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1.
With a production of 208.2 m3/d, heavy oil was produced by drill stem test (DST) from three shallow reservoirs in Sand Group Nos. Ⅰ and Ⅲ of the Neogene Guantao Formation (NgⅠ and NgⅢ) and the Eogene Dongying Formation (Ed) in an exploratory well Ban-14-1 within the Qianmiqiao region, Bohai Bay Basin, northern China. Based on the GC and GC-MS data of the NgⅠ and NgⅢ heavy oil samples, all n-alkanes and most isoprenoid hydrocarbons are lost and the GC baseline appears as an evident "hump", implying a large quantity of unresolved complex mixture (UCM), which typically revealed a result of heavy biodegradation. However, there still is a complete series of C14-C73 n-alkanes in the high-temperature gas chromatograms (HTGC) of the heavy oil, among which, the abundance of C30- n-alkanes are drastically reduced. The C35-C55 high molecular weight (HMW) n-alkanes are at high abundance and show a normal distribution pattern with major peak at C43 and an obvious odd-carbon-number predominance with CPI37-55 and  相似文献   

2.
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   

3.
《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   

4.
The distribution patterns of Organic Sulphur Compounds (OSC), occurring in certain sediments and immature crude oils, were compared with those of the corresponding hydrocarbons. Because of the complexity of the OSC mixtures, they were desulphurized to hydrocarbons (n-alkanes, isoprenoid alkanes, steranes, triterpanes and branched alkanes). The hydrocarbons produced by desulphurization of the OSC exhibited distribution patterns different from those of the hydrocarbons originally present. Therefore reaction of elemental sulphur with these hydrocarbons at elevated temperatures must be considered as an unlikely origin for these OSC. Sulphur incorporation reactions on an intramolecular basis with suitable functionalized precursors at the early stages of diagenesis are probably the major origin for these OSc. Desulphurization of high molecular weight fractions also produced hydrocarbons, dominated by n-alkanes up to C40. Therefore it is assumed that these substances contain n-alkanes, 2,5-dialkyl-thiophenes and -thiolanes linked to each other by sulphur briddges. These findings stronly suggest that sulphur-containing high molecular weight substances are formed by the same sulphur incorporation reactions as OSC, but in an intermolecular fashion.  相似文献   

5.
High-temperature gas chromatography (HTGC) has enhanced our ability to characterize hydrocarbons extending to C120 in crude oils. As a result, hydrocarbons in waxes (> C20) have been observed to vary significantly between crude oils, even those presumed to originate from the same source. Prior to this development, microcrystalline waxes containing hydrocarbons above C40 were not characterized on a molecular level due to the analytical limitations of conventional gas chromatography. Routine screenings of high pour-point crude oils by high-temperature gas chromatography has revealed that high molecular weight hydrocarbons (> C40) are very common in most oils and may represent 2% of the crude oil. Precise structures, origins, and significance of these high molecular weight compounds remain elusive. As a preliminary step to expand our knowledge of these compounds their general molecular structures and formulas have been investigated in this study. Initial results suggest that the major high molecular weight compounds include a homologous series of n-alkanes, methylbranched alkanes, alkylcyclopentanes, alkylcyclohexanes, alkylbenzenes and alkylcycloalkanes.  相似文献   

