Hydropyrolysis of algae, bacteria, archaea and lake sediments; insights into the origin of nitrogen compounds in petroleum

Nitrogen compounds are ubiquitous in fossil fuels and yet our understanding of their origins in the geosphere is limited. In this study, high hydrogen pressure pyrolysis was performed on sample material representing potential contributors to sedimentary organic matter (algae, bacteria and archaea) and sediments representing early diagenetic accumulations from Lake Pollen (Norway) and Priest Pot (UK). Previous workers demonstrated the structurally conservative nature of high hydrogen pressure pyrolysis in that the technique maximizes the yields of covalently bound hydrocarbon biomarkers from organic matter without adversely affecting their stereochemistry (Love et al., 1995). Release of covalently bound biomarkers in high yields from kerogen via catalytic hydropyrolysis. In this study, the types and distributions of organic nitrogen compounds in the hydropyrolysates were characterised under similar conditions to such experiments where biomarker hydrocarbons undergo minimal rearrangement. Compounds identified by gas chromatography–mass spectrometry included alkyl-substituted indoles, carbazoles, benzocarbazoles, quinolines and benzoquinolines.Indoles are present in all hydropyrolysates, suggesting a common origin. A potential source of indoles is represented by tryptophan which was shown to degrade through a series of alkylated intermediates to indole. Carbazole, quinoline and benzoquinoline were also found in the hydropyrolysates of algae, bacteria and archaea. The presence of these petroleum-related nitrogen compounds in hydropyrolysates generated from biomass suggests an early origin for petroleum nitrogen compounds. A potential source of naturally occurring nitrogen compounds such as that in the alkaloids has yet to be realised.Benzocarbazoles were absent from hydropyrolysates of algae, bacteria and archeae, but present in those from recent sediments, suggesting their presence may be related to processes occurring during early diagenesis at, or immediately below, the sediment–water interface. In sediments from Lake Pollen, changes in the benzocarbazole ratio [a]/([a] + [c]) ratio coincides with the interval described as a transition from fjord to lake environment, suggesting that benzocarbazoles are sensitive to changes in depositional environment and may have potential to act as a marker for environmental conditions.     

低熟油、烃源岩中含氮化合物分布规律及其地球化学意义

对八面河地区原油、油砂以及不同成熟度和岩性有异的烃源岩中吡咯类化俣物的分布特征及其潜在的地球化学意义进行了探讨。结果表明，随着成熟度的增加，八面河地区烃源岩中吡咯类化合物中屏蔽化程度高的异构体比屏蔽化程度低的异构体富集，咔唑及苯并咔唑系列中低相对分子质量的同系物比高相对分子质量的同系物富集，苯并[a]咔唑比苯并[c]咔唑富集。八面河地区原油中吡咯类化合物有较为明显的运移分馏效应，观察到高、低相对分子质量的同系物间的运移分馏效应比不同类型异构体间的分馏效应更为显著，而前者受成熟度的影响可能相对小于后者；发现吡咯类化合物的油气运移分馏效应不仅体现于不同氮官能团屏蔽、半屏蔽和暴露三类异构体间，同样体现于同一类型的同分导师构体之间，指示此类化合物有较强的活性。吡咯类化俣物与烃类化合物一样具有多方面潜在的地球化学意义。     

煤和泥岩不同温度条件下非烃中咔唑类化合物的演化特征

在密闭体系中,200～450℃的条件下进行了煤和泥岩加水的模拟实验,分析了实验产物非烃中咔唑类化合物的演化特征,并讨论了不同温度条件下非烃中咔唑类化合物的演化的机理.实验结果表明,非烃中咔唑类化合物是沉积有机质成岩过程的产物,其含量和组成受有机质成熟度影响较大.该研究结果不仅加深了对非烃中咔唑类化合物演化机理的认识,而...     

准噶尔盆地北三台地区湖相原油咔唑类化合物的地球化学特征

准噶尔盆地北三台地区北 82井、北 83井和沙 10 8井湖相原油咔唑类化合物的分布特征表明,油气运移作用是影响该区原油中咔唑类化合物分布的重要因素。沿油气运移方面,咔唑类化合物具有明显的分馏效应。总的趋势是原油中咔唑类化合物、高分子量的烷基咔唑化合物和苯并咔唑化合物的含量逐渐减少,低分子量咔唑和甲基咔唑,以及具屏蔽效应的二甲基咔唑和苯并 [a]咔唑异构体的相对含量则逐渐增加。利用咔唑类化合物的分馏效应,可有效地指示油气运移方向。     

