples at concentrations below 50 ppm oil charge.  相似文献   

2.
Deeply buried heavy oils from the Tabei Uplift of the Tarim Basin have been investigated for their source origin, charge and accumulation time, biodegradation, mixing and thermal cracking using biomarkers, carbon isotopic compositions of individual alkanes, fluid inclusion homogenization temperatures and authigenic illite K–Ar radiometric ages. Oil-source correlation suggests that these oils mainly originated from Middle–Upper Ordovician source rocks. Burial history, coupled with fluid inclusion temperatures and K–Ar radiometric ages, suggests that these oils were generated and accumulated in the Late Permian. Biodegradation is the main control on the formation of these heavy oils when they were elevated to shallow depths during the late Hercynian orogeny. A pronounced unresolved complex mixture (UCM) in the gas chromatograms together with the presence of both 25-norhopanes and demethylated tricyclic terpanes in the oils are obvious evidence of biodegradation. The mixing of biodegraded oil with non-biodegraded oil components was indicated by the coexistence of n-alkanes with demethylated terpanes. Such mixing is most likely from the same phase of generation, but with accumulation at slightly different burial depths, as evidenced by overall similar oil maturities regardless of biodegradation level and/or amount of n-alkanes. Although these Ordovician carbonate reservoirs are currently buried to over 6000 m with reservoir temperatures above 160 °C, no significant secondary hydrocarbon generation from source rocks or thermal cracking of reservoired heavy oil occur in the study area. This is because the deep burial occurred only within the last 5 Ma of the Neogene, and there has not been enough heating time for additional reactions within the Middle–Upper Ordovician source rocks and reservoired heavy oils.  相似文献   

3.
Hydrogen isotopic composition of n-alkanes was measured in sediments from an excavated profile of the Early Cretaceous Yixian Formation in Liaoning Province, NE China, aiming to assess the significance of the δD value of n-alkanes in ancient lacustrine sediments as the indicator for determining the source inputs of organic matters and paleoclimatic conditions. The δD values of n-alkanes are in the range of − 250‰ to − 85‰ and display an obvious three-stage variation pattern through the profile, which is consistent with the distribution of the dominated n-alkanes and the profile of their δ13C values. The δD and δ13C values of n-alkanes suggest that short-chain n-alkanes are primarily derived from photosynthetic bacteria and algae; n-C29 and n-C31 are mainly originated from terrestrial higher plants; n-C28 and n-C30 may be derived from the same precursor but via the different biological mechanism of hydrogen isotopic fractionation; while the source inputs of medium-chain n-alkanes are more complicated, with n-C23 being derived from some specific algae or biosynthesized by various aquatic organisms. The paleoclimatic conditions are reconstructed via two approaches. The reconstructed hydrogen isotopic values of lake water and meteoric water (expressed as δDLW and δDMW, respectively) were at the intervals of − 51.8‰ to 17.0‰ and − 118.1‰ to − 43.5‰, respectively, indicating a general climate transition from semi-arid to arid. The calculated ΔδDLW-MW values vary from 37.0‰ to 89.1‰ and display a similar but a significant large-scale variation trend with the ΔδDC23 ‐ long (− 28.8‰ to 85.0‰; long represents long-chain n-alkanes) and ΔδDmid-long (− 15.4‰ to 43.4‰; mid represents medium-chain n-alkanes) values. The discrepancy may be attributed to the source input overlap for n-alkanes and the uncertainties of εwater/lipid values. The coupling of ΔδDC23 ‐ long, ΔδDmid-long and ΔδDLW-MW values with the paleoclimatic evidence indicates that the δD values of n-alkanes could be more sensitive to the change of paleoclimatic conditions.  相似文献   