6.
The kerogen of a sample of Estonian Kukersite (Ordovician) was examined by spectroscopic (solid state 13C NMR, FTIR) and pyrolytic (“off-line”, flash) methods. This revealed an important contribution of long, linear alkyl chains in Kukersite kerogen. The hydrocarbons formed upon pyrolysis are dominated by n-alkanes and n-alk-1-enes and probably reflect a major contribution of selectivity preserved, highly aliphatic, resistant biomacromolecules from the outer cell walls of Gloeocapsomorpha prisca. This is consistent with the abundant presence of this fossilized organism in Kukersite kerogen. In addition high amounts of phenolic compounds were identified in the pyrolysates. Series of non-methylated, mono-, di- and trimethylated 3-n-alkylphenols, 5-n-alkyl-1,3-benzenediols and n-alkylhydroxybenzofurans were identified. All series of phenolic compounds contain long (up to C19), linear alkyl side-chains. Kukersite kerogen is, therefore, an aliphatic type II/I kerogen, despite the abundance of free phenolic moieties. This study shows that phenol-derived moieties are not necessarily associated with higher plant-derived organic matter.The flash pyrolysate of Kukersite kerogen was also compared with that of the kerogen of the Guttenberg Oil Rock (Ordovician) which is also composed of accumulations of fossilized G. prisca. Similarities in the distributions of hydrocarbons and sulphur compounds were noted, especially for the C1–C6 alkylbenzene and alkylthiophene distributions. However, no phenolic compounds were detected in the flash pyrolysate of the Guttenberg kerogen. Possible explanations for the observed similarities and differences are discussed.  相似文献   

7.
A Pliocene oil shale (Pula, Hungary), a C3 plant Triticum aestivum and a C4 plant Zea mays were compared using isotopic composition of bulk organic matter, along with distributions and individual carbon isotope ratios of n-alkanes from organic extracts. The microalga Botryococcus braunii (A race) was thus shown to be the main source of the predominant 27, 29 and 31 n-alkanes of Pula sediment Therefore, the dominance of odd carbon-numbered n-alkanes in the range C25–35 in extracts from immature sediments shall not be systematically assigned to higher plant contribution but algal input is also possible. In fact, the long chain n-alkanes with an odd predominance previously observed in extracts of various immature sediments are likely to be derived at least partially, from algae.  相似文献   

8.
Simultaneous mass spectral detection and stable carbon isotope analysis was performed on individual indigenous n-alkanes isolated from single C4 and C3 plant species and on a series of aliphatic and polycyclic aromatic hydrocarbons (PAH) produced from the combustion of these same biomass materials. The analysis technique used a combined gas chromatograph-mass spectrometer/combustion/isotope ratio mass spectrometer (GC-MS/C/IRMS). Precision (2σ) for replicate measurements of individual compounds in standard solutions using this novel configuration ranged between 0.2 and 0.5‰ for n-alkanes and 0.3 and 0.8‰ for PAH. Accuracy of the n-alkane measurements ranged between 0.1 and 0.4‰ and that of the PAH measurements ranged between 0.2 and 0.9‰. Replicate GC-MS/C/IRMS measurements on the combustion-derived n-alkene/alkane pairs were performed to within a precision of between 0.1 and 1.1‰ and the precision for the combustion PAH was similar to the standard PAH solution. No notable isotopic effects were observed when altering the temperature of the combustion process from 900 to 700°C, or as a result of the individual n-alkenes/alkanes partitioning between the gaseous and condensate fractions. Combustion-derived n-alkenes/alkanes ranged from C11 to C31, and the C4-derived n-alkenes/alkanes were approx. 8‰ more enriched in 13C than the C3-derived compounds. Both the C4 and C3-derived n-alkenes/alkanes (C20-C30) were isotopically similar to the indigenous n-alkanes and were 2-3‰ more depleted in 13C than the lower mol. wt (C1111-C19) n-alkenes/alkanes, suggesting an independent origin for the lower mol. wt compounds. Combustion-generated C4 and C3-derived 2-, 3-, and 4-ring PAH were also isotopically distinct (Δδ = 10‰). Unlike the n-alkenes/alkanes, no compound-to-compound variations were observed between the low and high mol. wt PAH. This study demonstrates that the isotopic composition of original plant biomass material is mainly preserved in the aliphatic hydrocarbons and PAH generated by its combustion. Consequently, analyses of these compounds in sediments impacted by fire occurrences may provide useful information about paleo-fire activity that may help elucidate the impact biomass burning may have had and could have on climate-biosphere interactions.  相似文献   

