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1.
Biodegradation of crude oil causes volumetrically important compositional changes, which lead to significant deterioration in quality, in particular during the early stages of alteration. To better understand these effects we focussed on a detailed assessment of light to moderate levels of alteration. We investigated a suite of 40 crude oil samples from five different petroleum systems to evaluate the extent of alteration occurring in reservoirs. Based on a comprehensive geochemical characterization, five individual crude oil sequences were defined, where compositional variability is mainly due to microbial activity in the reservoir. In particular, samples from the Gullfaks field (offshore Norway) and from a petroleum system offshore Angola illustrate that conventional molecular biodegradation parameters, such as the Pr/n-C17 and Ph/n-C18 alkane ratios are not suitable for defining the extent of biodegradation in petroleum reservoirs. Here, we suggest a new molecular biodegradation parameter, the degradative loss, that can be used to quantify depletion in individual crude oil constituents. The approach allows improved assessment of the extent of biodegradation in crude oil samples by means of the mean degradative loss. It is demonstrated that crude oil quality, as assessed from API gravity, can be predicted directly from the molecular composition of crude oils. Our data clearly indicate that the degradation patterns of light hydrocarbons and n-alkanes differ in different petroleum systems. This suggests that microbial communities are different and therefore generate different molecular degradation patterns which have to be evaluated individually for each system.  相似文献   

2.
A quantitative kinetic model has been proposed to simulate the large D and 13C isotope enrichments observed in individual n-alkanes (C13-C21) during artificial thermal maturation of a North Sea crude oil under anhydrous, closed-system conditions. Under our experimental conditions, average n-alkane δ13C values increase by ∼4‰ and δD values increase by ∼50‰ at an equivalent vitrinite reflectance value of 1.5%. While the observed 13C-enrichment shows no significant dependence on hydrocarbon chain length, thermally induced D-enrichment increases with increasing n-alkane carbon number. This differential fractionation effect is speculated to be due to the combined effect of the greater extent of thermal cracking of higher molecular weight, n-alkanes compared to lower molecular weight homologues, and the generation of isotopically lighter, lower molecular weight compounds. This carbon-number-linked hydrogen isotopic fractionation behavior could form the basis of a new maturity indicator to quantitatively assess the extent of oil cracking in petroleum reservoirs. Quantum mechanical calculations of the average change in enthalpy (ΔΔH‡) and entropy (ΔΔS‡) as a result of isotopic substitution in n-alkanes undergoing homolytic cleavage of C-C bonds lead to predictions of isotopic fractionation that agree quite well with our experimental results. For n-C20 (n-icosane), the changes in enthalpy are calculated to be ∼1340 J mol-1 (320 cal mol-1) and 230 J mol-1 (55 cal mol-1) for D-H and 13C-12C, respectively. Because the enthalpy term associated with hydrogen isotope fractionation is approximately six times greater than that for carbon, variations in δD values for individual long-chain hydrocarbons provide a highly sensitive measure of the extent of thermal alteration experienced by the oil. Extrapolation of the kinetic model to typical geological heating conditions predicts significant enrichment in 13C and D for n-icosane at equivalent vitrinite reflectance values corresponding to the onset of thermal cracking of normal alkanes. The experimental and theoretical results of this study have significant implications for the use of compound-specific hydrogen isotope data in petroleum geochemical and paleoclimatological studies. However, there are many other geochemical processes that will significantly affect observed hydrogen isotopic compositions (e.g., biodegradation, water washing, isotopic exchange with water and minerals) that must also be taken into consideration.  相似文献   

