首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 15 毫秒
1.
In addition to the previously reported 25-norhopanes and 25-norhopanoic acids, for the first time we report the identification of 25-norbenzohopanes. The hydrocarbon composition of the bitumen from Palaeozoic carbonates in northern Alberta displays molecular evidence for severe levels of biodegradation characterised by the removal of C30–C35 hopanes. Biodegradation is also indicated by the removal of C32 and C33 benzohopanes. The appearance of C31 and C32 25-norbenzohopanes corresponds to the decrease in C32 and C33 benzohopanes, suggesting that 25-norbenzohopanes originate by demethylation of benzohopane counterparts. Demethylation at C-10 in the hopanoids affects a broader class of compounds that so far includes the hopanes and hopanoic acids, as well as the benzylated hopanoid species.  相似文献   

2.
Origin and occurrence of 25-norhopanes: a statistical study   总被引：1，自引：0，他引：1  
The alkane fraction of more than 200 rocks, biodegraded oils and non-biodegraded oils, have been analysed by means of computerized GC-MS, in order to investigate the effect of natural biodegradation on the occurrence of “demethylated hopanes”, i.e. 17α-25-norhopanes. The results obtained indicate that 25-norhopanes are preexisting biomarkers the concentration of which is enhanced by selective biodegradation of more readily degradable homologs, i.e. regular hopanes, rather than by demethylation of hopanes in reservoirs. However, the use of 25-norhopane enrichment as a palaebiodegradation indicator in apparently non-biodegraded oils is still valuable providing the initial background content in the corresponding source rocks is known. Furthermore, 25-norhopanes appear to be diagnostic of specific environmental conditions (marine and lacustrine source rocks, dysoxic and not very hypersaline). Lastly, one other (novel) bacterially resistant rearranged hopanoic compound, namely a C29 neohopane, is applicable for both biodegradation and maturation evaluation.  相似文献   

3.
The formation or generation of hopanes are important processes during both the natural heating of organic-rich sediments and laboratory pyrolysis experiments. Molecular maturity parameters as well as the amounts (ng/g rock) of the C31 hopanes and C30–C32 hopanoic acids were quantified in a Jurassic silty shale horizon (Isle of Skye, Scotland) as a function of distance from an igneous intrusion. The maturity profiles of the homohopanes and the hopanoic acids are comparable. There is also a correlation between the decreasing amounts of C30–C32 hopanoic acids and concomitant increases in C29–C31 hopanes suggesting that free hopanoic acids could be one potential source of hopanes in this particular horizon. Other possible sources could include hopanoic acids that are bound into the macromolecular fraction.  相似文献   

4.
Novel side chain methylated and hexacyclic hopanes have been identified in coals and oils from around the world. Extended hopanes (>C32) with an additional methyl in the side chain (“isohopanes”) were identified by comparison with synthetic standards. The major C33-C35 isohopanes are 31-methylbishomohopanes, 32-methyltrishomohopanes and 33-methyltetrakishomohopanes. Extended hopanes methylated at C-29 were not detected. The 17α(H),21β(H)-31-methyltrishomohopanes show four peaks on gas chromatography because of the extra asymmetric carbon at C-31. Like regular hopanes, the isohopanes extend beyond C35. Low concentrations of novel hexacyclic hopanes having 35 or more carbons were also detected in oils and coal extracts. The C35 hexacyclic hopanes were identified as 29-cyclopentylhopanes. Isohopanes are released from the kerogen by hydrous pyrolysis and hydropyrolysis. The 22S/(22S + 22R) ratio for 31-methylbishomohopanes and other isohopanes is around 0.60 at equilibrium in geological samples. They isomerize slightly more slowly than regular C33 hopanes. Isohop-17(21)-enes, 2α-methylisohopanes and two series of rearranged isohopanes were tentatively identified. Isohopanes can be biodegraded to form the corresponding 25-norhopanes. When 25-norhopanes are not formed, the isohopanes are much more resistant to biodegradation than regular hopanes. In biodegraded oil seeps from Greece, 30-norisohopanes were tentatively assigned. The composition and relative abundance of C33 and C34 isohopanes in a worldwide set of coals and crude oils was determined. Isohopanes are abundant in coal and coal-generated oils, where they can account for more than 5% of all extended hopanes, and low in abundance in oils from source rocks deposited under anoxic conditions.  相似文献   