9.
The chemical composition of Cretaceous leaf remains showing exceptionally well preserved cuticles was investigated using pyrolysis gas chromatography–mass spectrometry (Py-GC–MS) and thermally assisted hydrolysis methylation (THM)-GC–MS. Samples of Coniferales (Frenelopsis) and Ginkgoales (Nehvizdya penalveri) leaf remains were collected from freshwater and coastal marine depositional environments. Material for pyrolysis included (i) untreated leaves and cuticles obtained after extraction from mineral rock matrix and bleaching, (ii) kerogen fraction from both materials, (iii) non-hydrolysable fraction from kerogen. The THM-GC–MS data from untreated leaves and bleached cuticles show that the fossil cuticle geopolymer essentially released aliphatic components upon thermal treatment, with a dominance of fatty acids (FAs) and n-alkanes/n-alkenes. The FAs are essentially resistant to bleaching and remain after solvent extraction. They occur mainly as short chain compounds ranging from C6 to C16 and with maximum abundance at C8–C9. The n-alkanes/n-alkenes from kerogen and the non-hydrolysable residue occur mainly as short chain compounds in the range C10–C16, with the highest abundance at C9–C12. The THM-GC–MS pyrograms of the fossil cuticles differ from those of cutan from fresh living plants. They support the preservation model via polymerization of monomers derived from cutin or from unsaturated cell FAs.  相似文献   

10.
The geochemical composition of lake bed sediments of a tropical reservoir in Brazil have been investigated. The C, N, P composition showed almost no variation between the different sampling points. All samples contained inorganic phosphorus (IP) ranging from 52 to 70%. The Redfield ratios show that the lake is without significant anthropogenic inputs. Most of the organic matter is composed by higher plants decomposition revealed in the total organic carbon (TOC):nitrogen (N) ratio ranging from 15.4 to 57.2. Also, TOC:organic phosphorus (OP) ranged from 265.3 to 933.6, suggesting that most of the organic matter has terrestrial origin from wood plants. The ratios suggest that most organic matter is influenced by the terrestrial characteristics of the watershed. Furthermore, the relative abundance of n-alkane homologues in the sediments was investigated. All samples have been analyzed for the n-alkanes C8 to C40. The sediments were dominated by n-alkanes C25–C38. It is concluded that n-alkane in sediments mainly come from terrestrial plants, however there is a contribution of submerged aquatic plants, especially in those sites in deeper areas of the lake. On the basis of Paq index the n-alkanes in sediments comes from terrestrial plants, however there is a contribution from emerged/floating plants.The investigated lake seems to be considered as meso to eutrophic. Because of the relatively high primary productivity in the lakes, there is a substantial organic-matter flux to the sediments, which rapidly becomes anoxic. According to the pristine/phytane ratio the sediments represent an anoxic environment with values found between 0.38 and 1.72.  相似文献   

11.
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   

12.
Hydrocarbon results from gas chromatography of 60 recent sediment and 10 benthic algae samples delineate two distinct shelf environments in the northeastern Gulf of Mexico.Sediments off Florida (shell hashes and sands) have moderate amounts of lipids/total sediment (average 113ppm ± 80%) but low hydrocarbon levels (average 3.06 ppm ± 41%). Aliphatic hydrocarbons are dominated by a series of branched or cyclic, unsaturated C25 isomers. The major n-alkane is n-C17. The n-alkane and isoprenoid patterns are consistent with a marine hydrocarbon source.Sediments closer to the Mississippi River (silts and clays) contain large amounts of lipids (average 232 ppm ± 53%) and hydrocarbons (average 11.7 ppm ± 55%) to total sediment. Aliphatic hydrocarbons are mainly odd carbon number high molecular weight n-alkanes, indicating a terrigenous hydrocarbon source. Isoprenoids are present in greater abundance than in sediments off Florida (n-C17/ pristane and n-C18/phytane ratios ~2to 3). Relatively large amounts of n-C16, together with an even distribution of n-alkanes in the range C14–C20 and a substantial unresolved envelope all point to a fossil fuel input to the Mississippi samples.Samples off the Alabama coast show intermediate characteristics.  相似文献   