3.
Biodegradation is one of the main natural attenuation processes in groundwater contaminated with petroleum hydrocarbons. In this work, preliminary studies have been carried out by analyzing the concentrations of total petroleum hydrocarbons (TPH), dissolved inorganic carbon (DIC), dominant terminal electron accepters or donors, as well as δ 13CDIC and δ 34SSO4, to reveal the biodegradation mechanism of petroleum hydrocarbons in a contaminated site. The results show that along groundwater flow in the central line of the plume, the concentrations of electron acceptors, pH, and E h increased but TPH and DIC decreased. The δ 13CDIC values of the contaminated groundwater were in the range of ?14.02 to ?22.28 ‰PDB and ?7.71 to 8.36 ‰PDB, which reflected a significant depletion and enrichment of 13C, respectively. The increase of DIC is believed to result from the non-methanogenic and methanogenic biodegradation of petroleum hydrocarbon in groundwater. Meanwhile, from the contaminated source to the downgradient of the plume, the 34S in the contaminated groundwater became more depleted. The Rayleigh model calculation confirmed the occurrence of bacterial sulfate reduction as a biodegradation pathway of the petroleum hydrocarbon in the contaminated aquifers. It was concluded that stable isotope measurements, combined with other biogeochemical measurements, can be a useful tool to prove the occurrence of the biodegradation process and to identify the dominant terminal electron-accepting process in contaminated aquifers.  相似文献   

4.
Stable carbon (δ13C) and hydrogen (δD) isotopic compositions of n-alkanes, anteiso-alkanes, n-alkanoic acids, n-alkanols, phytol and sterols in raw leaves of Acer argutum and Acer carpinifolium, their fallen leaves, mold and soils from a natural Acer forest were measured in order to: (1) understand isotopic variation of the plant biomarkers in a plant-soil system and (2) evaluate which biomarker is the most effective recorder of soil vegetation. Long-chain (> C24) n-alkanes, n-alkanoic acids and n-alkanols are gradually enriched in 13C up to 12.9‰ (average of 4.3‰) and depleted in D up to 94‰ (average of 55‰) from raw leaves to soils. However, anteiso-alkanes, phytol and sterols show little variation in both δ13C (< ± 1‰) and δD (< ± 2‰) from raw leaves to soils. These isotope signatures in a plant-soil system indicate that isoprenoid plant biomarkers such as sterols in soils faithfully preserve the isotopic compositions of dominant higher plants growing on the soils without a diagenetic effect upon the isotopic compositions. In contrast, long-chain n-alkyl molecules in soils undergo specific isotopic modification during biodegradation associated with early diagenesis and/or a significant contribution from heterotrophic reworking.  相似文献   

5.
《Applied Geochemistry》1998,13(7):851-859
Emerging acceptance of the limitations of separate phase product recovery has spawned interest in the intrinsic alteration of residual separate phase petroleum products. In this study the geochemical changes in a continuous core through soil containing a separate phase diesel fuel #2 (SPD) in contact with groundwater are investigated. Chemical heterogeneities are shown to exist which can be attributed to weathering, particularly intrinsic biodegradation. The results show that the aliphatic hydrocarbon content is reduced and the δ13C ratio of the aliphatic hydrocarbons increased from top to bottom in the core. Both changes are thought to be due to preferential biodegradation of (isotopically lighter) n-alkanes. A slight increase in the relative abundance of shorter chain n-alkanes (<n-C17) was also observed. The distribution of the dominant aromatic hydrocarbons (C0–C3 alkyl-naphthalenes) is remarkably consistent throughout the core, although naphthalene is depleted below the oil–water interface. In spite of low oil saturation (S0), little or no evidence of biodegradation is noted at the uppermost boundary of the SPD. However, intrinsic biodegradation is evident approximately 0.3 m above the oil–water interface in spite of higher S0. The extent of the chemical changes attributable to biodegradation (described above) gradually increases below the oil–water interface, eventually reaching a maximum at the bottom of the SPD profile (∼1.2 m below the interface) where S0 is again reduced. The relatively higher level of biodegradation observed at and below the oil–water interface may be attributed to the reduced S0 in this zone. An estimate of the mass reduction in diesel fuel between the uppermost and bottommost parts of the core is calculated to be 23% (by weight), due predominantly to the biodegradation of n-alkanes.  相似文献   

