不同环境烃源岩和原油中C_(19)～C_(23)三环萜烷的分布特征及地球化学意义

三环萜烷系列(tricyclicterpanes)普遍存在于不同沉积环境烃源岩抽提物及其原油中。基于不同沉积环境的104件烃源岩和113件原油样品中C_(19)～C_(23)三环萜烷系列化合物分布特征的研究,初步建立了一个关于C_(19)～C_(23)三环萜烷相对百分含量(C_(19+20)TT、C_(21)TT、C_(23)TT)的沉积环境判识图版,利用该图版可以很好地区分海相/咸水湖相、淡水湖相、河流/三角洲以及沼泽相等四种不同沉积环境的烃源岩和原油。进一步研究表明,C_(19)～C_(23)三环萜烷相对含量分布受热成熟度和生物降解作用的影响较小,可作为有效的油-油对比、油-源对比及有机质沉积环境的判识指标。     

准噶尔盆地玛湖凹陷西斜坡百口泉组含油储集岩分子与碳同位素地球化学特征及其意义

在准噶尔盆地玛湖凹陷斜坡区下三叠统百口泉组地层中发现大面积连片岩性油气藏。对西斜坡百口泉组一段和二段8个含油储集岩样分别提取自由态组分、束缚态组分和油气包裹体组分,并进行GC、GC-MS和GC-IRMS分析。结果表明,3类油气组分三环萜烷主峰含量远高于C_(30)藿烷,反映充注到百口泉组(T1b)储层的原油均具有很高的成熟度,相当于Ro1.20%。生物标志物指标β-胡萝卜烷/(β-胡萝卜烷+nC20)、伽马蜡烷/(C_(30)藿烷+伽马蜡烷)、三环萜烷/(三环萜烷+藿烷)、Ts/(Tm+Ts)、C_(29)Ts/(C_(29)藿烷+C_(29)Ts)、C_(30)重排藿烷/(C_(30)重排藿烷+C_(30)藿烷)和甾烷C_(21)/(C_(21)+∑C_(29))比值均显示从自由态组分、束缚态组分至油气包裹体组分成熟度依次降低的特征,表明油气组分在油藏充注过程中不断升高。与此相反,Pr/nC_(17)和Ph/nC_(18)比值以及正构烷烃单体碳同位素组成显示从自由态组分、束缚态组分至油气包裹体组分成熟度依次升高的特征,表明油气组分从成藏早期至晚期成熟度降低。由于充注的原油为不同成熟度油气组分混合产物,生物标志物在原油中属于微量及痕量组分,而Pr、Ph和正构烷烃属于常量组分,Pr/nC_(17)和Ph/nC_(18)比值以及正构烷烃单体δ~(13)C值更能代表主体原油的成熟度。因此,对于玛湖凹陷西斜坡油气藏,主体原油成熟度从成藏早期至晚期是降低的。根据8个油砂样3类油气组分的成熟度可以推断油气充注期介于晚侏罗世-白垩纪之间。盖层密封性是控制这类油气藏成藏的重要因素。在晚侏罗世之前,下伏二叠系盖层一直阻止了油气沿断层运移。至晚侏罗世-白垩纪,下伏盖层的密封性遭受破坏,油气才从下伏二叠系沿断层运聚至百口泉组一段和二段储层(T_1b~(1–2))。由于上覆百三段(T_1b~3)和克拉玛依组(T_2k)盖层自始至终保持密封状态,百口泉组一段和二段储层(T_1b~(1–2))成为玛湖凹陷油气的最终归宿。     

