Novel side chain methylated and hexacyclic hopanes: Identification by synthesis, distribution in a worldwide set of coals and crude oils and use as markers for oxic depositional environments

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.     

Constraining the genetic relationships of 25-norhopanes,hopanoic and 25-norhopanoic acids in onshore Niger Delta oils using a temperature-dependent material balance

Analysis of oil samples from the Niger Delta (Nigeria) revealed a range of structurally related hopanes, including 25-norhopanes, and hopanoic and 25-norhopanoic acids. 25-Norhopanes were detected in all medium and heavily biodegraded oils and were most abundant in the heavily degraded oils. Hopanoic acids (C₃₀-C₃₃) and 25-norhopanoic acids (C₃₀-C₃₁) were most abundant in moderately degraded oils and occurred in reduced concentration in heavily degraded oils but were absent from, or in trace concentration in, slightly degraded oils. Consideration of the structures suggests that 25-norhopanoic acids form via carboxylation of 25-norhopanes or demethylation of hopanoic acids. Mass balance for the onshore Niger Delta oils suggests that formation of 25-norhopanes operates independently of 25-norhopanoic acid formation and that 25-norhopanoic acids are likely transient intermediates for only a small proportion of the 25-norhopanes.     

The effect of minor to moderate biodegradation on C5 to C9 hydrocarbons in crude oils

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% R_c 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.     

Demethylated hopanes in crude oils and their applications in petroleum geochemistry

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.     

Investigation of hydrocarbon biodegradation from a downhole profile in Bohai Bay Basin: Implications for the origin of 25-norhopanes

A suite of reservoir cores (oil sands) from a single well in Bohai Bay Basin, East China, displayed a progressive increase in petroleum biodegradation extent on the basis of bulk composition and 25-norhopane content. This fits with the proposal that subsurface petroleum biodegradation is dominantly an anaerobic process and usually occurs at the oil–water contact. It is likely that sequential microbial degradation of hydrocarbons under anoxic conditions does not occur in a true stepwise fashion, but is controlled by various factors such as concentration and solubility of hydrocarbons and their diffusion rate to the oil/water contact. In fact, 25-norhopanes were formed prior to the complete elimination of the acyclic, and mono- and bicyclic alkanes. An inverse response of the 22S/(22S + 22R) ratio between each extended 17α(H)-hopane and its corresponding 25-norhopane was observed as severe biodegradation occurred, supporting the proposal that the 25-norhopanes originate from demethylation of hopanes. Field observation revealed that biomarkers without extended alkyl side chains, such as oleanane, gammacerane and β-carotane, have significant resistance to biodegradation and can be used as naturally occurring “internal standards” to evaluate variations in other biomarkers. The results suggest that the quantity of 25-norhopanes showed a minor increase as the hopanes decreased significantly, i.e. only partial hopane conversion to the corresponding 25-norhopanes. Alternative degradation pathways for hopanes might occur in reservoirs, in addition to C-25 demethylation.     

Geochemical analysis of mixed oil in the Ordovician reservoir of the Halahatang Depression, Tarim Basin, China

In this study,12 crude oil samples were collected and analyzed from the Ordovician reservoir in the Halahatang Depression,Tarim Basin,China.Although the density of oil samples varies considerably,based on saturated hydrocarbon gas chromatographic(GC),saturated and aromatic hydrocarbon gas chromatographic-mass spectrometric(GC/MS) and stable carbon isotopic composition analyses,all the samples are interpreted to represent a single oil population with similar characteristics in a source bed or a source kitchen,organic facies and even in oil charge history.The co-existence of a full suite of n-alkanes and acyclic isoprenoids with UCM and 25-norhopanes in the crude oil samples indicates mixing of biodegraded oil with fresher non-biodegraded oil in the Ordovician reservoir.Moreover,according to the conversion diagram of double filling ratios for subsurface mixed crude oils,biodegraded/non-biodegraded oil ratios were determined as in the range from 58/42 to 4/96.Based on oil density and oil mix ratio,the oils can be divided into two groups:Group 1,with specific density>0.88(g/cm3) and oil mix ratio>1,occurring in the north of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines,and Group 2,with specific density<0.88(g/cm3) and oil mix ratio<1,occurring in the south of the pinchout lines.Obviously,Group 2 oils with low densities and being dominated by non-biodegraded oils are better than Group 1 oils with respect to quality.It is suggested that more attention should be paid to the area in the south of the Upper Ordovician Lianglitage and Sangtamu Formation pinchout lines for further exploration.     