6.
The mechanism of thermochemical sulfate reduction (TSR) was investigated by separately heating n-C24 with three different sulfates (CaSO4, Na2SO4, MgSO4) in sealed gold tubes at 420 °C and measuring the stable carbon isotope values of hydrocarbon (C1-C5) and non-hydrocarbon (CO2) products. Extensive TSR was observed with the MgSO4 reactant as reflected by increasing concentrations of H2S, 13C depleted CO2 and relatively low concentrations of H2 (compared to the control). H2S yields were already very high at the first monitoring time (12 h) when the temperature had just reached 420 °C, suggesting that TSR had commenced well prior to this temperature. Only trace amounts of n-C24 and secondary C3-C5 alkanes were detected at 12 h, reflecting the efficient TSR utilization of the reactant and lower molecular weight alkane products. Ethane levels were still relatively high at 12 h, but declined thereafter as it was subject to TSR in the absence of higher molecular weight alkanes which had already been utilized. Methane yields were consistently high throughout the 48 h MgSO4 treatment. The temporal decrease in the concentrations of alkanes available for TSR may also contribute to the sharp enhancement of CO2 after 36 h. Absence or dampening of the molecular and isotopic trends of MgSO4 TSR was observed with Na2SO4 and CaSO4 respectively, directly reflecting the levels of TSR reached using these sulfate treatments.For all treatments, the δ13C values of C1-5n-alkanes showed an increase with both molecular weight and treatment time. MgSO4 TSR led to a 5-10‰ increase in the δ13C values of the C1-C5 hydrocarbons and a 20‰ decrease in the δ13C value of CO2. The significant 13C depletion of the CO2 may be due to co-production of 13C enriched MgCO3, although this remains unproven as the δ13C of MgCO3 was not measured. The difference in the δ13C values of ethane and propane (Δδ13CEP) increased in magnitude with the degree of TSR, and this trend could be used to help evaluate the occurrence and extent of TSR in subsurface gas reservoirs.  相似文献   

7.
The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δDEPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ18OEPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ18O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δDFLUID of ∼ −79‰ and −89‰, respectively, show δDEPIDOTE values of −115‰ and −125‰. In contrast, δDEPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δDEPIDOTE from well RN-10 at Reykjanes.Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote-water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ18O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δDFLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ18OFLUID values are within range of the observed fluid isotope composition. We propose that modern δDEPIDOTE and δDFLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δDROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δDFLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock-fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system.  相似文献   

8.
The carbon geochemistry of serpentinized peridotites and gabbroic rocks recovered at the Lost City Hydrothermal Field (LCHF) and drilled at IODP Hole 1309D at the central dome of the Atlantis Massif (Mid-Atlantic Ridge, 30°N) was examined to characterize carbon sources and speciation in oceanic basement rocks affected by long-lived hydrothermal alteration. Our study presents new data on the geochemistry of organic carbon in the oceanic lithosphere and provides constraints on the fate of dissolved organic carbon in seawater during serpentinization. The basement rocks of the Atlantis Massif are characterized by total carbon (TC) contents of 59 ppm to 1.6 wt% and δ13CTC values ranging from −28.7‰ to +2.3‰. In contrast, total organic carbon (TOC) concentrations and isotopic compositions are relatively constant (δ13CTOC: −28.9‰ to −21.5‰) and variations in δ13CTC reflect mixing of organic carbon with carbonates of marine origin. Saturated hydrocarbons extracted from serpentinites beneath the LCHF consist of n-alkanes ranging from C15 to C30. Longer-chain hydrocarbons (up to C40) are observed in olivine-rich samples from the central dome (IODP Hole 1309D). Occurrences of isoprenoids (pristane, phytane and squalane), polycyclic compounds (hopanes and steranes) and higher relative abundances of n-C16 to n-C20 alkanes in the serpentinites of the southern wall suggest a marine organic input. The vent fluids are characterized by high concentrations of methane and hydrogen, with a putative abiotic origin of hydrocarbons; however, evidence for an inorganic source of n-alkanes in the basement rocks remains equivocal. We propose that high seawater fluxes in the southern part of the Atlantis Massif likely favor the transport and incorporation of marine dissolved organic carbon and overprints possible abiotic geochemical signatures. The presence of pristane, phytane and squalane biomarkers in olivine-rich samples associated with local faults at the central dome implies fracture-controlled seawater circulation deep into the gabbroic core of the massif. Thus, our study indicates that hydrocarbons account for an important proportion of the total carbon stored in the Atlantis Massif basement and suggests that serpentinites may represent an important—as yet unidentified—reservoir for dissolved organic carbon (DOC) from seawater.  相似文献   