Effect of artificial maturation on carbazole distributions, as revealed by the hydrous pyrolysis of an organic-sulphur-rich source rock (Ghareb Formation, Jordan)

Hydrous pyrolysis experiments were performed on the Ghareb Formation (Upper Cretaceous, Jordan), a carbonate- and organic-rich (TOC 19.6%) source rock, using a temperature range of 200 to 360°C (72 h). The original sediment contains only low amounts of carbazoles, (maximum 2.2 μg/g bitumen for 1-methylcarbazole). With increasing thermal maturation, intense generation begins at temperatures only in excess of 300°C, reaching a maximum at 360°C. Likewise, during natural maturation, generation occurs at later stages of maturity (e.g. for Tithonian source rocks at >0.81% R_r and for Posidonia Shale at >0.88% R_r). Some isomeric changes during hydrous pyrolysis do not resemble those in nature whereas others do. The relative abundances of selected C₁- and C₂-alkylcarbazoles on ternary diagrams reveal differences, whereas the benzo[a]carbazole/benzo[a]carbazole+benzo[c]carbazole ratio is closely similar. The latter result supports the contention that maturation plays a key role in controlling carbazole distributions in source rocks. However, the results for alkylcarbazoles, especially the C₂-carbazoles, are not easy to interpret.     

Carbazoles and benzocarbazoles confirm migration of leaked petroleum through caprocks and overlaying formations of Valhall Well 2/8-8 in the North Sea

A wildcat may not result in a petroleum prospect; however, it may indicate potential need for frontier exploration, since petroleum could leak into economic accumulations. Carbazoles and benzocarbazoles are nitrogen compounds in petroleum and can be employed to explicitly explain migration direction and distance of leaking petroleum. The hypothesis of this study is that the mechanism of the reaction involving the attraction of carbazoles to clay minerals on the matrix of the walls along their migration pathways determines the isomer that is preferentially attached to the mineral matrix. The objectives of this study are (a) to produce a profile for carbazoles and benzocarbazoles ratios in the cap rock overlying the reservoir, (b) to compare carbazole and benzocarbazole ratios to 20S/(20S + 20R) αααC₂₉ Sterane maturity ratios (c) to infer migration direction from the carbazoles and benzocarbazoles ratio. Samples used were side wall cores and drill cuttings of water-based mud drilling. The EOM (extractable organic matter) from various formations overlying the reservoir was obtained using Soxtherm Automatic Equipment. The TPH (total petroleum hydrocarbon) was obtained from the EOM by using a precleaned isolute C₁₈ 500 mg / 3 mL column, eluted with hexane, while the polars were eluted with dichloromethane. Fractionation into saturates and aromatics was done using a silver nitrate–silica gel column. The profiles of 1,8/1,3 and 1,8/2,4 dimethyl carbazoles and the benzocarbazoles ratios show a vertical gradient of decreasing ratio with increasing vertical distance from the reservoir. This corroborates the %VRo equivalent of 20S/(20S + 20R) αααC₂₉ Sterane and the carbazole ratios infer vertical migration of leaked petroleum.     

Applicability of Carbazole Migration Indices in Continental Rift Basins: A Case Study of Western Lujiapu Depression in Kailu Basin, NE China

<正>Kailu Basin in which the Western Lujiapu Depression is located is a typical continental rift basin.Biomarker parameters of the oils indicate that depositional facies and environments vary between the Bao 1 and Bao 14 fault blocks with a higher saline environment in the Bao 1 fault block, but such difference has no significant impact on carbazole abundance and distribution.Maturity and migration distance are the main controls on carbazole abundance and distribution in the Western Lujiapu Depression.The commonly used migration indices,such as ratios of nitrogen shield isomers to nitrogen exposed isomers(1-/4-methylcarbazole ratio,1,8-/2,4-dimethylcarbazole(DMC) ratio and half-shield/exposed-DMC ratio),absolute concentrations of alkylated carbazoles and BC ratio(=benzo [a]carbazole/(benzo[a]carbazole+benzo[c]carbazole)) increase at the low mature range and decrease at a higher mature range with increasing maturity.At relatively low maturity stage(Rc0.77%), maturation has reversal effects with migration on the ratios of nitrogen shield isomers to nitrogen exposed isomers,which may cover migration influence and makes these parameters fail to indicate migration effects.Valid migration indicators at this maturity stage are concentrations of alkylated carbazoles and BC ratios,which can provide ideal tools for migration direction assessment even within short migration distance.Maturity effects should be taken into account when carbazole compounds are applied to indicate migration direction,and at different maturity stages,these commonly used parameters have different validity in tracing migration direction.Coupled with our previous study in the Eastern Lujiapu Depression,a conceptual model of the variation of nitrogen migration indices can be established for terrestrial rifted basins,that is,strong fractionation lateral migration model through sandy beds,weak fractionation vertical migration model along faults,and maturity impacts on migration assessment.     