生物降解原油中吡咯氮化合物组成的变化

渤海海域地区近50个原油样品中性氮组分的GC／MS定量分析资料表明,油藏中的生物降解作用对原油的吡咯氮化合物含量和分布有明显影响。经与同源未降解原油比较,各种烷基咔唑和苯并咔唑在3。4级中轻度降解油中就出现明显降解迹象,随生物降解程度增高其含量逐渐减少,在6—8级严重降解油中它们的总含量降低到原有的五分之一左右。在3—4级中轻度降解油中,裸露型甲基咔唑异构体更容易被微生物侵袭而代谢,抗生物降解能力按1-甲基咔唑〉4-甲基咔唑〉2-、3-甲基咔唑顺序递减;当降解程度更高时,这些化合物降解速率相当,1-／4-MCA等比值相对稳定。低-中等降解阶段,不同类型二甲基咔唑异构体的抗生物降解能力也存在明显差异性,呈屏蔽型〉半屏蔽型〉裸露型降低;在生物降解水平进一步增高时,这些异构体之间的相对含量变化不大。生物降解作用对苯并咔唑系列化合物分布的影响具有不确定性,且随降解程度的增加变得更为显著,降解油中【a】／[c】苯并咔唑比值或增高或降低。生物降解原油中吡咯氮化合物的组成变化,使降解油的二次运移示踪面临新的问题。     

Petroleum geochemistry of the Potwar Basin, Pakistan: 1. Oil-oil correlation using biomarkers, δC and δD

Geochemical characterisation of 18 crude oils from the Potwar Basin (Upper Indus), Pakistan is carried out in this study. Their relative thermal maturities, environment of deposition, source of organic matter (OM) and the extent of biodegradation based on the hydrocarbon (HC) distributions are investigated. A detailed oil-oil correlation of the area is established. Gas chromatography-mass spectrometry (GC-MS) analyses and bulk stable carbon and hydrogen isotopic compositions of saturated and aromatic HC fractions reveals three compositional groups of oils. Most of the oils from the basin are typically generated from shallow marine source rocks. However, group A contains terrigenous OM deposited under highly oxic/fluvio-deltaic conditions reflected by high pristane/phytane (Pr/Ph), C₃₀ diahopane/C₂₉Ts, diahopane/hopane and diasterane/sterane ratios and low dibenzothiophene (DBT)/phenanthrene (P) ratios. The abundance of C₁₉-tricyclic and C₂₄-tetracyclic terpanes are consistent with a predominant terrigenous OM source for group A. Saturated HC biomarker parameters from the rest of the oils show a predominant marine origin, however groups B and C are clearly separated by bulk δ¹³C and δD and the distributions of the saturated HC fractions supporting variations in source and environment of deposition of their respective source rocks. Moreover, various saturated HC biomarker ratios such as steranes/hopanes, diasteranes/steranes, C₂₃-tricyclic/C₃₀ hopane, C₂₈-tricyclic/C₃₀ hopane, total tricyclic terpanes/hopanes and C₃₁(R + S)/C₃₀ hopane show that two different groups are present. These biomarker ratios show that group B oils are generated from clastic-rich source rocks deposited under more suboxic depositional environments compared to group C oils. Group C oils show a relatively higher input of algal mixed with terrigenous OM, supported by the abundance of extended tricyclic terpanes (up to C₄₁₊) and steranes.Biomarker thermal maturity parameters mostly reached to their equilibrium values indicating that the source rocks for Potwar Basin oils must have reached the early to peak oil generation window, while aromatic HC parameters suggest up to late oil window thermal maturity. The extent of biodegradation of the Potwar Basin oils is determined using various saturated HC parameters and variations in bulk properties such as API gravity. Groups A and C oils are not biodegraded and show mature HC profiles, while some of the oils from group B show minor levels of biodegradation consistent with high Pr/n-C₁₇, Ph/n-C₁₈ and low API gravities.     