9.
Various aquatic plants from Lake Qinghai, the largest inland saline lake in China, and terrestrial plants from the surrounding area were investigated for the distribution of n-alkanes and their δD values. The n-alkanes in the samples range from C15 to C33 with C preference index (CPI) values of 4.0–29.7. The n-C23 or n-C25 alkane is the dominant compound in the aquatic submerged plants. The aquatic emergent and terrestrial plants have an abundance maximum at n-C27, n-C29 or n-C31. The average chain length (ACL) values, ranging from 26.0 to 29.6, are closely related to the plant species. The n-alkanes from the aquatic plants have mean δD values of −169‰ to −121‰ and those from the terrestrial plants values of −173‰ to −109‰. The H isotopic composition (δD) and fractionation differ significantly among the plants studied. Comparison shows that additional evaporative enrichment of the lake water associated with saline lakes and humidity influence the δD values of the n-alkanes in aquatic and terrestrial plants, respectively. The mean δD values of n-alkanes in the plants decrease with increasing ACL value. The n-alkanes from the different types of plants are more depleted in D relative to environmental water and those from aquatic plants (with a mean value of −143‰) have a greater isotopic fractionation than terrestrial plants (mean value −113‰).  相似文献   

10.
We measured molecular distributions and compound-specific hydrogen (δD) and stable carbon isotopic ratios (δ13C) of mid- and long-chain n-alkanes in forest soils, wetland peats and lake sediments within the Dorokawa watershed, Hokkaido, Japan, to better understand sources and processes associate with delivery of terrestrial organic matter into the lake sediments. δ13C values of odd carbon numbered C23-C33n-alkanes ranged from −37.2‰ to −31.5‰, while δD values of these alkanes showed a large degree of variability that ranged from −244‰ to −180‰. Molecular distributions in combination with stable carbon isotopic compositions indicate a large contribution of C3 trees as the main source of n-alkanes in forested soils whereas n-alkanes in wetland soil are exclusively derived from marsh grass and/or moss. We found that the n-alkane δD values are much higher in forest soils than wetland peat. The higher δD values in forest samples could be explained by the enrichment of deuterium in leaf and soil waters due to increased evapotranspiration in the forest or differences in physiology of source plants between wetland and forest. A δ13C vs. δD diagram of n-alkanes among forest, wetland and lake samples showed that C25-C31n-alkanes deposited in lake sediments are mainly derived from tree leaves due to the preferential transport of the forest soil organic matter over the wetland or an increased contribution of atmospheric input of tree leaf wax in the offshore sites. This study demonstrates that compound-specific δD analysis provides a useful approach for better understanding source and transport of terrestrial biomarkers in a C3 plant-dominated catchment.  相似文献   

11.
Biodegradation, one of the most important weathering processes, alters the composition of spilled oil, making it difficult to identify the source of the release and to monitor its fate in the environment. A laboratory experiment was conducted to simulate oil spill weathering process of microbial degradation to investigate compositional changes in a range of source- and weathering-dependent molecular parameters in oil residues, and the conventional diagnostic ratios for oil spill identification were also evaluated. The conventional diagnostic ratios of n-alkane displayed obvious changes after biodegradation, especially for Pr/n-C17 and Ph/n-C18 with relative standard deviation more than 118.84 %, which suggests they are invalid for oil source identification of the middle-serious spill. Many polycyclic aromatic hydrocarbons (PAHs) are more resistant to biodegradation process than their saturated hydrocarbon counterparts, thus making PAHs to be one of the most valuable fingerprinting classes of hydrocarbons for oil identification. Biomarker ratios of hopanes and steranes were also useful for source identification even after moderate biodegradation, and the diagnostic ratios from them could be used in tracking origin and sources of hydrocarbon pollution. Finally, the carbon isotopic type curve may provide another diagnostic means for correlation and differentiation of spilled oils, and be particularly valuable for lighter refined products or severely biodegraded oils, the source of which may be difficult to identify by routine biomarker techniques.  相似文献   