南阳凹陷原油含氮化合物组成及其控制因素

原油的含氮化合物总量及其组成特征不仅受原油运移距离影响,而且还受成油母质类型和成熟度影响.对南阳凹陷27个原油含氮化合物的研究表明,其油气运移指向的规律性不强,原因主要是由于原油运移距离较短,地质色层效应不明显.魏岗南部和北部原油的成熟度差异较大,但含氮化合物组成特征总体上非常相似,表明南阳凹陷原油的含氮化合物总量及其组成主要受控于成油母质类型.     

Study on secondary migration of hydrocarbons in Tazhong area of Tarim Basin in terms of carbazole compounds

Fractionated carbazoles have been detected for the first time in crude oils from the Tazhong area of the Tarim Basin, and these nitrogen compounds were successfully utilized in the study of petroleum migration. Alkylcarbazoles are quite abundant in all the samples analyzed; small amounts of benzocarbazoles were detected only in some of the sample, and dibenzocarbazoles were not found in the oils. Based on the distributions of G1, G2 and G3 types of C₂-alkylcarbazoles, the ratio of C₃-/C₂-carbazoles and the relative concentrations of alkylcarbazole and alkylbenzocarbazole, oils in the Carboniferous C_III reservoir in the Tazhong uplift are thought to have laterally migrated to the high level of Tazhong structure No. 4 from both northwest and southeast. The study here also shows that oils in the area may have undergone long-distance migration. This project is financially supported by the China National Petroleum Corporation and the State Educational Commission.     

Source rock potential (TOC + hydrogen index) evaluation by integrating well log and geochemical data

Evaluation of source rock potential using well log data alone, e.g. density or sonic log, is limited to the assessment of total organic carbon (TOC) content. Well logs that directly measure the most important source quality parameter, the hydrogen content of the kerogen, do not exist. Therefore, a method is proposed that combines log-derived TOC data with calibration schemes relating TOC content to Hydrogen Index and maturity for individual source rock sets. The method has been applied to the compositionally heterogeneous Kimmeridge Clay Formation of the North Sea and the homogeneous Lower Toarcian Shale of N.-Germany. The combination of well log and geochemical data greatly improves the accuracy with which the lateral and vertical distribution of source rocks and their potential can be predicted on a basin and prospect level.     

Hydrous pyrolysis of sediments: Composition and proportions of aromatic hydrocarbons in pyrolysates

Hydrous pyrolysis (closed vessel autoclaving in the presence of excess water) of organic-rich rocks is said to generate oils which closely resemble natural crude oils in their broad characteristics and composition. However there are only a few accounts of the proportions and compositions of hydrocarbons in hydrous pyrolysates and none of these discuss the aromatic hydrocarbon composition in detail. The present paper presents some data on the latter.Hydrous pyrolysis (3 days) of a dolomitic siltstone (Permian, Marl Slate) at 280, 300,320, 340 and 360°C produced significant amounts of oils in which the aromatic hydrocarbons were one and a half to two times as abundant as the saturated hydrocarbons.The overall composition of the aromatic hydrocarbons was similar to most crude oils; the major components isolated by our methods from natural oils and from pyrolysates were C_1–4 alkylnaphthalenes. At the lowest pyrolysis temperature (280°C) the distributions of the more minor components of the pyrolysates (e.g. alkylphenanthrenes, aromatic steroids) were also generally similar to those found in natural crudes. However, a number of components (e.g. methylanthracenes, Diels' hydrocarbon) which are not usually reported in crudes, were also detected and the relative proportions of these increased at the higher temperatures. Hydrous pyrolysis (340°C) of an organic-rich oil shale (Jurassic, Kimmeridge) and an asphaltic-material containing no minerals produced pyrolysates in which many of these unusual compounds were also present. In addition the pyrolysate of the oil-shale contained higher proportions of organic sulphur compounds. It appears that the formation of the unusual compounds is not simply a function of the type of organic matter or mineralogy but rather of the high temperatures or fast heating rates employed.     