The occurrence of ultra-deep heavy oils in the Tabei Uplift of the Tarim Basin,NW China

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

川北中、下侏罗统烃源岩重排藿烷组成变化与油源对比

利用色谱—质谱定量分析技术,对40余个取自四川盆地北部石龙场和元坝地区不同成熟度的中、下侏罗统湖相烃源岩和原油样品进行分析,以此揭示其重排藿烷组成和分布的变化特征,并进行精细的油源对比。研究结果表明,不同成熟度的烃源岩中重排藿烷的丰度相差悬殊。石龙场地区R_o值为0.8%~1.0%的中侏罗统千佛崖组和下侏罗统自流井组大安寨段泥岩中,各类重排藿烷异常丰富,检出了17α（H）-重排藿烷、18α（H）-新藿烷和早洗脱重排藿烷3个完整碳数系列。它们的相对含量大都数倍于规则藿烷,是该层系烃源识别的分子标志。而在相邻的元坝地区相带、岩性相近的这两层段高成熟（R_o值主要在1.4%~1.9%）烃源岩中,藿烷类化合物出现异常变化,重排藿烷的相对含量很低,可能与干酪根在高热演化阶段生成的规则藿烷比例较高有关。烃源岩中高丰度重排藿烷并不取决于其绝对含量的高低,而在于它们相对于规则藿烷的富集。弱氧化的沉积环境是导致重排藿烷相对富集的一个重要原因,而有机质生源中的细菌组成可能更是一个关键因素。当烃源岩达到高成熟阶段时,藿烷类化合物的组成和分布不再受控于热化学动力学机制,各类重排藿烷均按一定比例分布,基本失去了其地球化学属性。区内中、下侏罗统原油中重排藿烷的组成和分布也随热演化程度而变化。它们在成熟原油中极丰富,而在高成熟原油中则很少,与烃源岩存在对应关系。经油—岩对比,认为这些原油来源于所在层位的烃源岩。     

Identification and occurrence of 25-norbenzohopanes in biodegraded bitumen from Palaeozoic carbonates in northern Alberta

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.     

塔河油田混源油地球化学及多元数理统计学对比研究

       混源油的定量判识是当前石油地质地球化学研究的热点与难点。以塔里木盆地塔河油田奥陶系中聚集的混源油为典型研究实例,通过地质地球化学与数理统计学相结合的方法,探索了定量研究混源油的方法,取得良好效果。原油地球化学研究结果表明,塔河油田原油普遍混源,并表现出多期充注特征,早期充注原油遭受了生物降解,因此目前原油中的轻烃、链状烃、规则甾烷等生物标志物主要反映的是后期充注原油的特征,不能很好地指示早期充注原油。据此,选择受生物降解影响相对较小的三环萜烷和藿烷定量数据,采用多元数理统计学交替最小二乘算法进行了原油成因研究,综合分析后认为现今混源油中可划分出4个端元,其中端元1和2可能主要代表了中上奥陶统烃源岩的贡献,而端元3和4则可能主要代表了寒武系烃源岩的贡献。塔河主体区以寒武系原油聚集为主,而外围地区则以中上奥陶统原油聚集为主,并且在整个塔河油田,总体上以寒武系原油的贡献比例相对最高。这一综合对比研究表明,多元数理统计学方法在混源油的比例计算、端元分析等方面具有重要作用,是对传统地球地球化方法研究的有效补充,值得推广应用,此外,研究认识还为区域油气勘探提供了新的参考信息。     

A geochemical investigation of crude oils and source rocks from Biyang Basin, China

China has a number of petroliferous lacustrine sedimentary basins of varying salinity and age (mainly Eocene). A geochemical investigation has been undertaken on several oils and source rocks from the Eocene lacustrine Biyang Basin. The distributions of n-alkanes, isoprenoids, steranes, and terpanes have been studied and used to characterize the sedimentary environment of deposition, maturity, biodegradation and undertake possible correlations. The ratios of C₃₀-hopane/gammacerane, 4-methyl-steranes/regular steranes, steranes/hopanes, C₂₁ tricyclic/C₃₀ hopane are proposed to be indicative of the depositional environment whereas ß-carotane appears to be a source related indicator. The geochemical data obtained in this study suggest that the major source rocks in the Biyang Basin were deposited in a saline/hypersaline depositional environment.     

