The effect of biodegradation on steranes and terpanes in crude oils

Steranes and terpanes biodegrade at a slower rate than isoprenoids and survive moderate biodegradation. Heavy biodegradation results in destruction of regular steranes, survival of diasteranes (20R better than 20S) and tricyclic terpanes and transformation of hopanes to Ring A/B demethylated hopanes. These survivors can be used as source fingerprints in biodegraded crudes. The structure of predominant steranes in undegraded to moderately degraded fossil fuels was proven to be 14β, 17β(H) (20R + S) by molecular spectroscopy. These compounds plus the 20S epimers of regular 5α-steranes (20R) were identified as major constituents and their 5β-counterparts as minor components in a cholestane isomerizate (300°C, Pt on C), allowing assessment of relative thermodynamic stabilities. An observed increase of optical activity in heavily degraded crudes from three different basins is interpreted to be the result of bacterial transformation of terpanes and steranes to new optically active species plus enrichment of the latter by n + isoparaffin depletion rather than total bacterial synthesis. Diagnostic ion profiling by GC-MS-C is a convenient tool for surveying the relative abundance of individual diasteranes and regular steranes plus distinguishing epimeric and ring skeletal isomeric series in complex fossil fuel mixtures. A new practical method of determining the absolute quantities of individual steranes by spiking with 5β-cholane and integration of mass chromatograms is described.     

Molecular parameters of maturation in the Toarcian shales,Paris Basin,France—I. Changes in the configurations of acyclic isoprenoid alkanes,steranes and triterpanes

The changes in configuration at a number of chiral centres in certain acyclic isoprenoid alkanes, steranes, rearranged steranes and triterpanes of the hopane type in a suite of fourteen Toarcian shales (Paris Basin) have been determined by gas chromatography and combined gas chromatography-mass spectrometry. A sequence of changes in the ratios of various stereoisomers occurs both with increasing maximum depth of burial and in a North-South direction for shallow samples (maximum burial depth < 1000m). These changes cover the full range of maturity shown by the samples. The presence of 5α(H), 14β(H), 17β(H)-steranes in immature samples indicates the presence of a reworked component. The extent of epimerisation at a number of the chiral centres suggests that it relates to the extent of steric hindrance at the centres.     

Steranes and terpanes in kerogen pyrolysis for correlation of oils and source rocks

Comparison of biological marker alkanes in the kerogen pyrolyzate and bitumen from a sediment is a useful test for the indigenous nature of sediment extracts. For the pyrolysis conditions used, the bulk of the hydrocarbons is released from the kerogen matrix between 375° and 550°C; and its steriochemistry is almost the same as that observed in the extractable bitumen in a genuine source rock. Examples are given to demonstrate that, during pyrolysis, the sterane/terpane ratio decreases and secondary terpanes are generated at the expense of primary ones.The mechanism of artificial petroleum generation by pyrolysis differs from ‘natural’ diagenesis during geological time and is reflected in the composition of certain C₂₇-C₂₉ steranes, as demonstrated by simulation experiments and C₂₉-C₃₀ moretanes and hopanes. The $\text{5α}\text{5β}$-sterane ratios, jointly with $\text{17α(H)-hopane}\text{17β(H)-moretane}$ ratios, tricyclic terpane concentrations and $\text{17α(H)}\text{17β(H)}$-trisnorhopane ratios, allow the differentiation of kerogens from adjacent stratigraphies.     

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.     

Steranes and triterpanes generated from kerogen pyrolysis in the absence and presence of minerals

Steranes and triterpanes generated from pyrolysis of immature Monterey Formation kerogen in the presence and absence of calcite, illite and montmorillonite reveal results that are both consistent and divergent with published data that reflect the use of these biological markers as maturation indicators. The extent of isomerization of biomarkers generated from pyrolysis of kerogen at 300°C for 2 hours, at C-20 in 14α(H),17α(H)-steranes, at C-22 in 17α(H),21β(H)-hopanes and of 17β(H),21β(H)-hopanes correspond to early diagenetic stages in rock extracts from sedimentary basins. Isomerization increases with heating time and, after 1000 hours, attains values which correspond to the catagenetic stage in sedimentary basins, or equivalent to that of mature oil. Stepwise pyrolysis of the kerogen indicates faster isomerization rates for steranes and triterpanes in the bitumen than for those retained in the kerogen structure, confirming earlier studies.Presence of a mineral matrix can influence the isomerization of steranes and triterpanes considerably. Comparisons with results from kerogen heated alone, for a given maturation stage, show that calcite inhibits, illite catalyzes slightly and montmorillonite has a pronounced catalytic effect on these reactions. This effect results in early isomerization of steranes and hopanes corresponding to the catagenetic stage in the presence of montmorillonite, while kerogen or kerogen with calcite held at the same temperature (300°C) and time (10 hours) only yield isomerized products which correspond to a diagenetic stage. Further, illite and montmorillonite affect various isomerization reactions differently. The fastest reaction is the isomerization at C-20 in 14α(H),17α(H)-steranes followed by that at C-22 in 17α(H),21β(H)-hopanes and the slowest is the formation of 14β(H),17β(H) steranes.These results show that maturation measurements of rock or oil samples from sedimentary basins which use biological markers have to take into account the mineral matrix effects, which have been largely ignored until present.     