5.
The distributions of hopanoic acids, ranging from C30 to C34, in the Messel oil shale were characterized in both the free and bound states. The bound acids were released by thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH). These were compared with the distributions of the hopanoic acids and hopanes released or generated from Messel oil shale kerogen following closed system microscale pyrolysis. This comparison revealed that epimerization had occurred at C-17, C-21 and C-22 during heating. It was also clear that the residual bound hopanoic acids had undergone configurational isomerization. During the pyrolysis there is a large loss of hopanoic acids following their rapid release from the kerogen into the free fraction even at 250 °C. In these particular experiments this loss does not appear to result in exclusive formation of hopanes, by way of decarboxylation or reduction reactions, unless the resulting hopanes are either themselves rapidly transformed into other compounds or the reaction rates are a function of the total number of carbon atoms in each hopanoic acid precursor.  相似文献   

6.
The analyses, by gas chromatography and gas chromatography/mass spectrometry, of the triterpane concentrate of crude oils sampled from various oil fields of the Tertiary Niger delta have revealed the ubiquitous presence of a series of C24–C27 tetracyclic alkanes likely to be novel degraded triterpanes. The presence in the crude oils of a C25 tricyclic alkane, apparently structurally related to the tetracyclanes, seemed consistent with the hypothesis of sequential cleavages of the terminal rings of precursor pentacyclic triterpenoid derivatives with increasing thermal transformation of the respective petroleums.The degraded triterpanes might be useful for assessing the stages of thermal evolution of petroleum in the reservoir. A possible application, to oil exploration, of the expected variations in the concentration of the polycyclanes in crude oils with different thermal histories would be in distinguishing primary (immature) oils from mature but biodegraded oils.  相似文献   

7.
Biomarker and n-alkane compound specific stable carbon isotope analyses (CSIA) were carried out on 58 crude oil samples from shallow water and deepwater fields of the Niger Delta in order to predict the depositional environment and organic matter characteristics of their potential source rocks. Using a source organofacies prediction approach from oil geochemistry, the presence in the western deepwater oils relatively abundant C27 steranes, C30 24-n-propyl cholestane, low oleanane index, relatively low pr/ph ratios, gammacerane, and positive to nearly flat C12–C30 n-alkane compound specific stable carbon isotope profiles, suggests that the source facies that expelled these oils contain significant marine derived organic matter deposited under sub-oxic and stratified water column conditions. This contrasts with the terrigenous organic matter dominated source rocks accepted for shallow water Niger Delta oils. Oils in the shallow water accumulations can be separated into terrigenous and mixed marine-terrigenous families. The terrigenous family indicates expulsion from source rock(s) containing overwhelmingly higher plant source organic matter (average oleanane index = 0.48, high C29 steranes) as well as having negative sloping n-alkane isotope profiles. Oxic source depositional conditions (pr/ph > 2.5) and non-stratified conditions (absence to low gammacerane content) are inferred for the terrigenous family. The mixed marine-terrigenous family has biomarker properties that are a combination of the deepwater and terrigenous shallow water oils. Bitumen extracts of the sub-delta Late Cretaceous Araromi Formation shale in the Dahomey Basin are comparable both molecularly and isotopically to the studied western deepwater oil set, but with an over all poor geochemical correlation. This poor geochemical match between Araromi shale and the western deepwater oils does not downgrade the potential of sub-delta Cretaceous source rock contribution to the regional oil charge in the deepwater Niger Delta.  相似文献   

8.
Based on gas chromatography and gas chromatography-mass spectrometry analyses, an amazing amount of hopanoids was detected in the peat deposits in the Dajiuhu National Wetland Park in central China. The hopanoids identified included hopanes (C27-C31 αβ, C27-C32 ββ, C29 βα), hopenes (hop-22(29)-ene, 22,29,30-trinorhop-17(21)-ene, hop-17(21)-ene, hop-13(18)-ene, etc.), hopanoic acids (C31-C34 ββ, C32-C33 βα, C32 αβ), hopanols (C32 ββ and αβ) and hopanone (22,29,30-trinorhop-21-one). C31 αβ-22R hopane was found to be the dominant hopanoid, more abundant than individual nalkanes derived from higher plants. These hopanoids, exclusive of some hopenes, are proposed to be primarily from bacteria. The dominant C31 αβ-22R hopane in young sediments, without any thermal maturation, might be formed through microbial epimerization under acidic conditions in the peatland as suggested before, or directly from aerobic bacteria. This finding highlights the importance of microbes in the formation of peatland as well as in the reconstruction of paleoenvironments.  相似文献   