松辽盆地梨树断陷原油高碳数三环萜烷分布特征及主控因素

对梨树断陷71个湖相原油样品的饱和烃和芳烃进行了气相色谱和色谱—质谱(GC-MS)分析,根据其生物标志化合物组合特征的差异将梨树断陷的原油划分为三种类型,不同成因类型的原油样品中三环萜烷的分布特征差异明显,其中Ⅰ类原油三环萜烷的浓度明显高于Ⅱ类原油,而Ⅱ类原油Σ三环萜烷/17α(H)-藿烷比值明显高于Ⅰ类原油,但不同类型原油中均存在较高含量的高碳数三环萜烷(C₂₈TT-C₂₉TT),其相对百分含量均高于低碳数三环萜烷(C₁₉TT-C₂₀TT、C₂₁TT、C₂₃TT)。从沉积环境、母质来源和成熟度三方面对高碳数三环萜烷的主控因素进行了探讨,结果表明,梨树断陷不同类型原油的母质来源都是以细菌及低等水生生物为主,(C₂₈TT-C₂₉TT)/17α(H)-藿烷与伽马蜡烷指数有一定的正相关性,说明偏还原性的咸水环境是高碳数三环萜烷富集的次要影响因素。就该研究区而言,Σ三环萜烷/17α(H)-藿烷比值与成熟度参数Ts/(Ts...     

准噶尔盆地车排子地区原油油源分析

车排子地区紧邻沙湾凹陷和四棵树凹陷,油源条件优越,具有多层系含油特点,前人对该区原油油源认识存在较大分歧.本次研究在研究区原油生物标志物分析基础上,结合金刚烷异构化指标和正构烷烃单体碳同位素分布,对车排子地区原油油源进行了分析.结果表明,车排子地区存在3类原油:(1)Ⅰ类原油为新近系沙湾组轻质油,表现为侏罗系来源特征,Pr/Ph比值大于2.0,原油碳同位素值偏重,δ13C值大于?28.0‰,姥鲛烷(Pr)、植烷(Ph)和三环萜烷含量较低,Pr/nC17和Ph/nC18比值小于0.3,不含β-胡萝卜烷,三环萜烷/藿烷比值分布在0.08～0.18之间,C24四环萜烷/C26三环萜烷比值分布在1.61～3.81之间,ααα20R规则甾烷具有C27和C29规则甾烷优势,呈C27>C28C28相似文献   

羌塘盆地二叠系展金组烃源岩生物标记化合物特征及意义

本文对羌塘盆地二叠系展金组烃源岩的岩心样品进行了烃源岩基本特征及生物标志化合物特征研究,重点探讨了烃源岩生烃母质来源、沉积环境特征及成熟度特征。研究显示,展金组烃源岩富含正构烷烃、类异戊二烯烷烃、萜类化合物和甾类化合物。其中,正构烷烃多为前高后低的单峰型,具有低碳数优势,n C_(15)、n C_(16)或n C_(17)为主峰碳;规则甾烷中C_(27)-C_(28)-C_(29)呈"V"字型分布,C_(27)占优势;萜烷具有三环萜烷五环三萜烷四环萜烷的相对丰度,三环萜烷/五环三萜烷值在1.1~1.96之间;生烃母质主要来源于海相低等水生生物。Pr/Ph分布范围在0.53~1.16,平均值为0.85,植烷优势较明显;五环三萜烷的Ts/(Tm+Ts)值表明烃源岩形成于还原环境。而伽马蜡烷的含量以及伽马蜡烷/αβ-C30藿烷比值,证明烃源岩沉积时水体盐度为正常盐度。甾烷的C29ααα20S/(20S+20R)和C29αββ/(αββ+ααα)、萜烷的C31αβ~(22)S/(22S+22R)、Ts/(Tm+Ts)值等反映烃源岩成熟度的参数显示,烃源岩处于成熟阶段。     

原油裂解过程中正构烷烃的组成变化特征及其地球化学意义

采用高温模拟技术,对原油样品进行模拟实验,并对其裂解过程中正构烷烃的组成和变化特征进行了地球化学研究。结果表明,原油大量裂解生成气态烃之前,原油中的高分子量正构烷烃已经开始裂解,以C1+5烃类裂解成C6～C14化合物为主。随成熟度的增加,C6～C14化合物进一步转化为C1～C5化合物,并伴随苯系物的产出,最终形成甲烷和裂解沥青。在正构烷烃的裂解过程中,苯及其同系物的丰度呈明显增加趋势,可以作为原油裂解程度的潜在判识标志。此外,利用同样的实验条件对正十六烷进行了对比模拟实验,其产物的组成和变化特征与原油中正构烷烃的基本相同。     