12.
The stable hydrogen isotopic compositions (δD) of selected aliphatic hydrocarbons (n-alkanes and isoprenoids) in eight crude oils of similar source and thermal maturity from the Upper Indus Basin (Pakistan) were measured. The oils are derived from a source rock deposited in a shallow marine environment. The low level of biodegradation under natural reservoir conditions was established on the basis of biomarker and aromatic hydrocarbon distributions. A plot of pristane/n-C17 alkane (Pr/n-C17) and/or phytane/n-C18 alkane (Ph/n-C18) ratios against American Petroleum Institute (API) gravity shows an inverse correlation. High Pr/n-C17 and Ph/n-C18 values and low API gravity values in some of the oils are consistent with relatively low levels of biodegradation. For the same oils, δD values for the n-alkanes relative to the isoprenoids are enriched in deuterium (D). The data are consistent with the removal of D-depleted low molecular weight (LMW) n-alkanes (C14–C22) from the oils. The δD values of isoprenoids do not change with progressive biodegradation and are similar for all the samples. The average D enrichment for n-alkanes with respect to the isoprenoids is found to be as much as 35‰ for the most biodegraded sample. For example, the moderately biodegraded oils show an unresolved complex mixture (UCM), loss of LMW n-alkanes (<C15) and moderate changes in the alkyl naphthalene distributions. The relative susceptibility of alkyl naphthalenes at low levels of biodegradation is discussed. The alkyl naphthalene biodegradation ratios were determined to assess the effect of biodegradation. The dimethyl, trimethyl and tetramethyl naphthalene biodegradation ratios show significant differences with increasing extent of biodegradation.  相似文献   

13.
We measured hydrogen isotope compositions (δD) of high-molecular-weight n-alkanes (C27-C33) from grasses grown in greenhouses and collected from the US Great Plains. In both cases, n-alkanes from C4 grasses are enriched in D by more than 20‰ relative to those from C3 grasses. The apparent enrichment factor (εC29-GW) between C29n-alkane and greenhouse water is −165 ± 12‰ for C3 grasses and −140 ± 15‰ for C4 grasses. For samples from the Great Plains, δD values of C29n-alkanes range from −280 to −136‰, with values for C4 grasses ca. 21‰ more positive than those for C3 grasses from the same site. Differences in C3 and C4 grass n-alkane δD values are consistent with the shorter interveinal distance in C4 grass leaves, and greater back-diffusion of enriched water from stomata to veins, than in C3 grass leaves. Great Plains’ grass n-alkane isotopic ratios largely reflect precipitation δD values. However, the offset or apparent fractionation between n-alkanes and precipitation is not uniform and varies with annual precipitation and relative humidity, suggesting climatic controls on lipid δD values. The dryer sites exhibit smaller absolute apparent fractionation indicative of D-enrichment of source waters through transpiration and/or soil evaporation. To explore the relationship between climate and n-alkane δD values, we develop three models. (1) The ‘direct analog’ model estimates δDC29 values simply by applying the apparent enrichment factors, εC29-GW, observed in greenhouse grasses to precipitation δD values from the Great Plains. (2) The ‘leaf-water’ model uses a Craig-Gordon model to estimate transpirational D-enrichment for both greenhouse and field sites. The transpiration-corrected enrichment factors between C29 and bulk leaf-water, εC29-GW, calculated from the greenhouse samples (−181‰ for C3 and −157‰ for C4) are applied to estimate δDC29 values relative to modeled bulk leaf-water δD values. (3) The ‘soil- and leaf-water’ model estimates the combined effects of soil evaporation, modeled by analogy with a flow-through lake, and transpiration on δDC29 values. Predictions improve with the addition of the explicit consideration of transpiration and soil evaporation, indicating that they are both important processes in determining plant lipid δD values. D-enrichment caused by these evaporative processes is controlled by relative humidity, suggesting that important climatic information is recorded in leaf wax n-alkane δD values. Calibration studies such as this one provide a baseline for future studies of plant-water-deuterium systematics and form the foundation for interpretation of plant wax hydrogen isotope ratios as a paleo-aridity proxy.  相似文献   