4.
A combined geochemical, geological and microbiological analysis of an actively biodegrading 24.5 m thick oil column in a Canadian heavy oil reservoir has been carried out. The reservoir properties associated with the cored vertical well are characterised by a 15.75 m thick oil column and an 8.75 m zone of steadily decreasing oil saturation below the oil column, referred to as the oil–water transition zone (OWTZ), grading down into a thin water leg. The oil column exhibits systematic gradients in oil physical properties and hydrocarbon composition and shows variations in biodegradation level throughout the reservoir consistent with the notion that the biodegradation of oil is focussed in a bioreactor zone at the base of the oil column. Through the oil column, the dead oil viscosity measured at 20 °C ranged from 50,000 cP (0.05 McP) at the top of the oil column to 1.4 McP at the oil–OWTZ contact, and continued to increase to 10.5 McP within the OWTZ. The saturated and aromatic hydrocarbons are characterised by systematically decreasing bulk fraction and component concentrations down through the oil column. Different compound classes decreased to levels below their detection limit at different depths within the OWTZ, defining a likely bioreactor extent of over 5 m in depth with, for example, n-alkanes being reduced to their detection limit concentration at the bottom of the oil column/top of the OWTZ, while branched isoprenoid alkanes were not completely degraded until well into the OWTZ.Core samples from the oil column and the lower part of the OWTZ were estimated to contain ca. 104–105 bacterial cells/g, based on qPCR of bacterial 16S rRNA genes, while samples from a narrow interval in the OWTZ immediately below the oil column contained on the order of 106–107 cells/g of sediment. Interestingly, these latter numbers are typical of those observed in active deep subsurface biosphere systems with the notion that microbial activity and abundance in the deep subsurface is elevated at geochemical interfaces. The numbers of organisms are not constant throughout the OWTZ. The highest bacterial abundance and geochemical gradients of, for example, methylphenanthrene biodegradation define a zone near the oil–water contact as likely the most active in terms of biodegradation. The largest bacterial abundances in the upper part of the OWTZ are in line with the trend of bacterial abundance with depth that has emerged from extensive analysis of microbial cells in deep subsurface sediments, implying that in terms of deep biosphere cell abundance, oil reservoirs are similar to other deep subsurface microbial environments. This is puzzling, given the atypical abundance of organic carbon in petroleum reservoirs and may imply a common large scale control on microbial abundance and activity in the deep biosphere, including in oilfields.  相似文献   

5.
The Mundaú–Manguaba estuarine–lagoon system (MMELS) constitutes one of the most representative ecosystems in the state of Alagoas, NE Brazil. Approximately 30% or 60,000 ha of the lower surrounding drainage basin of the MMELS are covered by sugar cane and a total of 250,000 inhabitants contribute untreated effluent to the system. Short sediment cores from MMELS were collected in 2007 at three sites: Manguaba Lagoon (C03), Mundaú Lagoon (C07) and Mundaú River (C08) in order to characterize the delivery and deposition of n-alkanes over the past 40 yr. The n-alkanes ranged from C15 to C35, with total aliphatic hydrocarbon (TAH) concentration in the range 27.8–139.5 μg g−1. An unresolved complex mixture (UCM) was observed in all sediments. The terrigenous/aquatic ratio (TAR), carbon preference index (CPI) and average chain length (ACL) showed that the terrigenous input dominated. The (δ13C) values of individual n-alkanes (C16–C33) varied between −22.6‰ and −34.2‰, suggesting a dominance of 12C-enriched n-alkanes that originated from C3 plants and lacustrine algae. The data reflect how anthropogenic input (via sewage, industrial pollution and agriculture) has influenced the organic content of the system through time.  相似文献   