Study on diagenesis of organic matter in immature rocks

Since many immature oils have been found in a number of Tertiary basins of China, a series of cores (Oligocene) and several immature rocks after thermal simulation have been investigated for their biomarker distributions by GC and GC-MS. The presence of biomarkers in the cores seem to follow a rule of less to greater stability of hopenes, ββ-hopanes, diasterenes with increasing the depth of cores, and subsequently the 22R, 22S configuration of hopanes reaches equilibrium. The thermal simulation experiments with immature rocks demonstrated that it is possible to generate some immature oils from immature rocks during the diagenesis stage. The tricyclic terpanes generated from source rocks during diagenesis stage tended to be enriched in the oils compared to their source rocks and the relative abundance of lower molecular weight tricyclic terpanes to their higher molecular weight homologues may be useful for the subdivision of diagenesis.     

极端生物降解沥青中8,14- 开环藿烷系列及其地球化学意义

借助色谱- 质谱（GC- MS）和色谱- 质谱- 质谱（GC- MS- MS）分析,对黔南坳陷凯里残余油藏凯棠和洛棉剖面上储层沥青中的烃类组成进行了系统分析,以判断其所遭受生物降解作用的程度,探寻在极端降解原油中是否还存在原生生物标志物,为这类原油的油源研究开拓新的途径和方法。结果表明：凯棠剖面上的储层沥青中尽管仍可检测到较为完整的C19- 30三环萜烷和C27- 35藿烷系列,但C19- 29脱甲基三环萜烷系列和C26- 34 25- 降藿烷系列丰富而完整,甾烷系列中C21- 22低分子量甾烷和重排甾烷优势明显,这一系列特征表明这些沥青遭受了剧烈生物降解作用的改造。但三芳甾类仍保存完好,依据原油生物降解程度的评判标准,判断其生物降解级别介于8~9级之间。洛棉剖面上的储层沥青中藿烷系列基本消失殆尽,三环萜烷系列及其脱甲基产物和25- 降藿烷系列的分布因极端生物降解作用而发生显著变异,某些化合物如C23T、C24T、C23NTE和C28- 29NH成为优势成员;甾烷系列中C21- 22低分子量甾烷占绝对优势,三芳甾类完全消失,据此判断该剖面上沥青的生物降解级别已达到10级或更严重。由于这两个剖面上的沥青遭受了极端生物降解作用的改造,常用的甾、萜类生物标志物完全失去了实用价值。但是,在所分析的这些沥青中都检测到三个系列的C27- 35 8,14- 开环藿烷系列,它们与塔里木盆地塔中地区海相端元油中存在的同类标志物的分布特征相似。正常海相端元油和极端生物降解沥青中同时检测到这三个系列的8,14- 开环藿烷,这一事实表明这类生物标志物在成因上具有原生性,而与生物降解作用无关。此外,在极端生物降解作用沥青中的完好保存,表明它们具有极强的抗生物降解能力,因而它们在此类原油的油源研究中可能具有潜在的实用价值。     

Geochemical Characteristics of Different Kinds of Crude Oils in the Tarim Basin,Northwest China

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.     

Delineating compositional variabilities among crude oils from Central Montana, USA, using light hydrocarbon and biomarker characteristics

Three compositionally distinctive groups of oils identified in central Montana by biomarker analyses are also recognized by the unique compositions of their light hydrocarbon (gasoline range) fraction. The majority of oils produced from Paleozoic pools (Pennsylvanian Tyler–Amsden interval) group into one broad category based on the distribution of C₂₀–C₄₀ biomarkers. These oils not only have the lowest Paraffin Indices and relative concentrations of normal heptane, but are readily distinguishable from the other compositional groups by using selected “Mango” parameters. However, the biomarker-based subdivision of this group into at least two sub-families is not reflected in the gasoline range fraction, suggesting little effect of source rock host lithology on the distribution of C₅–C₈ hydrocarbons. Oils occurring predominantly in Jurassic–Cretaceous reservoirs display different biomarker and gasoline range characteristics, including Paraffin Indices, K1 parameter and relative concentrations of C₇ compounds, and are classified in two separate compositional categories. In contrast to oils from the Tyler–Amsden interval, the oils produced from the Mesozoic strata are amongst the most mature oils in the study area. The unique biomarker/light hydrocarbon signatures are likely due to different source organic matter. Secondary alteration of oil due to biodegradation and migration, although recognized, appears less significant. The results indicate the overall usefulness of gasoline range compositions in delineating compositional affinities of crude oils in central Montana, clearly suggesting that the oils found in Paleozoic and Mesozoic reservoirs belong to different petroleum systems.     