13.
Organic matter (OM) associated with the Dongsheng sedimentary U ore hosting sandstone/siltstone was characterized by Rock-Eval, gas chromatography–mass spectrometry and stable C isotope analysis and compared to other OM in the sandstone/siltstone interbedded organic matter-rich strata. The OM in all of the analyzed samples is Type III with Ro less than 0.6%, indicating that the OM associated with these U ore deposits can be classified as a poor hydrocarbon source potential for oil and gas. n-Alkanes in the organic-rich strata are characterized by a higher relative abundance of high-molecular-weight (HMW) homologues and are dominated by C25, C27 or C29 with distinct odd-to-even C number predominances from C23 to C29. In contrast, in the sandstone/siltstone samples, the n-alkanes have a higher relative abundance of medium-molecular-weight homologues and are dominated by C22 with no or only slight odd-to-even C number predominances from C23 to C29. Methyl alkanoates in the sandstone/siltstone extracts range from C14 to C30, maximizing at C16, with a strong even C number predominance, but in the organic-rich layers the HMW homologues are higher, maximizing at C24, C26 or C28, also with an even predominance above C22. n-Alkanes in the sandstone/siltstone sequence are significantly depleted in 13C relative to n-alkanes in most of the organic-rich strata. Diasterenes, ββ-hopanes and hopenes are present in nearly all the organic-rich sediments but in the sandstone/siltstone samples they occur as the geologically mature isomers. All the results indicate that the OM in the Dongsheng U ore body is derived from different kinds of source materials. The organic compounds in the organic-rich strata are mainly terrestrial, whereas, in the sand/siltstones, they are derived mainly from aquatic biota. Similar distribution patterns and consistent δ13C variations between n-alkanes and methyl alkanoates in corresponding samples suggest they are derived from the same precursors. The OM in the organic-rich strata does not appear to have a direct role in the precipitation of the U ore in the sandstone, but an indirect role cannot be excluded. The OM in the U hosting sandstone shows a relatively low hydrogen index, presumably due to oxidation or radiolytic damage.  相似文献   

14.
Previous studies on lipid biomarkers preserved in Chinese stalagmites have indicated that ratios of low‐molecular‐weight (LMW) to high‐molecular‐weight (HMW) n‐alkanes, n‐alkan‐2‐ones, n‐alkanols and n‐alkanoic acids can be used as an index of vegetation versus microbial organic matter input to the system and, by extension, a marker of climatic changes, with increases in the proportion of LMW compounds coinciding with colder periods. Here we test whether this hypothesis is equally applicable to a different geographical region (north‐west Scotland), by examining a stalagmite record of the past 200 years, and a wider range of lipid markers. We also test the applicability of other lipid proxies in this context, including the use of n‐alkane ratios, to interpret vegetation changes, and unsaturated alkanoic acid ratios as climatic indicators. The results show that lipid proxies preserved in stalagmites, and especially those related to vegetation, are potentially extremely useful in palaeoenvironmental research. Of particular value is the use of C27/C31 n‐alkane ratios as a proxy for vegetation change, clearly indicating variations between herbaceous and arboreal cover. This proxy has now been successfully applied to samples from diverse environments, and can be considered sufficiently robust to be of use in analysing future stalagmite records. It will be of particular value in areas where reliable pollen records are not available, as is often the case with deeper cave deposits. However, the division between LMW and HMW aliphatic compounds is not a clear‐cut case of microbial versus plant activity, with the changes in LMW compounds relating more closely to those in their HMW analogues than in specific bacterial biomarkers. The use of unsaturated alkanoic acid ratios here gives conflicting results, with the observed variation through time depending on the isomer measured. The discrepancies between the findings of this study and previous work are likely to be due to the varying controls on the lipids (original organic matter input, and compound degradation), which in turn will be affected by whether the main climatic limiting factor on the soil is temperature or precipitation. This suggests that lipid proxies preserved in stalagmites must be interpreted with care, particularly in the case of bacterial compounds which may be derived from within the cave or from the soil. However, many of these issues can be resolved by the use of multi‐proxy studies. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