6.
We investigated the effect of ionizing radiation on organic matter (OM) in the carbonaceous uranium (U) mineralization at the Mulga Rock deposit, Western Australia. Samples were collected from mineralized layers between 53 and 58.5 m depths in the Ambassador prospect, containing <5300 ppm U. Uranium bears a close spatial relationship with OM, mostly finely interspersed in the attrinite matrix and via enrichments within liptinitic phytoclasts (mainly sporinite and liptodetrinite). Geochemical analyses were conducted to: (i) identify the natural sources of molecular markers, (ii) recognize relationships between molecular markers and U concentrations and (iii) detect radiolysis effects on molecular marker distributions. Carbon to nitrogen ratios between 82 and 153, and Rock–Eval pyrolysis yields of 316–577 mg hydrocarbon/g TOC (HI) and 70–102 mg CO2/g TOC (OI) indicate a predominantly lipid-rich terrigenous plant OM source deposited in a complex shallow swampy wetland or lacustrine environment. Saturated hydrocarbon and ketone fractions reveal molecular distributions co-varying with U concentration. In samples with <1700 ppm U concentrations, long-chain n-alkanes and alkanones (C27–C31) reveal an odd/even carbon preference indicative of extant lipids. Samples with ⩾1700 ppm concentrations contain intermediate-length n-alkanes and alkanones, bearing a keto-group in position 2–10, with no carbon number preference. Such changes in molecular distributions are inconsistent with diagenetic degradation of terrigenous OM in oxic depositional environments and cannot be associated with thermal breakdown due to the relatively low thermal maturity of the deposits (Rr = 0.26%). It is assumed that the intimate spatial association of high U concentrations resulted in breakdown via radiolytic cracking of recalcitrant polyaliphatic macromolecules (spores, pollen, cuticles, or algal cysts) yielding medium chain length n-alkanes (C13–C24). Reactions of n-alkenes with OH− radicals from water hydrolysis produced alcohols that dehydrogenated to alkanones or through carbonylation formed alkanones. Rapid reactions with hydroxyl radicals likely decreased the isomerization of n-alkenes and decreased alkanone diversity, such that the alkan-2-one isomer is predominant. This specific distribution of components generated by natural radiolysis enables their application as “radiolytic molecular markers”. Breaking of C–C bonds through radiolytic cracking at temperatures much lower than the oil window (<50 °C) can have profound implications on initiation of petroleum formation, paleoenvironmental reconstructions, mineral exploration and in tracking radiolysis of OM.  相似文献   

7.
Novel ecosystem development is occurring within the western boreal forest of Canada due to land reclamation following oil sand surface mining. Sphagnum peat is the primary organic amendment used to reconstruct soil in these novel ecosystems. We hypothesised that ecosystem recovery would be indicated by an increasing similarity in the biomolecular characteristics of novel soil organic matter (SOM) derived from peat to those of natural boreal ecosystems. We evaluated the use of the homologous series of long chain (⩾ C21) n-alkanes with odd/even predominance to monitor the re-establishment of boreal forest on these anthropogenic soils. The lipids were extracted from dominant vegetation inputs and SOM from a series of natural and novel ecosystem reference plots. Twice the concentration of n-alkanes was extracted from natural than from novel ecosystem SOM (p < 0.01). We observed unique n-alkane signatures for the source vegetation, e.g. peat material was dominated by C31, and aspen (Populus tremuloides Michx.) leaves by C25. The n-alkane distribution differed between the two systems (p < 0.001) and reflected the dominant vegetation input, i.e. peat or tree species. Our results indicate that further research is required to clarify the influence of vegetation or disturbance on the signature of n-alkanes in SOM; however, the use of n-alkanes as biomarkers of novel ecosystem development is a promising application.  相似文献   

8.
Future climatic conditions may coincide with an increased potential for wildfires in grassland and forest ecosystems, whereby charred biomass would be incorporated into soils. Molecular changes in biomass upon charring have been frequently analysed with a focus on black carbon. Aliphatic and aromatic hydrocarbons, known to be liberated during incomplete combustion of biomass have been preferentially analysed in soot particles, whereas determinations of these compounds in charred biomass residues are scarce. We discuss the influence of increasing charring temperature on the aliphatic and aromatic hydrocarbon composition of crop grass combustion residues. Straw from rye, representing C3 grasses and maize, representing C4 grasses, was charred in the presence of limited oxygen at 300, 400 and 500 °C. Typical n-alkane distribution patterns with a strong predominance of long chain odd-numbered n-alkanes maximising at C31 were observed in raw straw. Upon combustion at 300 °C aliphatic hydrocarbons in char were dominated by sterenes, whereas at 400 °C sterenes disappeared and medium chain length n-alkanes, maximising around n-C20, with a balanced odd/even distribution were present. At a charring temperature of 500 °C n-alkane chain length shifted to short chain homologues, maximising at C18 with a pronounced predominance of even homologues. Even numbered, short chain n-alkanes in soils may thus serve as a marker for residues of charred biomass. Aromatic hydrocarbons indicate an onset of aromatization of biomass already at 300 °C, followed by severe aromatization upon incomplete combustion at 400–500 °C. The diagnostic composition of aliphatic and aromatic hydrocarbons from charred biomass affords potential for identifying residues from burned vegetation in recent and fossil soils and sediments.  相似文献   