云南金顶铅锌矿床的生物标志物特征及意义

采用GC-MS分析方法对金顶铅锌矿床进行研究,结果表明:该矿床正构烷烃呈前高后低的单峰型分布,nC16或nC22为主峰碳,轻烃组分占有绝对优势,OEP值0.837～0.976,接近平衡值1.00,Pr/Ph为0.1～0.88,显示植烷优势;萜烷丰度顺序为五环三萜烷>三环萜烷>四环萜烷;规则甾烷呈不对称的“V”字形分布,表现为C29>C27>C28的分布特征。这些特征显示,金顶铅锌矿床有机质(生物)来源物质以菌藻类生物为主,可能有部分高等植物的贡献;矿床形成于缺氧的高盐度环境。结合金顶铅锌矿床下含矿层(原地沉积岩系)沉积于水深较浅的蒸发盐湖环境(即氧化环境),可以推断该矿床成矿流体中丰富的有机质形成的还原条件以及外来推覆岩系形成的封闭条件是金顶铅锌矿床形成的必要条件,矿床属于后期充填作用为主的矿床。     

The occurence of unusual C27 and C29 sterane predominances in two types of Oman crude oil

Two types of Oman crude oils reveal unusual sterane distributions. Type “A”, which is the more common (74 examples), is characterised by a predominance of C₂₉ iso- and normal-steranes and generally none or only very low relative concentrations of rearranged-steranes. The triterpanes are characterised by the predominance of the C₂₉ 17αH, 21βH norhopane over the C₃₀ 17αH, 21βH hopane and non-predominant C₂₀–C₃₀ tricyclic terpanes. The C₂₉ steranes of this type of crude were not derived from the C₂₉ sterols of land-plant origin (frequently proposed as the source of C₂₉ steranes) since there is good geological evidence that these crudes were generated from a pre-Cambrian source rock, a geological period when land-plants did not exist.The type “B” crude oil (11 known examples) is characterised by a strong predominance of C₂₇ iso-, normal- and rearranged-steranes, relatively lower concentrations of 17αH, 21βH hopanes and relatively high concentrations of C₂₀–C₃₀ tricyclic terpanes.The remarkably different biomarker characteristics of these crude oils imply that the organisms active in the depositional environment of the respective source rocks were significantly different.     

Structure proof and significance of stereoisomeric 28,30-bisnorhopanes in petroleum and petroleum source rocks

Two C₂₈H₄₈-pentacyclic triterpanes were isolated from Monterey shale. X-ray crystallography of a crystal containing both compounds proved their structures as 17β,18α,21α(H)-28,30-bisnorhopane and 17β,18α,21β(H)-28,30-bisnorhopane. Several differences are found between 28,30-bisnorhopanes and the regular hopanes. Unlike the regular hopane epimers, for practical purposes the three epimeric 28,30-bisnorhopanes [17α,21β(H)-, 17β,21α(H)-, and 17β,21β(H)-]cannot be distinguished by their mass spectra. Special conditions are needed to separate them by gas chromatography. The diagenetically first-formed epimer is thought to be 17α,21β(H)- because it predominates in immature shales. The order of thermodynamic stability is 17β,2lα(H) < > 17α,21β(H) > 17β,21β(H), and all three epimers are present in petroleum. 25,28,30-Trisnorhopanes can be analyzed in similar fashion and are found to have similar thermodynamic characteristics. The percent of the ring D/E cis epimer of 28,30-bisnorhopane and/or 25,28,30-trisnorhopane is a useful maturation parameter similar to the 20S/20R sterane ratio. Evidence indicates 25-demethylation of 28,30-bisnorhopane to 25,28,30-trisnorhopane during advanced stages of biodegradation. Hence, percent ring $\text{D}\text{E}$cis 25,28,30-trisnorhopane has an application to maturation assessment in heavily biodegraded oils.     