14.
Mg isotope ratios (26Mg/24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/24Mg ratio (expressed as δ26Mg) at −0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ26Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (δ26Mggrass-δ26Mgrain) of 0.21‰. The deep pore-waters (1-15 m deep) have δ26Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of 24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.  相似文献   

15.
This study investigates the extent of post-depositional alteration of δD values of n-alkyl lipids, isoprenoids, and kerogen isolated from a continuous 450 m core that covers the transition from thermally immature to early mature sediments in the lacustrine Kissenda Formation, Lower Cretaceous, Gabon Basin. Large variations in δD values (up to 40‰ for nC17 and up to 30‰ for nC29 alkanes as well as up to 10‰ for kerogen) in closely spaced samples are evident throughout the core and remain preserved even at the bottom of the section. δD values of individual n-alkanes show a slight overall D-enrichment with depth, and a general trend of increasing δD values with increasing n-alkane chain length characterizes all samples, particularly in those below 600 m depth. Hydrogen isotopic compositions of kerogen samples overlap with those of n-alkanes throughout the section. δD values of pristane and phytane are more negative than those of nC17 alkane by as much as 120‰ at shallow depths but increase dramatically and approach δD values of nC17 alkane in the samples closest to the oil window. Integration of analytical and computational results indicates that: (1) n-alkanes and isoprenoids have the potential to preserve the original biological signal before the onset of oil generation; (2) isomeric and structural rearrangements taking place at the beginning stages of oil generation do not influence significantly the δD values of n-alkanes and kerogen. However, these processes have a major effect on the isotopic composition of isoprenoids, causing isotopic D-enrichment up to 90‰.  相似文献   

16.
A suite of natural gases from the northern Songliao Basin in NE China were characterized for their molecular and carbon isotopic composition. Gases from shallow reservoirs display clear geochemical evidence of alteration by biodegradation, with very high dryness (C1/C2+ > 100), high C2/C3 and i-C4/n-C4 ratios, high nitrogen content and variable carbon dioxide content. Isotopic values show wide range variations (δ13CCH4 from −79.5‰ to −45.0‰, δ13CC2H6 from −53.7‰ to −32.2‰, δ13CC3H8 from −36.5‰ to −20.1‰, δ13CnC4H10 from −32.7‰ to −24.5‰, and δ13CCO2 from −21.6‰ to +10.5‰). A variety of genetic types can be recognized on the basis of chemical and isotopic composition together with their geological occurrence. Secondary microbial gas generation was masked by primary microbial gas and the mixing of newly generated methane with thermogenic methane already in place in the reservoir can cause very complicated isotopic signatures. System openness also was considered for shallow biodegraded gas accumulations. Gases from the Daqing Anticline are relatively wet with 13C enriched methane and 13C depleted CO2, representing typically thermogenic origin. Gases within the Longhupao-Da’an Terrace have variable dryness, 13C enriched methane and variable δ13C of CO2, suggesting dominant thermogenic origin and minor secondary microbial methane augment. The Puqian-Ao’nan Uplift contains relatively dry gas with 13C depleted methane and 13C enriched CO2, typical for secondary microbial gas with a minor part of thermogenic methane. Gas accumulations in the Western Slope are very dry with low carbon dioxide concentrations. Some gases contain 13C depleted methane, ethane and propane, indicating low maturity/primary microbial origin. Recognition of varying genetic gas types in the Songliao Basin helps explain the observed dominance of gas in the shallow reservoir and could serve as an analogue for other similar shallow gas systems.  相似文献   

17.
The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in 18O and 13C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ13C and δ18O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ13C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ18O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ18O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C4 type plants, supporting a scenario of expansion of C4 plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The 87Sr/86Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic 87Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.  相似文献   