9.
《Applied Geochemistry》2006,21(6):1006-1015
We investigated the ability of accelerated solvent extraction (ASE) to extract selected lipid biomarkers (C19–C34 n-alkanes, n-alcohols and n-fatty acids as well as dehydroabietic acid and β-sitosterol) from a sandy soil profile under Corsican pine. Two organic layers (moss and F1) as well as two mineral soil horizons (EA and C1) were sampled and extracted with DCM/MeOH (93:7 v/v) by ASE at 75 °C and a pressure of 6.9 × 106 Pa or 17 × 106 Pa. Soxhlet extractions were used as the established reference method. After clean-up and derivatization with BSTFA, the extracts were analyzed on GC/MS.Using Soxhlet as a reference, we found ASE to extract all compounds adequately. The n-alkanes, especially, were found to be extracted very efficiently from all horizons studied. Only the n-fatty acids and β-sitosterol from the organic layers seemed to be extracted at a slightly lower efficiency by ASE. In all but two instances the relative abundance of extracted lipids within a component class was the same regardless of the extraction method used.Using a higher pressure in the ASE extractions significantly increased the extraction efficiency for all component classes in the moss layer, except β-sitosterol. The effect was most pronounced for the n-alkanes. In the EA horizon, a higher pressure slightly reduced the extraction efficiency for dehydroabietic acid. The observed differences between ASE and Soxhlet extractions as well as the pressure effect can be explained by a decrease in polarity of the extractant due to the elevated pressure and temperature applied during ASE extractions as compared to Soxhlet extractions. This would mainly increase the extraction efficiency of the least polar biomarkers: the n-alkanes as was observed. In addition, a better penetration of still partially water-filled micro pores under elevated pressure and temperature may have played a role.  相似文献   

10.
Parasound profiles across the Shaban Deep in the Red Sea indicate turbiditic transport of surface sediments from the topographic height (basalt ridge) into the interior of the deep. This is supported by petrographical and (isotope-) geochemical evidence in the East Basin of the Shaban Deep where the presence of variable mixtures of authochtonous and allochthonous sediment compounds had been found.The uppermost 170 cm of both sediment cores 17008-1 and 17009-3 reveal “normal” stable oxygen isotope values for the planktonic foraminifera Globigerinoides ruber near ?1‰ which is indicative for carbonate formation in Red Sea surface water around 27 °C. However, below 182 cm in core 17008-1 highly variable δ 18O values for G. ruber between 0.26 and ?10.68‰ occur which are not the result of temperature-controlled oxygen isotope fractionation between foraminiferal carbonate and Red Sea surface water. The lowest δ18O values of ?10.68‰ measured for highly-altered foraminifera shells suggests carbonate precipitation higher than 90 °C.Organic petrographical observations show a great diversity of marine-derived macerals and terrigenous organic particles. Based on petrographical investigations sediment core 17008-1 can be subdivided in intervals predominantly of authochtonous character (i.e. 1, 3, 5 corresponding to core depths 0–170 cm, 370–415 cm, 69–136 cm), and allochthonous/thermally altered character (e.g. 2 and 4 corresponding to core depths 189–353 cm and 515–671 cm). Allochthonous/thermally altered material displays a wide to an extremely wide range of maturities (0.38–1.42% Rr) and also natural coke particles were found.Similarly, the organic geochemical and pyrolysis data indicate the predominance of well-preserved, immature algal and bacterial remains with a minor contribution of land plant material. Sediments below 170 cm (core 17008-1) contain contributions of re-sedimented pre-heated material most likely from the area of the basaltic ridge. This is documented by individual coke particles reduced hydrogen indices and elevated Tmax values up to 440 °C.An “oil-type” contribution (evidenced by mature biomarkers, hopene/hopane ratios, elevated background fluorescence, n-alkane distribution) is also present in the sediments which most likely originated at greater depth and impregnated the surface sediments.The heat source responsible for recrystallisation of foraminiferal carbonate and maturation of organic particles in Shaban Deep sediments most likely is attributed to modern basalt extrusions which now separate the Shaban Deep subbasins.  相似文献   