Biomarkers in crude oil revealed by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry: Depositional paleoenvironment proxies

Crude oil samples from two basins were analyzed using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC×GC–TOFMS) to better understand the compositional heterogeneity of branched-cyclic hydrocarbons. GC×GC–TOFMS and conventional GC–MS results were compared. GC×GC–TOFMS revealed a wide range of compounds, including tricyclic, tetracyclic and pentacyclic terpane series, rearranged hopanes, methyl hopanes, secohopanes, onoceranes and steranes. Assignment of methyl hopane and 8,14-secohopane series other than onocerane isomers was only possible due to the high peak capacity and sensitivity of GC×GC. The oils comprised a mixture of two end members: non-biodegraded oil with abundant tricyclic terpanes and hopanes, and severely biodegraded oil with abundant 8,14-secohopane and demethylated tricyclic terpanes. A predominance of two distinct series, 3β-methylhopane and onocerane, was detected only in the lacustrine samples (classification based on biomarker parameters). In contrast, the predominance of a 2α-methylhopane series and lack of onocerane were found only for the marine oil sample. The results suggest that the distribution of 3β- and 2α-methylhopane series and the presence or absence of onocerane isomers reflect genetic differences in the source organic matter and that these compounds are new classes of biomarkers that can used as depositional paleoenvironment proxies.     

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.     

Evolution characteristics of saturated hydrocarbons of enclosed organic matter in carbonate minerals in Tieling limestone under high temperature and high pressure

The enclosed organic matter chiefly releases lower carbon-number n-alkanes under high temperature and high pressure,while the kerogen mainly produces higher carbon-number n-alkanes.The rsidual hydrocarbons generated by both kerogen and enclosed organic matter in the Tieling limestone contain abundant tricyclic terpanes,pentacyclic triterpanes and steranes,but the contents of tetracyclic terpanes and 25-norhopane are lower.The residual enclosed orgainc matter shows the same distribution characteristics of n-alkanes,steranes and terpanes as that of the original bitumaen A,i.e.,the higher contents of triterpanes and tetracyclic terpanes,the higher ratios of 25-norhopanes over regular hopanes and markedly degraded steranes.By comparing the residual hydrocarbon.residual enclosed orgainc matter and original enclosed orgainc matter.it can be concluded that steranes and terpanes in the residual hydrocarbons are produced mainly by the kerogen and subordinately by the residual enclosed organic matter,the steranes and terpanes do not enter into the residual enclosed organic matter,and the thermal evolution of the residual enclosed organic matter maintains its unique character.Furthermore,pressure retards the pyrolysis of higher carbon-number alkanes and influences the isomerization ratios of C29-steranes,making 20S/(20S 20R) lower under the higher pressure than that under lower pressure,Higher pressure retards the thermal evolution of organic matter.     

Typification of oils in the Timan-Pechora province according to the composition of hydrocarbon biomarkers (steranes and terpanes)

Distinctive compositional features of cyclic saturated hydrocarbon biomarkers have been established in oils from the main petroliferous lithostratigraphic complexes of various structural zones in the Timan-Pechora petroliferous province (TPPP). Four geochemical families (types) of oils in TPPP are recognized based on the variations in the geochemical parameters of steranes and terpanes including sterane ratios C₂₇/C₂₉ and C₂₈/C₂₉, K¹ mat and K² mat, diasterane/regular sterane, pregnane (C_21–22)/sterane (C_21–22 + C_27–29), as well as terpane Ts/Tm parameters, adiantane C₂₉/hopane C₃₀, neoadiantane/adiantane, tryciclic terpane/pentacyclic terpane, hopane/sum of C₂₉ steranes, etc. The distribution of various types of oil in the sedimentary sequence of TPPP makes it possible to infer source rocks for each of the four selected types.     