The characteristics of the biomarkers and δ13C of n-alkanes released from thermally altered solid bitumens at various maturities by catalytic hydropyrolysis

Solid bitumen occurs extensively in the paleo-reservoirs of marine sequences in southern China. The fluids in these paleo-reservoirs have usually experienced severe secondary alteration such as biodegradation and/or thermal maturation. The concentrations of extractable organic matter (EOM) in the resulting solid bitumens are too low to satisfy the amount required for instrumental analysis such as GC–MS and GC–IRMS. It is also difficult to get enough biomarkers and n-alkanes by dry pyrolysis or hydrous pyrolysis directly because such solid bitumens are hydrogen poor due to high maturities. Catalytic hydropyrolysis (HyPy) can release much more EOM from solid bitumen at mature to highly over-mature stages than Soxhlet extraction, dry pyrolysis and hydrous pyrolysis. However, whether the biomarkers in hydropyrolysates can be used for bitumen-source or bitumen–bitumen correlations has been questionable. In this study, a soft biodegraded solid bitumen sample of low maturity was thermally altered to various maturities in a closed system. HyPy was then employed to release bound biomarkers and n-alkanes. Our results show that the geochemical parameters for source and maturity based on biomarkers released from these thermally altered bitumen residues by HyPy are insensitive to the degree of thermal alteration. Furthermore, the maturity parameters are indicative of lower maturity than bitumen maturation products at a corresponding temperature. This suggests that biomarker source and maturity parameters, based on the products of HyPy, remain valid for bitumens which have suffered both biodegradation and severe thermal maturation. The distributions of δ¹³C of n-alkanes in hydropyrolysates are also insensitive to the temperature used for bitumen artificial maturation. Hence, the δ¹³C values of n-alkanes in hydropyrolysates may also provide useful information in bitumen–bitumen correlation for paleo-reservoir solid bitumens.     

Migration Fractionation of Neutral Nitrogen Compounds of Crude Oils from Tabei Oilfield in the Tarim Basin, China

The absolute amounts and relative distributions of neutral nitrogen compounds in the Tabei oilfield (e. G. Blocks Ln1-Ln1 1) showed remarkable migration fractionation in the vertical direction. From Ordovician reservoirs (O) to oil legs TⅢ and T Ⅰ of Triassic reservoirs in blocks LN1-LN11, the concentrations of [a] [c] decreased from 1. 59μg/g, 0.49μg/g to 0.17 μg/g (oil). The ratios of various alkylcarbazole isomers, such as 1,8-dimethylcarbazole/nitrogenpartially shielded isomers and 1,8-dimethylcarbazole/nitrogen-exposed isomers, were adopted as the indicators of petroleum migration. The ratios increased from 0.13, 0.20 to 0.67 and from going from the south to the north of the Tabei oilfield, the absolute concentrations of neutral nitrogen compounds decreased drastically, and the nitrogen-shielded isomers were enriched relative to nitrogen-exposed isomers and nitrogen-partially shielded isomers. Crude oils in the Tabei oilfield migrated laterally from the Jilake structure to the Sangtamu fault uplift and Lunnan fault uplift, and crude oils in the same fault uplift migrated and remigrated vertically from Ordovician reservoirs, to oil legs TⅢ to TⅠ of Triassic reservoirs.     

Similarities and differences in hydrous pyrolysis of biomass and source rocks

In laboratory simulation of oil generation, products from closed systems pyrolysis of immature source rocks in the presence of water gives the closest match to petroleum compositions observed in nature. Fresh biomass can also be converted to fluids by pyrolysis, but in the absence of the sedimentary diagenetic transformations, the initially much higher oxygen content gives high yields of oxygen containing products. In this work, the reactions that occur during hydrous pyrolysis of a Kimmeridge source rock, a brown coal and two polymeric waste materials from alginate production are compared in terms of quantities of the main products and kinetic models of the reaction systems. The biomass pyrolysis and the simulated maturation are described in similar reaction networks. Conversion of biomass to fluids occurred with reaction networks and activation energy distributions comparable to the brown coal, while for the Kimmeridge source rock reactions a simpler reaction network could be used. The biomass samples gave a high degree of conversion to fluid products, and higher yields of bitumen than the coal.     