11.
This study contributes to the paleoenvironmental reconstruction of the loess–paleosol sequence of Nussloch, Germany, by using n-alkanes as plant leaf-wax-derived lipid biomarkers. We found that n-alkane patterns and concentrations in the Saalian loess and the last interglacial Eemian paleosol of Nussloch point to very strong degradation and prevailing deciduous vegetation. Degradation effects in the overlying paleosols and loess layers are less pronounced and allow for the application of an end-member mixing model to estimate vegetation changes semi-quantitatively. Our findings highlight the potential for the interpretation of degradation-corrected n-alkane ratios. n-Alkane modelling results for loess layers, paleosols and an in-filled paleochannel dated to ~ 60–32 ka suggest that up to ~ 50% of the n-alkanes were derived from deciduous trees or shrubs. This finding is in agreement with the abundant occurrence of wood fragments and indicates a highly variable and dynamic landscape dominated by tundra shrubland. On the other hand, deciduous trees or shrubs did not contribute significantly to the soil organic matter in the late Weichselian loess layers and the intercalated Gelic Gleysols (~ 32–18 ka).  相似文献   

12.
Based on paleoclimatic reconstructions using various proxies, the Holocene Climate Optimum (10.5–6 ka) has been characterized as a warmer and wetter period in most of East Asia. The summer monsoons associated with the East Asian Monsoon evidently intensified and extended further inland from the Pacific Ocean, a source region of moisture. A notable exception to this general pattern exists in northeast China, where less wet conditions are recorded. We determined molecular compositions of individual plant wax hydrocarbons and their hydrogen isotope compositions (δD values) in a radiocarbon-dated peat core recovered from the Hani marsh in Jilin Province (China) and confirmed that the temperature-dependent effective precipitation in northeast China decreased during the Holocene Climate Optimum. A combination of Paq, an indicator of the relative contribution of aquatic to terrestrial plants, and the difference in δD between low (C23, C25 and C27) and high molecular weight (C31) n-alkanes in the Hani peat bog indicates a dramatic change in vegetation from the deglaciation to the Holocene. No significant differences were observed between the δD values of low and high molecular weight n-alkanes with relatively high δD values and low Paq during the early Holocene, indicating that all n-alkanes were produced by evapotranspiration-sensitive terrestrial plants during that time. However, lower δD values of mid-chain n-alkanes (C23, C25 and C27) relative to the long chain n-alkane (C31), together with higher Paq values during the deglaciation (14–11 ka), suggest an increase in the contribution of aquatic plants and a higher water level during the period. The study demonstrates that northeast China was under a markedly wetter climate condition during the late deglaciation. For the 16 kyr record in the Hani peat sequence, we infer that moisture delivery by the East Asian Monsoon was relatively invariable in northeast China, but increased evaporation during the warmer Holocene Climate Optimum reduced the effective precipitation, defined by the balance between precipitation and evaporation.  相似文献   

13.
Pore waters of natural clays, which are investigated as potential host rock formations for high-level nuclear waste, are known to contain large amounts of low-molecular weight organic compounds. These small organic ligands might impact the aqueous geochemistry of the stored radionuclides and, thus, their migration behavior. In the present work, the complexation of Cm(III) with formate in aqueous NaCl solution is investigated by time-resolved laser fluorescence spectroscopy (TRLFS) as a function of the ionic strength (0.5–3.0 mol/kg), the ligand concentration (0–0.2 mol/kg) and the temperature (20–90 °C). The Cm(III) speciation is determined by deconvolution of the emission spectra. The obtained distribution of Cm(III) species is used to calculate the conditional stability constants (log K′(T)) at a given temperature and ionic strength which are extrapolated to zero ionic strength by using the specific ion interaction theory (SIT). Thus, the thermodynamic log K0n(T) values for the formation of [Cm(Form)n](3−n)+ (n = 1, 2) and the ion interaction coefficients (ε(i,k)) for [Cm(Form)n](3−n)+ (n = 1, 2) with Cl− are obtained. The log K01(T) (2.11 (20 °C)–2.49 (90 °C)) and log K02(T) values (1.17 (30 °C–2.01 (90 °C)) increase continuously with increasing temperature. The log K0n(T) values are used to derive the standard reaction enthalpies and entropies (ΔrH0m, ΔrS0m) of the respective complexation reactions according to the Van’t Hoff equation. In all cases, positive ΔrH0m and ΔrS0m values are obtained. Thus, both complexation steps are endothermic and entropy-driven.  相似文献   