18.
Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources or fluid histories. The former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, and the latter allows the reconstruction of precipitation and redox processes. These processes take place during ore formation or alteration. The aim of this contribution is to investigate the suitability of this new isotope method as a probe of ore-related processes. For this purpose 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, the ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300 °C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100 °C). Measured iron isotope compositions are variable and cover a range in δ56Fe between −2.3‰ and +1.3‰. Primary hematite (δ56Fe: −0.5‰ to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: −0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: −1.4 to −0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitating from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethites from oolitic sedimentary iron ores were also analyzed. Their compositional range appears to indicate oxidative precipitation from relatively uniform Fe dissolved in coastal water. This comprehensive iron isotope study illustrates the potential of the new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.  相似文献   

19.
This study is a comprehensive, stable isotope survey of the marine carbonate-dominated, upper Paleo- to lower Neoproterozoic stratigraphy of Jixian County, China. Carbonate-associated sulfate (CAS) was extracted and measured for δ34SCAS using the same samples analyzed for δ13Ccarbonate. This integrated proxy approach is a step towards a more comprehensive picture of secular variation in the composition of Proterozoic seawater. We specifically sampled marine carbonate intervals from the lower section of the Chuanlinggou Formation, Changcheng Group (ca. 1700 Ma) to the top of the Jingeryu Formation, Qingbaikou Group (ca. 800 Ma). δ13Ccarbonate values are mostly negative in the upper Paleoproterozoic Changcheng Group, with an ascending trend from −3‰ to 0‰. We observed variation of approximately 0 ± 1‰ in the Mesoproterozoic Jixian Group, and positive values of +2 ± 2‰ characterize the lower Neoproterozoic Qingbaikou Group. Stratigraphic variations in δ34SCAS are more remarkable in their ranges and magnitudes, including conspicuously high values exceeding +30‰ in the three intervals at ca. 1700 Ma, 1300-1100 Ma, and 1000-900 Ma. In the Changcheng Group, δ34SCAS values are typically higher than +25‰, with only a few values of less than +15‰. In contrast, most of the data spanning from the Mesoproterozoic Tieling Formation of the Jixian Group to the lower Neoproterozoic Jingeryu Formation of the Qingbaikou Group are highly variable between +10‰ and +25‰, with some values exceeding +25‰.In the late Paleoproterozoic (1700-1600 Ma), a >10‰ decrease in δ34SCAS and ∼3‰ increase in δ13Ccarbonate are coincident with, and likely related to, the breakup of Columbia, a supercontinent that predated Rodinia. Carbon and sulfur isotope data from the Mesoproterozoic, when global tectonic activity was comparatively weaker, fall mostly in the ranges of +15 ± 10‰ and 0 ± 1‰, respectively, but fluctuations of >20‰ for δ34SCAS and >3‰ for the δ13Ccarbonate at ca. 1450-1400 Ma may reflect subduction and large-scale magmatic activity in island arcs marking the end of Columbia breakup. From the late Mesoproterozoic (ca. 1300-1100 Ma) to the early Neoproterozoic (ca. 800 Ma), the δ13C and δ34S of seawater increased gradually with increasing variability. Most impressive areδ34SCAS values that exceed +30‰ in two intervals at ca. 1300-1100 Ma and ca. 1000-900 Ma, which may reflect the assembly and early breakup of Rodinia. Although gaps in the record remain, and studies of even higher resolution are warranted, our results suggest that changes in paleoceanographic conditions linked to global tectonics strongly influenced the biogeochemical cycles of C and S. Furthermore, periods of the Proterozoic previously noted for their isotopic invariability show clear isotopic expressions of this tectonic activity.  相似文献   

20.
In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography-isotope ratio mass spectrometry (GC-IRMS), was employed to determine compound specific carbon isotopic values (δ13C) of gasoline range hydrocarbons. The reproducibility of the method was found to be satisfactory. By comparison with the δ13C values of the twelve target compounds determined using direct injection of their n-C16 solution, no obvious isotopic fractionation was observed during the HS-SDME procedures. Some parameters that could affect the carbon isotopic fractionation, such as ionic strength of working solutions and inlet split ratio, were examined. The results also suggest that these factors had no significant effect on the carbon isotopic determination of gasoline range hydrocarbons. The application of HS-SDME to a crude oil sample proved that this method could be a promising tool for the determination of carbon isotopic values of gasoline range hydrocarbons in oils or aqueous samples.  相似文献   

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