庙西凹陷严重生物降解原油序列中三环萜烷的异常分布成因初探

对渤海湾盆地一系列生物降解原油的色谱-质谱分析结果表明,庙西凹陷PL15-8D与PL9-4井四个严重生物降解原油三环萜烷系列分布较为异常,主要表现为以C₂₃为主峰的后峰型、C₂₀与C₂₃为主峰的微弱双峰型以及以C₂₀与C₂₄为主峰的双峰型分布模式。强烈的生物降解作用导致C₁₉~C₂₃三环萜烷优先于C₂₄₊三环萜烷被不同程度地侵蚀,是形成这一异常分布的根本原因。三环萜烷系列相对丰度与绝对浓度的变化规律表明,不同碳数三环萜烷的生物降解作用同时发生,但其降解速率有明显差别,即抗生物降解能力不同。三环萜烷系列化合物（除C₂₀三环萜烷以外）的抗生物降解能力具有随碳数增加而增强的趋势,而C₂₀三环萜烷抗降解能力似乎强于C₂₁~C₂₃三环萜烷。原油中未检测到脱甲基三环萜烷,表明三环萜烷的降解并非通过微生物的脱甲基化作用,推测其降解途径是微生物氧化三环萜烷C环支链末端的甲基,形成对应的羧酸化合物。四个原油样品甾烷、藿烷与三环萜烷被微生物严重侵蚀,不能用于油源对比研究,而三芳甾烷未受生物降解影响,可作为研究区严重生物降解原油油源对比的有效指标。     

Geochemical characteristics of saturate hydrocarbons in crude oils and source rocks of the Qaidam, Tarim, Tupran basins, NW China

Based on the systematic analyses of fifteen typical crude oils and ten typical potential source rocks col-lected from the Qaidam,Tarim and Turpan basins,Northwest China,the geochemical characteristics of the oils and source rocks were investigated and oil-source rock correlations undertaken.The oils and source rocks deposited in saline lacustrine environment from the western Qaidam Basin were characterized by n-alkanes with even car-bon-number preference in the C20-C28 range,low pristane/phytane(Pr/Ph) ratios(less than 0.5),and high abundances of C27 steranes,gammacerane and C35 hopanes.The oils and source rocks deposited in marine environment from the Tarim Basin were characterized by n-alkanes with even carbon-number preference in the C14-C18 range,relatively low Pr/Ph ratios(near to 1),high abundance of C28 steranes,and relatively high gammacerane.In contrast,the oils and source rocks deposited in terrigenous bog environment from the Turpan Basin were characterized by relatively high Pr/Ph ratios(oil samples greater than 6) high abundance of C29 steranes,and relatively low gammacerane and C31-35 hopanes.The higher amounts of C37 and C38 n-alkanes of source rocks from the western Qaidam Basin and the Tarim Basin suggest an origin of these alkanes from functionalized C37 and C38 n-alkadienes and alkenones in prymnesiophytes living in lacustrine and marine environments.Oil-source rock correlations suggest oils in the west-ern Qaidam Basin were derived from the Oligocene Lower Ganchaigou Formation(E3),oils in the Tabei and Tazhong uplifts from the Tarim Basin have a genetic relationship with the Middle-Upper Ordovician source beds.Oils in the Turpan Basin generally fall into two genetic types.Most oils in the Taibei depression from the Turpan Basin were derived from the Lower-Middle Jurassic coal measures,but the fewer oils in this region are a mixed source derived from the Lower-Middle Jurassic coal measure and the Upper Permian source rocks.     

25-降藿烷系列化合物及其在油藏地球化学研究中的应用

通过阐述烃源岩及原油中25-降藿烷系列化合物的结构、鉴定方法、成因（生物降解作用、原生作用）、降解机理,系统介绍了目前国内外25-降藿烷系列化合物的研究现状。认为25-降藿烷系列化合物在油源对比和油气成藏方面的应用和研究,有助于推动油藏地球化学的发展,同时也存在一些局限性。