9.
The aliphatic hydrocarbon composition (acyclic isoprenoids, hopanoids and steroids) of oils from the most productive fields in the southern geological Province of Cuba have been studied. This province is defined by its position with respect to the Cretaceous overthrust belt generated during the formation of oceanic crust along the axis of the proto-Caribbean Basin. The relative abundances of 18α(H)-22,29,30-trisnorneohopane, gammacerane and diasteranes suggest that Pina oils are related to the carbonate oils from the Placetas Unit in the northern province (low Ts/(Ts+Tm) and C27,29 rr/(rr+sd) ratios). The Cristales and Jatibonico oils exhibit some differentiating features such as higher Ts/(Ts+Tm) and absence of gammacerane. The oils from this province do not exhibit significant differences in either hopane, C32 22S/(S+R) and C30 αβ/(αβ+βα), or sterane, C29 αα 20S/(S+R), maturity ratios. However, the relative content of 5α(H),14β(H),17β(H)-cholestanes (C29 ββ/(ββ+αα) ratio) indicates that Pina oils are more mature than Cristales and Jatibonico oils. Several of these oils (Cristales, Jatibonico and Pina 26) are heavily biodegraded, lacking n-alkanes, norpristane, pristane and phytane (the two former oils do not contain acyclic isoprenoid hydrocarbons). Other biodegradation products, the 25-norhopanes, are found in all the oils. Their occurrence is probably due to mixing of severely biodegraded oil residues with undegraded crude oils during accumulation in the reservoir.  相似文献   

10.
Unusual short chain lanostanes (C24 and C25) and C30 lanostane were identified in sulfur rich crude oils from the Jinxian Sag, Bohai Bay Basin, northern China. Besides the regular steranes (C27-30), a series of 4-methyl steranes (C22−23, C27−30), 4,4-dimethyl steranes (C22−24, C28−30), short chain steranes (C23−26), abundant pregnanes (C21−22) and androstanes (C19−20), together with sulfur containing steroids (20-thienylpregnanes and thienylandrostanes) were detected in the aliphatic and branched-cyclic hydrocarbon fraction of these crude oils. A literature survey of some long chain sterane analogues (e.g., A-nor-steranes, norcholestanes, C30 steranes, lanostanes) and pregnanes seems to point to a sponge and/or dinoflagellate source. 4-Methyl, 4,4-dimethyl steroids and lanosterols (4,4,14-trimethyl steroids as the basic skeleton of lanostanes) can be derived from methanotrophic bacteria. Thus, a biological origin from a prokaryotic methylotroph can be used to explain the common source of abundant short chain steranes (C23-26), 4-methyl (C22-23) and 4,4-dimethyl steranes (C22-24), as well as lanostanes (C24-25 and C30 analogues) in our oil samples. Generally, the steroids appear to have been extensively sulfurized with sulfur substitution at the C-22 position in the side chain during the early stage of diagenesis, which was readily subject to attack by bacterial degradation (enzymatic cleavage) and/or abiotic oxidation. As a consequence, short chain sterane analogues (e.g., abundant pregnanes and androstanes in this study) and short chain lanostanes (C24−C25) might later be released through cleavage of weak C-S bonds at the C-22 carbon in the sulfurized steroids and lanostane sulfides. Finally, the formation of the short chain C24−C25 lanostanes and distinctive occurrence of short chain steranes in this study can be well explained by microbial biodegradation of sulfurized lanostanoids and steroids in the reservoir.  相似文献   