准噶尔盆地玛湖凹陷烃源岩和原油生物标志物与碳同位素组成及其意义

对准噶尔盆地玛湖凹陷6个下二叠统风城组(P_1f)烃源岩抽提物和10个原油样品进行了色谱、色谱-质谱和色谱-同位素比值质谱分析。依据分子和碳同位素地球化学特征,将10个原油样品分为三组。第一组具有如下特征:(1)Pr/nC_(17)和Ph/nC_(18)值较高;(2)几乎不含Ts、C_(29)Ts和C_(30)重排藿烷;(3)伽马蜡烷和胡萝卜烷相对含量高;(4)三环萜烷分布模式为C_(20)C_(21)C_(23)或C_(20)C_(21)C_(23);(5)正构烷烃单体δ~(13)C值随碳数增加而变重。该组油样是Ⅰ类原油的端元代表,来源于"典型"下二叠统风城组(P_1f)烃源岩。第二组油样的特征与第一组油样明显不同:(1)较低的Pr/nC_(17)和Ph/nC_(18)值;(2)相对较低的伽马蜡烷和胡萝卜烷含量;(3)三环萜烷分布模式为C_(20)C_(21)C_(23);(4)正构烷烃单体δ~(13)C值则随碳数增加而变轻。该组油样是Ⅱ类原油的端元代表。第三组油样为介于Ⅰ类和Ⅱ类原油之间的过渡类型。6个烃源岩样品抽提物分子指标与该凹陷"典型"下二叠统风城组(P_1f)烃源岩及Ⅰ类原油具有一定的差异,而与之前认为的中二叠统下乌尔禾组(P_2w)烃源岩及Ⅱ类原油类似。烃源岩抽提物之间正构烷烃单体δ~(13)C值具有较大的差异,并且比10个原油样相对偏轻。在玛湖凹陷至今尚未钻遇有较大生油潜力的下乌尔禾组烃源岩样品,风城组烃源岩既是Ⅰ类原油的油源,也可能是Ⅱ类原油的主要油源。由于有机岩相及成熟度的差异,导致来源于同一套烃源岩的原油具有不同的生物标志物和碳同位素组成。     

新疆三塘湖盆地马朗凹陷石炭系原油形成过程及勘探意义

新疆三塘湖盆地马朗凹陷牛圈湖构造带马25井石炭系火山岩储层中发现了原油和固体沥青共存,通过储层多级分离抽提和原油地球化学分析,并对这些原油(或固体沥青)的形成过程与其勘探意义进行了研究,结果表明这些原油和沥青可能主要来自下石炭统烃源岩,具有较低姥植比和伽玛蜡烷含量、高丰度三环萜、高Ts/Tm比值和重族组分碳同位素(δ13C-28.0‰)特征;现今油藏原油富含生物降解标志物(25-降藿烷),同时却无明显其它生物降解特征(正构烷烃、甾藿烷等保存完整),反映多期充注特征,即二叠纪中晚期成藏后遭受生物降解,白垩纪后期又再次充注形成;下石炭系有效烃源岩的存在预示着本区具有深部下石炭统源岩贡献的油气藏勘探前景。     