The occurrence of bisnorhopane,trisnorhopane and 25-norhopanes as free hydrocarbons in some Australian shales

The triterpenoid hydrocarbons of some West Australian shales have been examined by GC-MS. In addition to the common 17α(H),21β(H)-hopanes, 17β(H),21β(H)-hopanes and 17β(H),21α(H)-moretanes, 28,30-bisnorhopane, 25,28,30-trisnorhopane and 25-norhopanes were identified in the organic extracts. In contrast, pyrolysates of the solvent-extracted sediments contained only the common hopane and moretane series, indicating that 28,30-bisnorhopane, 25,28,30-trisnorhopane and 25-norhopanes are not bonded to kerogen, but rather are present in the sediments as free hydrocarbons.     

Laboratory biomarker fractionations and implications for migration studies

This paper discusses results from laboratory experiments designed to study possible effects of migration on the distribution of biomarkers in oils and source rock extracts and on the various biomarker indicators which have been used to discuss the source, maturity, biodegradation and depositional environments. The results of the study showed that lower molecular weight n-alkanes eluted from the alumina column faster than higher molecular weight n-alkanes, tricyclic terpanes faster than pentacyclic terpanes and 14β,17β steranes faster than 5α20R steranes. Gammacerane eluted extremely slowly and could be obtained almost as a pure component. Many of these observations duplicate those previously observed in nature and ascribed to migration. The evidence suggests that the effects of migration must be carefully considered when discussing biomarker-based maturity parameters of oils and source rocks. It is proposed that the biomarkers remaining in the source rock after expulsion of oil show low maturity values which might not reflect the actual maturity of the rock.     

辽河盆地过渡带有机地球化学（Ⅱ）──甾萜烷及芳烃系列化合物成熟度参数在过渡带内的系统变化特征

所研究的辽河盆地东部凹陷沙一、沙二段烃源岩主要处于未成熟演化阶段,其可溶有机质中甾萜烷立体化学参数显示出“地质型”异构体的“异常”富集。各参数在普遍高于各自门限值的背景上,大致以２２００-２３００ｍ、２５００ｍ、２７００ｍ和２７７０ｍ为界线,呈现出系统的变化规律。同时发现萘、菲、氧芴和硫芴等系列化合物的成熟度参数与甾萜烷的成熟度参数的变化规律十分相似。本文对比进行了深入研究,结合原始可溶有机质红外特征吸收参数的变化特征,指出过渡带内甾萜烷及芳烃成熟度参数的变化规律受控于原始可溶有机质的演化特征,定性地提出了辽河盆地东部凹陷过渡带的地球化学判识标志,并认为本研究结果同样有助于不同地区过渡带气或未成熟油、低成熟油的判识。     

Charactristics of the Biomarkers from Nonmarine Crude Oils in China—A Review

The present paper deals with the biomarker characteristics of crude oils and source rocks from different environments(fresh,fresh-brackish and salt waters)of nonmarine depositional basins of different ages in China.Their characters are summarized as follows:1)Souce rocks and crude oils derived from fresh-water lacustrine facies have an odd/even predominance of n-alkanes and high pristine/phytane ratios.Oils from the fresh-water lacustrine facies differ from typical marine oils in the relative contents of total steranes and terpanes,the concentrations of hopanes and organic sul-phur compounds and the values of methylphenanthrene indices and C,H,S stable isotopes.2)The source rocks and crude oils derived from saline lacustrine facies possess an even/odd predominance of n-alkanes and high phytane/pristine ratios.There are also some differences between saline lacustrine oils and freshwater lacustrine oils in the concentrations of steranes,tricyclic terpanes and organic sulphur compounds,as well as in the values of methylphenanthrene indices and C,H,S stable isotopes.3)Oils derived from fresh-brackish water lake facies differ from oils from fresh-water lacustrine or samline lacustrine environments in respect of some biomarkers.According to the various distributions of these biomarkers,a number of geochemical parameters can be applied synthetically to differentiating and identifying the nature of original depositional environments of crude oils and source rocks and that of organisms-primary source materials present in those environments.     