11.
A 10 m sediment core from Academician Ridge in Lake Baikal was analyzed for its molecular composition using on-line TMAH (tetramethylammomium hydroxide) thermochemolysis. Major products are lignin phenols, n-C14 to C30 fatty acids (alkanoic acids), cutin acids, hydroxy acids and aliphatic dicarboxylic acids. Lignin phenols are abundant in warmer periods (the interglacial: marine isotope stage (MIS) 5e and MIS 1), but extremely low in the other (colder) periods. This result coincides well with pollen records reported for a core near the present site, where an expansion of coniferous forests in sub-stage 5e and MIS 1 was implied. Normal C24–C30 alkanoic acids, important components of plant wax esters, are abundant in 5e and MIS 1 and are present in significant amounts in the other (colder) periods, unlike the lignin phenols. A high abundance of n-C24 to C30 alkanoic acids relative to lignin phenols in the Bølling–Allerød warm period suggests an enhanced development of herbs.It is implied from comparison of the sedimentary lignin phenol record with fossil pollen records and lignin phenol analysis of modern pollen that the ratio of cinnamyl phenols to vanillyl phenols may serve as an indicator of pollen contribution to sedimentary organic matter.  相似文献   

12.
Based on the compositions and distributions of biomarkers in thirty-five representative oil samples, oils from the Tarim Basin of northwestern China are mainly divided into two oil families. One oil family contains relatively low amounts of C15-C20 isoprenoid hydrocarbons and shows pristane predominance with Pr/Ph ratios ranging from 1.50 to 3.00. The GC/MS analytical data of these oils show the occurrence of abundant hopanes, and low concentrations of steranes and tricyclic terpanes with hopanes/steranes ratios from 6.25 to 12.24 and tricyclic terpanes/hopanes ratios from 0.03 to 0.24. These oils contain low drimane relative to homodrimane (C15/C16 < 1.0) and abundant rearranged bicyclanes in bicyclic sesquiterpanes. They are dominated by low carbon number (C19-C21) compounds in the tricyclic terpanes, and are rich in rearranged hopanes, C29Ts and an unknown C30 compound in pentacyclic triterpanes. These geochemical characteristics suggest that the oils were generated mainly from terrigenous organic matter. The other oil family shows remarkably different biomarker compositions and distributions. The oils revealed Pr/Ph ratios of about 1.0, high drimane/homodrimane ratios (>1.0), low hopanes/steranes ratios (0.65–2.50), high tricyclic terpanes/hopanes ratios (0.30–2.00) and a dominant peak at C23 in tricyclic tepanes, suggesting a marine organic origin. Oil-source rock correlation indicates that these two oil families seem to have been derived from Mesozoic Jurassic-Triassic terrestrial source rocks (shales and coal seams) and Lower Paleozoic Ordovician-Cambrian marine source rocks, respectively.  相似文献   

13.
This paper consists of two interrelated parts. In the first part, the influence of the composition of sediment organic matter on crude oil composition is discussed. The second part deals with the origin of normal paraffins in petroleum.Source beds with abundant terrestrial plant matter generate heavy hydrocarbons rich in five-ring naphthenes. Unless such source beds are exposed to a high temperature for a prolonged time, the oils released are also rich in five-ring naphthenes. Such oils are rare; thus far the only examples found are some Eocene Wilcox oils from the Texas Gulf Coast and some Eocene Green River oils from the Uinta Basin, Utah. Normally, oil source beds are not rich in terrestrial plant matter and the five-ring naphthene content of the source bed hydrocarbons, as well as that of the produced oils, is low.The n-paraffins generated by oil source beds rich in terrestrial plant matter are characterized by abnormally low (C21 + C22)/(C28 + C29) ratios of 0.6–1.2. In oils of dominantly marine origin, this ratio is in the range 1.5–5.0. The ratio of marine to terrestrial organic matter in source beds appears to influence both the naphthene composition and the n-paraffin composition of the generated oils.Evidence is presented that petroleum n-parainns originate from slow thermal cracking of fatty acids contained in fats and waxes. Reaction equations are discussed which explain the major geochemical observations, including the difference in carbon-number distribution of the assumed parental fatty acids and of their descendant n-paraffins. In normal oils, which originate mostly from fat rich marine organic matter, the n-paraffin concentration tapers off above C20. The molecular weight range of the fatty acids of plant waxes is considerably higher than that of fats. If plant waxes contribute strongly to the oil source material, the molecular weight distribution of the petroleum n-paraffins formed is abnormal and high carbon numbers in the C24-C32 range dominate.  相似文献   