14.
Four crude oil samples from the Sergipe–Alagoas Basin, northeastern Brazil, were analyzed using full scan gas chromatography–quadrupole mass spectrometry (GC–qMS) for biomarkers, in order to correlate them using aromatic carotenoids thereby enhancing knowledge about the depositional environment of their source rocks. The geochemical parameters derived from saturated fractions of the oils show evidence of little or no biodegradation and similar thermal maturation (Ts/(Ts + Tm) for terpanes, C29 αββ/(αββ + ααα), C27, and C29 20S/(20S + 20R) for steranes). Low pristane/phytane ratios and the abundance of gammacerane and β-carotane are indicative of an anoxic and saline depositional environment for the source rocks. Moreover, we identified a large range of diagenetic and catagenetic products of the aromatic carotenoid isorenieratene, including C40, C33, and C32 diaryl isoprenoids and aryl isoprenoid derivatives with short side chains and/or additional rings. These results indicate anoxia in the photic zone during the deposition of the source rocks.  相似文献   

15.
Although the effects of biodegradation on the composition and physical properties of crude oil have been well studied, effects of in-reservoir petroleum biodegradation on molecular and isotopic compositions of crude oils are not yet clearly understood. The Alberta Basin, in western Canada, is one of the world’s largest petroleum accumulations and constitutes an ideal example of a natural suite of sequentially biodegraded oils. The basin hosts moderately to severely biodegraded petroleum, regionally distributed and in single, more or less continuous, oil columns. In this study, a series of oil samples from the Alberta heavy oil and oil sands provinces, with varying degrees of biodegradation, were analyzed to assess the impact of progressive biodegradation on the molecular and C, H, N, and S isotopic compositions of oils. The results of the molecular characterization of the hydrocarbon fraction of the studied oils show that the oils have suffered biodegradation levels from 2 to 10+ (toward the Alberta–Saskatchewan border) on the Peters and Moldowan scale of biodegradation (abbreviated PM 2 to PM 10) and from tens to hundreds on the Manco scale. Within single reservoirs, increasing biodegradation was observed from top to bottom of the oil columns at all sites studied. The whole oil stable isotopic compositions of the samples varied in the ranges δ13C = −31.2‰ to −29.0‰, δ2H = −147‰ to −133‰, δ15N = 0.3–4.7‰ and δ34S = 0.4–6.4‰. The maximum differences between δ values of samples (Δ) within single oil columns were Δ13C = 1.4‰, Δ2H = 7‰, Δ15N = 1.7‰ and Δ34S = 1.0‰. Regional variations in the isotopic compositions of oil samples from different wells (averaged values from top to bottom) were 1.2‰ for δ13C, 12‰ for δ2H, 4.1‰ for δ15N and 5.5‰ for δ34S and hence generally significantly larger variations were seen than variations observed within single oil columns, especially for N and S. It appears that even severe levels of biodegradation do not cause observable systematic variations in carbon, nitrogen or sulfur isotope composition of whole oils. This indicates that sulfur and nitrogen isotopic compositions may be used in very degraded oils as indicators for oil charge from different source rock facies.  相似文献   