塔中4油田石炭系储层不同赋存态烃类分子和碳同位素对比研究

塔中421井和塔中402井石炭系油层2个原油样和8个油砂样连续抽提组分甾烷、萜烷分布特征和正构烷烃单体碳同位素组成具有明显的差异,具有不同的来源。塔中421井上石炭统3个油砂样自由态组分、束缚态组分和油气包裹体具有伽马蜡烷和C28甾烷相对含量高、正构烷烃单体碳同位素组成重的特征,划分为Ⅰ类原油,对比认为主要来源于寒武系-下奥陶统烃源岩。塔中421井和塔中402井上石炭统的2个油样具有伽马蜡烷和C28甾烷相对含量低、并且正构烷烃单体碳同位素组成轻的特征,划分为Ⅱ类原油,其来源尚不明确。塔中402井石炭系上、中和下统的5个油砂样各类组分具有介于Ⅰ、Ⅱ类原油之间的特征,为Ⅰ和Ⅱ类原油的混合物。5个油砂样从自由态组分、束缚态组分至油气包裹体Ⅰ类原油含量依次增高,Ⅱ类原油含量依次降低。2口井8个油砂样从自由态组分、束缚态组分至油气包裹体C23三环萜烷／（C23三环萜烷＋C30藿烷）和C21／（C21＋∑C29）甾烷比值都依次降低,反映了油气充注过程中,原油成熟度不断升高。塔中4井区储层油砂不同吸附态烃类分子与碳同位素的研究结果反映塔中4油田具有多种油气来源,经历长期油气充注过程,寒武系-下奥陶统烃源岩在地史上对该区具有成烃贡献。     

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。     

Selective enrichment of steroid and triterpenoid hydrocarbons from crude oil using gel permeation chromatography for stable carbon isotope analysis

Gel permeation chromatography (GPC) using a high performance liquid chromatography (HPLC) system was studied for the separation and enrichment of steroid and hopanoid hydrocarbons from crude oil for stable carbon isotope analysis. A crude oil sample was pretreated using silica gel chromatography and 5A molecular sieve to remove polycyclic aromatic hydrocarbons and n-alkanes. The GPC behavior of both the pretreated saturated hydrocarbon fraction of the oil and standard steroid [5α(H), 14α(H), 17α(H) C₂₇–C₂₉ steranes], hopanoid [17α(H) C₂₇ trisnorhopane, 17α(H), 21β(H) C₂₉–C₃₂ hopanes] and triterpenoid [18α(H)-oleanane, gammacerane] mixtures were examined. The results indicate that 17α(H), 21β(H) hopanes as well as steranes could be enriched efficiently using GPC and that they could be obtained without removing n-alkanes from the oil saturated hydrocarbon fraction. The GPC behavior of steroid and triterpenoid hydrocarbons was controlled by molecular size and shape.     

济阳拗陷特殊生物降解油的初步研究

济阳拗陷草古100平19井奥陶系储层原油,饱和烃馏分均已损失殆尽,仅残余藿烷系列化合物;同时,芳烃馏分化合物也已损失殆尽,仅残余三芳甾烷系列化合物。上述芳烃化合物俱已损失殆尽的蚀变特征,在相关文献中的报道较为少见。原油中不受水洗作用影响的正构烷烃和不受生物降解作用影响的有机硫化合物(苯并噻吩和二苯并噻吩)均已损失殆尽,显然暗示了原油蚀变是遭受水洗作用和生物降解作用共同作用的结果。原油芳烃馏分的“UCM”鼓包幅度明显大于饱和烃馏分的“UCM”鼓包,暗示了芳烃化合物的降解速率平行甚至超过饱和烃的降解速率。原油中低分子量芳烃化合物的快速降解,暗示了油藏可能是在氧化条件下而非在还原条件下发生蚀变的。     

Geochemical characteristics of Ordovician crude oils in the northwest of the Tahe oil field,Tarim basin

Forty-six crude oil samples were selected from the Ordovician in the northwestern part of the Tahe oilfield for detailed molecular geochemical and isotopic analysis, including group compositions, carbonhydrogen isotopes and gas chroma-tograms of saturated hydrocarbons, as well as the characteristics of terpane, sterane and other biomarkers, indicating that crude oils are of the same origin from different districts in the Tahe oilfield and were derived from the same source kitchen (or oil source formation), i.e., mainly stemming from marine hydrocarbons. Detailed studies of oil physical properties of 25-honpane revealed that such oils have heavy or thick oil qualities due to biodegradation. Comprehensive assessment in terms of five maturity parameters shows that the oils from the Ordovician with Ro values varying from 0.80% to 1.59% are widely distributed in the northwest of the Tahe oilfield.     