14.
《China Geology》2020,3(4):602-610
Thirty-nine crude oils and twenty-one rock samples from Niger Delta Basin, Nigeria have been characterized based on their isotope compositions by elemental analysis-isotope ratio mass spectrometry and gas chromatography-isotope ratio mass spectrometry. The bulk carbon isotopic values of the whole rock extracts, saturate and aromatic fractions range from –28.7‰ to –26.8‰, –29.2‰ to –27.2 ‰ and –28.5 ‰ to –26.7 ‰, respectively while the bulk carbon isotopic values of the whole oils, saturate and aromatic fractions range from –25.4 ‰ to –27.8 ‰, –25.9 ‰ to –28.4 ‰ and –23.5 ‰ to –26.9 ‰, respectively. The average carbon isotopic compositions of individual alkanes (nC12-nC33) in the rock samples range from –34.9‰ to –28.2‰ whereas the average isotopic values of individual n-alkanes in the oils range from –31.1‰ to –23.8‰. The δ13C isotope ratios of pristane and phytane in the rock samples range from –29.2 ‰ to –28.2 ‰ and –30.2 ‰ to –27.4 ‰ respectively while the pristane and phytane isotopic values range from –32.1‰ to –21.9‰ and –30.5‰ to –26.9‰, respectively. The isotopic values recorded for the samples indicated that the crude oils were formed from the mixed input of terrigenous and marine organic matter and deposited under oxic to sub-oxic condition in lacustrine-fluvial/deltaic environments. The stable carbon isotopic compositions were found to be effective in assessing the origin and depositional environments of crude oils in the Niger Delta Basin.  相似文献   

15.
Analyses of some Australian crude oils show that many contain varying concentrations of A/ B-ring demethylated hopanes. These range from C26 to C34 and have been identified from their retention times and mass spectral data as 17α(H)-25-norhopanes. Comparison of hopane and demethylated hopane concentrations and distributions in source-related, biodegraded oils suggests that demethylated hopanes are biotransformation products of the hopanes. Further, it appears that the process occurs at a late stage of biodegradation, after partial degradation of steranes has occurred. Demethylated hopanes are proposed as biomarkers for this stage of severe biodegradation. The presence of these compounds in apparently undegraded crude oils is thought to be due to the presence of biodegraded crude oil residues which have been dissolved by the undegraded crude oil during accumulation in the reservoir sands. The timing of hopane demethylation, relative to the degradation of other compounds, has been assessed and the progressive changes in crude oil composition with increasing extent of biodegradation have been identified. The use of demethylated hopanes as maturity parameters for severely biodegraded crude oils, and the applicability of established biomarker maturity parameters to such oils, are also discussed.  相似文献   

16.
Long-chain fatty acids (C10-C32), as well as C14-C21 isoprenoid acids (except for C18), have been identified in anhydrous and hydrous pyrolyses products of Green River kerogen (200–400°C, 2–1000 hr). These kerogen-released fatty acids are characterized by a strong even/odd predominance (CPI: 4.8-10.2) with a maximum at C16 followed by lesser amounts of C18 and C22 acids. This distribution is different from that of unbound and bound geolipids extracted from Green River shale. The unbound fatty acids show a weak even/odd predominance (CPI: 1.64) with a maximum at C14, and bound fatty acids display an even/odd predominance (CPI: 2.8) with maxima at C18 and C30. These results suggest that fatty acids were incorporated into kerogen during sedimentation and early diagenesis and were protected from microbial and chemical changes over geological periods of time. Total quantities of fatty acids produced during heating of the kerogen ranged from 0.71 to 3.2 mg/g kerogen. Highest concentrations were obtained when kerogen was heated with water for 100 hr at 300°C. Generally, their amounts did not decrease under hydrous conditions with increase in temperature or heating time, suggesting that significant decarboxylation did not occur under the pyrolysis conditions used, although hydrocarbons were extensively generated.  相似文献   