16.
The organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance, source and thermal maturity, which are essential prerequisites for a hydrocarbon source rock. In the present study, evaluation of organic matter in the outcrop shale samples from the Semri and Kaimur Groups of Vindhyan basin was carried out using Rock Eval pyrolysis. Also, the adsorbed low molecular weight hydrocarbons, methane, ethane, propane and butane, were investigated in the near surface soils to infer the generation of hydrocarbons in the Vindhyan basin. The Total Organic Carbon (TOC) content in shales ranges between 0.04% and 1.43%. The S1 (thermally liberated free hydrocarbons) values range between 0.01–0.09 mgHC/gRock (milligram hydrocarbon per gram of rock sample), whereas the S2 (hydrocarbons from cracking of kerogen) show the values between 0.01 and 0.14 mgHC/gRock. Based on the Tmax (temperature at highest yield of S2) and the hydrogen index (HI) correlations, the organic matter is characterized by Type III kerogen. The adsorbed soil gas, CH4 (C1), C2H6 (C2), C3H8 (C3) and nC4H10, (nC4), concentrations measured in the soil samples from the eastern part of Vindhyan basin (Son Valley) vary from 0 to 186 ppb, 0 to 4 ppb, 0 to 5 ppb, and 0 to 1 ppb, respectively. The stable carbon isotope values for the desorbed methane (δ13C1) and ethane (δ13C2) range between −45.7‰ to −25.2‰ and −35.3‰ to −20.19‰ (VPDB), respectively suggesting a thermogenic source for these hydrocarbons. High concentrations of thermogenic hydrocarbons are characteristic of areas around Sagar, Narsinghpur, Katni and Satna in the Son Valley. The light hydrocarbon concentrations (C1–C4) in near surface soils of the western Vindhyan basin around Chambal Valley have been reported to vary between 1–2547 ppb, 1–558 ppb, 1–181 ppb, 1–37 ppb and 1–32 ppb, respectively with high concentrations around Baran-Jhalawar-Bhanpur-Garot regions (Kumar et al., 2006). The light gaseous hydrocarbon anomalies are coincident with the wrench faults (Kota – Dholpur, Ratlam – Shivpuri, Kannod – Damoh, Son Banspur – Rewa wrench) in the Vindhyan basin, which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils.  相似文献   

17.
Calcite veins with fluid and solid bitumen inclusions have been discovered in the south-western shoulder of the Dead Sea rift within the Masada-Zohar block, where hydrocarbons exist in small commercial gas fields and non-commercial fields of heavy and light oils. The gas–liquid inclusions in calcite are dominated either by methane or CO2, and aqueous inclusions sometimes bear minor dissolved hydrocarbons. The enclosed flake-like solid bitumen matter is a residue of degraded oil, which may be interpreted as “dead carbon”. About 2/3 of this matter is soot-like amorphous carbon and 1/3 consists of n-C8C18 carboxylic acids and traces of n-alkanes, light dicarboxylic acids, and higher molecular weight (>C20) branched and/or cyclic carboxylic acids. Both bitumen and the host calcites show genetic relationship with mature Maastrichtian chalky source rocks (MCSRs) evident in isotopic compositions (δ13C, δ34S, and δ18O) and in REE + Y patterns. The bitumen precursor may have been heavy sulfur-rich oil which was generated during the burial compaction of the MCSR strata within the subsided blocks of the Dead Sea graben. The δ18O and δ13C values and REE + Y signatures in calcites indicate mixing of deep buried fluids equilibrated with post-mature sediments and meteoric waters. The temperatures of fluid generation according to Mg–Li-geothermometer data range from 55 °С to 90 °С corresponding to the 2.5–4.0 km depths, and largely overlap with the oil window range (60–90 °С) in the Dead Sea rift (Hunt, 1996; Gvirtzman and Stanislavsky, 2000; Buryakovsky et al., 2005). The bitumen-rich vein calcites originated in the course of Late Cenozoic rifting and related deformation, when tectonic stress triggers damaged small hydrocarbon reservoirs in the area, produced pathways, and caused hydrocarbon-bearing fluids to rise to the subsurface; the fluids filled open fractures and crystallized to calcite with entrapped bitumen. The reported results are in good agreement with the existing views of maturation, migration, and accumulation of hydrocarbons, as well as basin fluid transport processes in the Dead Sea area.  相似文献   