15.
Shallow surface sediment samples from the Mesopotamian marshlands of Iraq were collected and analyzed to determine the distribution, concentrations and sources of aliphatic lipid compounds (n-alkanes, n-alkanols, n-alkanoic acids, and methyl n-alkanoates) and molecular markers of petroleum in these wetlands. The sediments were collected using a stainless steel sediment corer, dried, extracted with a dichloromethane/methanol mixture and then analyzed by gas chromatography-mass spectrometry (GC–MS). The aliphatic lipid compounds included n-alkanes, n-alkanoic acids, n-alkanols and methyl n-alkanoates with concentrations ranged from 6.8 to 31.1 μg/g, 4.1 to 5.0 μg/g, 5.9 to 7.7 μg/g and from 0.3 to 5.9 μg/g, respectively. The major sources of aliphatic lipids were natural from waxes of higher plants (24–30%) and microbial residues (42–30%), with a significant contribution from anthropogenic sources (27–30%, petroleum), based on the organic geochemical parameters and indices. Further studies are needed to characterize the rate, accumulation and transformation of various organic matter sources before and after re-flooding of these wetlands.  相似文献   

16.
Hydrocarbon mixtures too complex to resolve by traditional capillary gas chromatography display gas chromatograms with dramatically rising baselines or “humps” of coeluting compounds that are termed unresolved complex mixtures (UCMs). Because the constituents of UCMs are not ordinarily identified, a large amount of geochemical information is never explored. Gas chromatograms of saturated/unsaturated hydrocarbons extracted from Late Archean argillites and greywackes of the southern Abitibi Province of Ontario, Canada contain UCMs with different appearances or “topologies” relating to the intensity and retention time of the compounds comprising the UCMs. These topologies appear to have some level of stratigraphic organization, such that samples collected at any stratigraphic formation collectively are dominated by UCMs that either elute early- (within a window of C15–C20 n-alkanes), early- to mid- (C15–C30 n-alkanes), or have a broad UCM that extends through the entire retention time of the sample (from C15–C42 n-alkanes). Comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC–MS) was used to resolve the constituents forming these various UCMs. Early- to mid-eluting UCMs are dominated by configurational isomers of alkyl-substituted and non-substituted polycyclic compounds that contain up to six rings. Late eluting UCMs are composed of C36–C40 mono-, bi-, and tricyclic archaeal isoprenoid diastereomers. Broad UCMs spanning the retention time of compound elution contain nearly the same compounds observed in the early-, mid-, and late-retention time UCMs. Although the origin of the polycyclic compounds is unclear, the variations in the UCM topology appear to depend on the concentration of initial compound classes that have the potential to become isomerized. Isomerization of these constituents may have resulted from hydrothermal alteration of organic matter.  相似文献   

17.
The molecular organic compounds have been identified by gas chromatography (GC) and GC-mass spectrometry (GC-MS) from Mesoproterozoic rocks in the Xuanlong depression in North China. The main saturated compounds are n-alkanes, monomethylalkanes, n-alkylcyclohexanes, acyclic isoprenoids, and hopanes. The dominant lower-molecular-weight n-alkanes are indicative of the main contribution of microorganisms, in particular, the chemosynthetic bacteria. The presence of abundant monomethylalkanes (mid- and end-branched) and the long chained (>C20) acyclic isoprenoids indicates the existence of abundant bacteria and/or archaea in ancient oceans. The low abundance of pristane and phytane is suggestive of the relatively low abundance of photosynthetic autotrophs in comparison with chemosynthetic bacteria in the Mesoproterozoic oceans in North China. The sedimentary environmental condition is suboxic/anoxic, as indicated by the low value of the Pr/Ph ratio as well as the presence of abundant sulfur-bearing organic compounds, consistent with the other geochemical data in North China and elsewhere in the world. Both the composition of the primary producers and the sedimentary environmental conditions are favorable for the formation of hydrocarbon source rocks.  相似文献   