17.
A sample of the sediment-water column interface which lies on the continental shelf under the Peru upwelling regime, has been examined for fatty acids, fatty alcohols, ketones and hydrocarbons. Fatty acids were the most abundant compound class, ranging from C12-C24, with 16:0 as the major component (765.5 μg/g dry sediment). The alcohols were dominated by 3,7,11,15-tetramethylhexadeca-2-en-ol (phytol), with even-chain n-alcohols in the range C14-C20. The ketones consisted of C37-C39 di- and tri-unsaturated alken-2-ones and alken-3-ones. Both alkanes and alkenes were present in the hydrocarbon fraction; the alkanes ranging from C13 — C20 and comprising both straight chain and isoprenoid compounds; the alkenes consisting of isomeric pairs of C25 branched trienes and tetraenes. The data indicate that the organic content has been contributed very largely from marine sources (probably mainly from phytoplankton and bacteria), showing little terrigenous influence. The presence of labile compounds such as polyunsaturated fatty acids (with two to six double bonds), implies that the sediment has undergone very little diagenetic alteration, and the lipids are probably largely unchanged from the state in which they actually reached the sediment. They may therefore serve as a useful baseline in assessing diagenesis in older sediments, where diagenetic transformations are more advanced.  相似文献   

18.
Fatty acids (FAs), β- and ω-hydroxy acids, α,ω-dicarboxylic acids and n-alkanes were studied in a 200 m sediment core taken from Lake Biwa, Japan. FAs showed bimodal distribution with peaks at C16 and C22-C28. Their distribution patterns clearly changed with depth from lower molecular weight (C12-C19) predominance to higher molecular weight (C20-C32) predominance in the upper 20 m interval. Analyses of related compounds (β- and ω-hydroxy acids and α, ω-dicarboxylic acids) suggest that β- and ω-oxidative degradation of C12-C19 FAs has occurred in the sediments.The ratio of bound C12-C19 to unbound FAs increases with depth in the upper 0–1 m sediments, suggesting that unbound FAs are more labile. However, the ratio varies significantly in deeper sections and may be associated with water temperature.In the sediments deeper than 20 m in depth, C12-C19 FAs gradually decrease. On the other hand, higher molecular weight FAs (HFAs: C20-C32), which were probably derived from terrestrial plants, increase in concentration from 20 m to 100 m, suddenly decrease at 100 m and show progressively lower concentration in deeper sediments. These fluctuations are interpreted in relation to paleolimnological changes of the lake and the drainage basin. ω-Hydroxy C20-C30 acids and C20-C30α, ω-dicarboxylic acids show a distribution pattern similar to HFAs. Branched chain FAs, ω-hydroxy acids and C9-C19α,ω-dicarboxylic acids show a major peak around 3–15 m in depth. This peak is probably caused by increased bacterial activity in the water column and surface sediments in the past, which may be associated with an increase in primary production of the lake.  相似文献   

19.
Abundant aryl isoprenoids have been detected in source rock extracts from the 25-65 Ma saline lacustrine formations in the western Qaidam Basin, NW China. Identification was based mainly on comparison of mass spectra and gas chromatographic behaviour with literature data. Two pseudohomologous series in the range C13-C40 were assigned as 2,3,6- and 2,3,4-trimethyl monoaryl isoprenoids. The C40 2,3,4-trimethylaryl isoprenoid, okenane, was the most abundant component of the series, while its C40 2,3,6-trimethylaryl isomer, chlorobactane, was present in low abundance. Two C40 monoaromatic isoprenoids with a cyclohexyl ring were tentatively assigned as β-isorenieratane and β-renierapurpurane. One C40 diaryl isoprenoid was identified as isorenieratane. Identification of okenane, chlorobactane and isorenieratane was confirmed by co-elution experiments using synthetic standards. In addition, two novel series of aryl isoprenoids in the C12-C39 range were tentatively assigned as 2,6- and 2,3-dimethylaryl isoprenoids. The C39 2,3-dimethylaryl isoprenoid homologue was the most abundant component of the series, while the C39 2,6-dimethylaryl isomer was only present in low abundance. Two C39 monoaromatic bicyclic compounds were tentatively assigned as 2,6- and 2,3-dimethylaryl isoprenoids with a terminal cyclohexyl ring. Furthermore, a minor C38 component was tentatively assigned as a diaryl isoprenoid with a 2,3-/2,3-dimethyl substitution pattern. These dimethylaryl isoprenoids seem to be formed by demethylation of their parent trimethylaryl isoprenoid counterparts, the location of the demethylated position being exclusively at C-3, C-4 or C-6 of the aromatic ring. This specificity points to a biologically-mediated process (as opposed to an abiotic, maturity-related transformation) that needs to be further investigated.  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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号