Chemical characteristics and pollution sources of petroleum hydrocarbons and PAHs in sediments from the Beiluohe River,Northern China

To determine the degree of hydrocarbon contamination and the contribution of local petroleum industries to contaminant loadings in sediments from the Beiluohe River, China, 12 surface sediment samples were collected for geochemical analysis in 2005. Sediment samples were extracted by organic solvents, separated by silica gel column chromatography and the profiles of n-alkanes, biomarkers and polycyclic aromatic hydrocarbons (PAHs) in sediments were analyzed by gas chromatography with flame ionization detector and gas chromatography/mass spectroscopy. Concentrations of total hydrocarbons in the sediments varied from 12.1 to 3,761.5 μg g⁻¹ dry wt, indicating that most sediments in Beiluohe River was only slightly to moderately contaminated by hydrocarbons. Concentrations of PAHs for six samples (sum of 16 isomers) varied from 17.7 to 407.7 ng g⁻¹ dry wt and at present low levels of PAHs did not cause adverse biological effects in Beiluohe River sedimentary environment. PAH compositions, n-alkanes and biomarker profiles all suggested that there were different sources of contaminations in studied areas. n-Alkanes reflect two distinct sources: a fossil n-alkane series from crude oil at sites S40, S43, S87 and plantwax n-alkanes at sites S39 and S45. Judged by their PAH ratios, the sediments at site S15 were pyrolytic, sediments at S17 and S43 were petrogenic, and sediments at S39, S40 and S64 had a mixture source of pyrolytic and petrogenic.     

Assessment of petroleum biodegradation using stable hydrogen isotopes of individual saturated hydrocarbon and polycyclic aromatic hydrocarbon distributions in oils from the Upper Indus Basin,Pakistan

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.     

Characterization of high molecular weight biomarkers in crude oils

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.     

Geochemical Evidence for Coal and Carbonaceous Mudstone as the Possible Major Oil Source Rock in the Jurassic Turpan Basin,Northwest China

Petroleum geologists have debated whether the hydrocarbons from Jurassic coal measures are derived from the coals, carbonaceous mudstones or coal-measure mudstones in the Turpan Basin. Based on the geochemistry analysis of the 20 crude oils and 40 source rocks from the Turpan Basin, some data have been obtained as follows: carbon preference index and methylphenanthrene index of the Jurassic oils are 1.16–1.45 and 0.28–0.80, and the ααα C29 sterane 20S/(20S+20R) and C29 sterane ββ/(ββ+αα) are 0.44–0.51 and 0.4–0.54 respectively, which show the normal maturity of oils; the vitrinite reflectance of the source rocks from the Xishanyao to Badaowan Formations range from 0.47% to 0.97%, which indicate immature to mature thermal evolutionary stage and sufficient conditions for generating mass mature oil. The effect of hydrocarbon expulsion should be considered when studying the source of coal-derived oil by using Biomarkers. Biomarkers in the Jurassic oils from the basin are similar to those in the coals and carbonaceous mudstones, with a strong predominant content of pristane, relatively high ratio of C15/C16 sesquiterpenoids (>1), a relatively high content of low carbon number tricyclic terpanes and C24 tetracyclic terpane, little gammacerane and C29 Ts detected, an absolute predominant content of C29 sterane and a relatively high content of diasterane. However, the opposite characteristics are shown in mudstones, with an approximately equal content of pristane and phytane, relatively low ratio of C15/C16 sesquiterpenoids (<1), a relatively high content of high carbon number tricyclic terpanes and a low content of C24 tetracyclic terpane, peaks of gammacerane and C29 Ts detected obviously and an increasing C27 sterane content. All of these characteristics identify the coals and carbonaceous mudstones as the possible major oil source rocks in this area, and they were formed in the stronger oxidizing environment with shallower water than mudstones.     

Origin of organic sulphur compounds and sulphur-containing high molecular weight substances in sediments and immature crude oils

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.     

Carbon isotopic composition of individual n-alkanes in asphaltene pyrolysates of biodegraded crude oils from the Liaohe Basin, China

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.     

Biological marker compounds as indicators of the depositions! history of the Maoming oil shale

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.