18.
To reconstruct past shifts in the upper forest line (UFL) in the Northern Ecuadorian Andes we are studying the applicability of plant-specific patterns of lipids preserved in soils as proxies for past vegetation along an altitudinal transect. Longer chain length n-alkanes, (C19–C35) were previously found to occur in plant-specific patterns in the dominant vegetation in the area as well as in preliminary soil samples, and may serve as such a proxy. In the present study, we assessed the preservation of n-alkane patterns with depth in soils from five excavations along an altitudinal transect 3500–3860 m above sea level (m.a.s.l) in the area. We used the carbon preference index (CPI) as well as chain length distributions of n-alkanes and their most likely degradation products, n-methyl (Me) ketones, n-alcohols and n-fatty acids. Clear n-alkane patterns were found in all the soils and at all depths, while a clear relationship with the observed patterns of n-Me ketones identified them as the primary degradation product of the former. Very low average n-Me ketone/n-alkane ratio values were found, ranging from 0.03 to 0.15 at the top of the mineral soil, to 0.05–0.20 at the interface with an underlying palaeosol several thousand years old. The concurrent high CPI values indicate very limited degradation of n-alkanes with depth. Except for C33, the shifts in n-Me ketone/n-alkane values were similar for all chain lengths investigated, signifying an absence of preferential degradation of individual n-alkanes. With one exception, all the soils showed a similar increase in n-Me ketone/n-alkane values with depth, indicating that the degradation rates were not influenced by altitude. This means that, even if the total concentration of n-alkanes decreases over time, the characteristic pattern remains intact, conserving their potential as a biomarker for past vegetation reconstruction in the area, as well as for investigation of degradation processes of soil organic carbon.  相似文献   

19.
In this study, the molecular composition and biomarker distribution of lacustrine sediments from Val-1 drillhole in the central zone of the western part of the Valjevo-Mionica basin were investigated at depth interval of 0–400 m. Former investigations have shown that the core material can be separated into six depth intervals based on bulk geochemical, mineralogical and sedimentological characteristics. Concerning the quality of organic matter, presence of specific minerals, and high salinity and anoxicity, or alkalinity, three zones are of highest interest, defined at depth intervals of 15–75 m (A), 75–200 m (B) and 360–400 m (F). The first aim of the study was to identify which biomarkers characterize these specific intervals. The second aim, addressing the transitions zones of these intervals, was to extend the changes in the characteristics of the organic substance, to reflect the changes of conditions in the depositional environment as well as to define biomarker parameters which are the most sensitive sedimentological indicators.The sediments from the hypersaline anoxic and alkaline environment show high contribution of algal precursor biomass, what is in accordance with the good quality of organic substances in the sediments from these zones. High squalane content and low content of regular isoprenoid C25 are typical for hypersaline anoxic environment, whereas sediments from alkaline environment have high regular isoprenoid C25 content.Transition to specific sedimentation zones is characterized by change in total organic matter content, and of both free and pyrolysis-derived, and change in hydrogen index value. In the biomarker distributions, more significant changes were detected in distributions of n-alkanes and isoprenoids, compared to polycyclic alkanes. The most intensive changes in alkane distribution are reflected in changes in n-C17 content compared to n-C27, and phytane compared to n-C18. In addition, significant sensitivity was seen in ratios between squalane and n-alkane C26 (hypersaline depositional environment), or isoprenoid C25 and n-alkane C22 for high alkalinity environment.This study showed that Sq/n-C26 ratio can be used to assess the quality of organic substance in immature lacustrine sediments.  相似文献   

20.
《Organic Geochemistry》2012,42(12):1269-1276
This study sought to characterize hydrogen isotopic fractionation during biosynthesis of leaf wax n-alkanes in succulent plants capable of crassulacean acid metabolism (CAM). The metabolic and physiological features of CAM represent crucial strategies for survival in hot and dry climates and have been hypothesized to impact hydrogen isotope fractionation. We measured the stable carbon and hydrogen isotopic compositions (δ13C and δD, respectively) of individual n-alkanes in 20 species of succulent plants from a global collection of the Huntington Botanical Gardens, San Marino, California. Greenhouse conditions and irrigation with water of constant δD value enabled determination of interspecies differences in net D/H fractionation between source water and leaf wax products. Carbon isotope ratios provide constraints on the extent of CAM vs. C3 photosynthesis and indicate a wide range of CAM use, with δ13C values ranging from −33.01‰ to −18.54‰ (C27–C33 n-alkanes) and −26.66‰ to −17.64‰ (bulk tissue). Despite the controlled growth environment, we observed ca. 90‰ interspecies range in δD values from −193‰ to −107‰. A positive correlation between δ13Cbulk and δDC31 values with R2 = 0.60 (δ13CC31 and δDC31 values with R2 = 0.41) implicates a metabolic isotope effect as the dominant cause of interspecies variation in the hydrogen isotopic composition of leaf wax n-alkanes in CAM-intermediate plants.  相似文献   

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