近代盐湖沉积物中的生物标志化合物

    生物标志化合物是在沉积有机质地质演化过程中可保留其原碳骨架的生物分子，具记录沉积环境和沉积有机质热演化历史的功能。用有机地球化学方法（气相色谱、气相色谱/质谱、热解色谱和同位素质谱）对采自我国西部盐湖的近代沉积物进行了大量研究，而且对盐湖沉积环境地球化学特征、有机质组成与早期成岩作用的关系和各种生物标志化合物指标进行了更加深入细致的探索。研究结果表明干酪根碳同位素组成（δ¹³C PDB）和盐湖类型有关系，例如，硫酸盐型湖的干酪根δ¹³C 值较重，而碳酸盐型盐湖的δ¹³C值则较轻。姥鲛烷/植烷比（Pr/Ph）、总三环萜烷/总萜烷（%）、孕甾烷/甾烷比（%）、伽玛蜡烷/αβ—藿烷和三芴系列化合物组成三角图等生物标志化合物指标可以用来判识沉积环境的盐度。通常，在盐湖或咸水沉积环境中，除额吉卓尔盐湖样品因采自边缘，Pr/Ph比值较大而外，其余各盐湖样品的Pr/Ph比值一般≤1.0，而其余各比值均明显大于在淡水湖中的比值；三芴系列化合物三角图中，各盐湖样品均集中分布在硫芴含量高的区域。与我国各典型沉积环境中的原油及古代岩样的分析结果作了对比，发现同样表现出上述规律，故认为是良好的识别标志。正构烷烃分布、甾烷相对含量及某些特殊生物标志化合物的含量如三环二萜烷可以用来划分可溶有机质类型。察汉卓和查干诺尔盐湖可溶有机质中，正烷烃以低碳数占优势，显示其组成主要来源于低等水生生物；额吉卓尔盐湖则以高碳数为主，显然与边缘相陆生高等植物的输入有关；而小柴旦盐湖正烷烃高低碳数分布均衡，说明高等植物与低等水生生物同时输入。C₂₇～C₂₉正常甾烷及三环二萜烷的组成与分布也显示了相似特征。
    另外，还发现了多种非常规生物标志化合物，报导了其 m/z 217、m/z 231、m/z 245、m/z 259离子色谱特征。这些特征离子大多由三芳甾烷产生。三芳甾烷是由生物分子（甾体）在地质条件下演化而成的。盐湖沉积物中大量该类化合物的发现意味着含盐沉积环境及其继后的早期成岩作用在甾体的这种芳化过程中起着积极作用。     

辽河盆地过渡带有机地球化学（Ⅱ）──甾萜烷及芳烃系列化合物成熟度参数在过渡带内的系统变化特征

所研究的辽河盆地东部凹陷沙一、沙二段烃源岩主要处于未成熟演化阶段,其可溶有机质中甾萜烷立体化学参数显示出“地质型”异构体的“异常”富集。各参数在普遍高于各自门限值的背景上,大致以２２００—２３００ｍ、２５００ｍ、２７００ｍ和２７７０ｍ为界线,呈现出系统的变化规律。同时发现萘、菲、氧芴和硫芴等系列化合物的成熟度参数与甾萜烷的成熟度参数的变化规律十分相似。本文对比进行了深入研究,结合原始可溶有机质红外特征吸收参数的变化特征,指出过渡带内甾萜烷及芳烃成熟度参数的变化规律受控于原始可溶有机质的演化特征,定性地提出了辽河盆地东部凹陷过渡带的地球化学判识标志,并认为本研究结果同样有助于不同地区过渡带气或未成熟油、低成熟油的判识。     

Geochemical characteristics and genetic types of crude oils from Qinjiatun and Qikeshu oilfields in the Lishu Fault Depression,Songliao Basin,northeastern China

The Qinjiatun and Qikeshu oilfields are new Mesozoic petroleum exploration targets in Lishu Fault Depression of Songliao Basin, northeastern China. Currently, researches on geochemistry of crude oils from Qinjiatun and Qikeshu oilfields have not been performed and the genesis of oils is still uncertain. Based on bulk analyses, the crude oils in the Qinjiatun and Qikeshu oilfields of Lishu Fault Depression from the Lower Cretaceous can be classified as three types. TypeⅠoils, from Quantou and Denglouku formations of Qikeshu oilfield, are characterized by high C24tetracyclic terpane/C26tricyclic terpanes ratios, low gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29norhopane ratios and 17α(H)-diahopane/17α(H)-hopane ratios, indicating a brackish lacustrine facies. TypeⅡoils, from Shahezi Formation of Qikeshu oilfield show low C24tetracyclic terpane/C26tricyclic terpanes, high gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29 norhopane and C30diahopane/C30hopane ratios, thus suggesting that they originated from source rocks deposited in a weak reducing brackish lacustrine environment, or clay-rich sediments. Type oilsⅢ, from some wells of Qikeshu oilfield have geochemical characteristics intermediate between those two types and may be mixture of typeⅠand Ⅱoils.