18.
The Eocene Maoming oil shale from Guangdong Province occurs as a laterally uniform stratigraphic section, typically 20–25 m thick, from which the aliphatic hydrocarbon constituents of six representative samples were investigated using GC and C-GC-MS. The sediments evaluated included the basal lignite, a vitrinite lens from the overlying claystone, and four intervals from the massive oil shale bed. As expected, the lignite and vitrinite differ markedly from the oil shales. The lignite is dominated by bacterial hopanoids and components of higher plant origin, including C29 steroids and triterpenoids such as oleanenes. Visually, the oil shale samples show corroded and degraded phytoclasts, spores, wispy particles of fluorescent organic material attributable to dinoflagellates and, especially in the uppermost sample, colonial algal bodies. The distributions of biological markers in the oil shales show many features in common, notably a dominance of dinoflagellate-derived 4-methylsteroids, and a significant proportion of higher-plant derived n-alkanes with marked odd-over-even carbon number predominance. Overall, they exhibit several features that resemble characteristics of the Messel shale. The hydrocarbons of the lowest shale horizon suggest that there may have been a gradual transition between deposition of the original peat and the subsequent oil shales. The aliphatic hydrocarbons of the uppermost shale are dominated by a number of C31 and C33 botryococcane homologues and other unusual branched alkanes possibly derived from green algae. All of the samples are immature. Overall, molecular and microscopic examination of the stratigraphic succession of the Maoming oil shale suggests a shallow, lacustrine environment within which peats were deposited. This lake subsequently deepened to support abundant algal populations, especially dinoflagellates, culminating in a dominance of botryococcoid algae.  相似文献   

19.
Biodegraded oils are widely distributed in the Liaohe basin, China. In order to develop effective oil-source correlation tools specifically for the biodegraded oils, carbon isotopic compositions of individual n-alkanes from crude oils and their asphaltene pyrolysates have been determined using the gas chromatography–isotope ratio mass spectrometry technique. No significant fractionation in the stable carbon isotopic ratios of n-alkanes in the pyrolysates of oil asphaltenes was found for anhydrous pyrolysis carried out at temperatures below 340°C. This suggests that the stable carbon isotopic distribution of n-alkanes (particularly in the C16–C29 range) in the asphaltene pyrolysates can be used as a correlation tool for severely biodegraded oils from the Liaohe Basin. Comparison of the n-alkane isotopic compositions of the oils with those of asphaltene pyrolysates shows that this is a viable method for the differentiation of organic facies variation and post-generation alterations.  相似文献   

20.
A suite of 18 oils from the Barrow Island oilfield, Australia, and a non-biodegraded reference oil have been analysed compositionally in order to detail the effect of minor to moderate biodegradation on C5 to C9 hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes, steranes, alkylnaphthalenes and alkylphenanthrenes indicate thermal maturities of about 0.8% Rc for most of the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C5 to C9 hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily biodegraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biodegraded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes (>95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C6 and C7 show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methylhexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C8 and C9 hydrocarbons, alkylcyclohexanes are more resistant to biodegradation than linear alkanes. Secondly, there is a trend of lower susceptibility to biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylbenzenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C3 alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methylalkanes are the most resistant and 4-methylalkanes have intermediate resistance. Therefore, terminal methyl groups are more prone to bacterial attack compared to mid-chain isomers, and C3 carbon chains are more readily utilised than C2 carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C5–C9 alkanes are isotopically light (depleted in 13C) relative to cycloalkanes and aromatic hydrocarbons. The effects of biodegradation consistently lead to enrichment in 13C for each remaining hydrocarbon, due to preferential removal of 12C. Differences in the rates of biodegradation of low molecular weight hydrocarbons shown by compositional data are also reflected in the level of enrichment in 13C. The carbon isotopic effects of biodegradation show a decreasing level of isotopic enrichments in 13C with increasing molecular weight. This suggests that the kinetic isotope effect associated with biodegradation is site-specific and often related to a terminal carbon, where its impact on the isotopic composition becomes progressively ‘diluted’ with increasing carbon number.  相似文献   

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