A comparison of the distributions of carbazoles and dibenzothiophenes in western Qaidam Basin,Northwest China

Variations in the distributions of carbazoles and dibenzothiophenes were investigated in a set of source rocks, which differ mainly in their maturity levels during immature-mature stages. A comparison of the distributions of carbazoles and dibenzothiophenes has revealed the main results as follows- ① variations in the relative amounts of C0-, C1-, C2-dibenzothiophenes show a low correlation with that of the corresponding carbazoles, with the for- mer＇s being much higher than the latter＇s; ② variations in the relative amounts of methyldibenzothiophene isomers also display a low correlation with that of the corresponding methylcarbazoles, with 4-/4- ＋1-methyldibenzothio- phene ratio ranging from 0.52 to 0.96, while the corresponding carbazole ratio of 1-/1- ＋4-methylcarbazole only being 0.71-0.05; ③ the maturity parameter for 4,6-/4,6- ＋l,4-dimethyldibenzothiohene, ranging from 0.34 to 0.75, shows a remarkable linear correlation with the corresponding ratio of 1,8-/1,8- ＋1,4-dimethylcarbazole （R2〉0.84）. The un-correlation may indicate some different geological-geochemical fates for some isomers of dibenzothiophenes and carbazoles. The high correlation may reveal a strong maturation dependence on the dimethylcarbazole distribu- tions, indicating that attention should be paid when 1,8-/1,8- ＋1,4-dimethylcarbazole is used as a petroleum migra- tion indicator.     

Degradation of 5α-cholestane into dimethylperhydrophenanthrenes: an experimental and theoretical study

A tetradeuteriated cholestane was heated with kerogen isolated from the Messel oil shale at 350 °C for 25 different time periods (ranging from 0.5 to 80 h) in borosilicate vessels. Three isomers of tetradeuteriated dimethylperhydrophenanthrene were formed and one of these was the major saturated hydrocarbon product under most conditions. The stereochemistry for these isomers was unknown but if the stable 5α,8β,9α(H),10β(CH₃) sterane configuration is inherited from the cholestane then there are four possible isomers namely: I and II [13β(H) and 13α(H) diastereomers, respectively, of 10,13-dimethylperhydrophenanthrene] as well as III and IV [14β(H) and 14α(H) diastereomers, respectively, of 10,14-dimethylperhydrophenanthrene]. Molecular mechanics revealed that I is the most stable form and therefore is the component that dominated the saturated hydrocarbon degradation products. The identification of only three isomers of deuteriated dimethylperhydrophenanthrene in the pyrolysis experiments is supported by the calculated stabilities (ΔΔG) as isomer IV is considerably more unstable (by 1.7 kcal/mol) than II which is the least stable of I, II and III. If these compounds can be detected in crude oils their distributions may be useful maturity indicators in oils that have been generated at high thermal maturity.     

运用原油吡咯类含氮化合物研究塔里木盆地塔中地区石油的二次运移

在塔里木盆地塔中地区的原油中分离和检测到咔唑类含氮化合物,并成功地将其应用于油气运移研究,烷基咔唑类化合物大量存在于所分析的样品中,苯并咔唑只在部分样品检测到且含量小,而二苯并卟唑则未发现,根据Ｃ２烷基咔唑的Ｇ１,Ｇ２和Ｇ３结构类型的分布,Ｃ３－咔唑／Ｃ２－咔唑比值以及烷基咔唑与苯并烷基咔唑的相对含量可知,塔中地区石炭系Ｃ３油组的石油分别从北西和南东两个方向向塔中４号构造的高点侧向运移,研究还表明     

A new peculiar muensterellid coleoid (Cephalopoda) from the Kimmeridge Clay Formation of Dorset (England)

A concretion from the lower Tithonian Kimmeridge Clay Formation (Pectinatus Zone) found by Steve Etches yielded a gladius of a coleoid cephalopod. It is peculiar in shape and has an unusual ornamentation of radiating ribs and tubercles. The new form is named Etchesia martilli n. gen. n. sp. and preliminarily placed within the octobrachian family Muensterellidae based on its limpet-like gladius. Through the presence of radiating ribs as well as the absence of a narrow anterior rachis E. martilli n. gen. n. sp. is similar to Pearceiteuthis buyi from the Oxford Clay Formation (Callovian). The new muensterellid is unique in having an enrolled patella apex, which is located close to the posterior gladius rim. E. martilli n. gen. n. sp. represents the first muensterellid coleoid from the Kimmeridge Clay Formation. A phylogenetic relationship of E. martilli n. gen. n. sp. (and Pearceiteuthis) with cirrate and incirrate octopods is discussed, although further information on soft parts such as the muscular mantle is necessary.     

Novel aryl polycyclic aromatic hydrocarbons: Phenylphenanthrene and phenylanthracene identification, occurrence and distribution in sedimentary rocks

Numerous reports have recognised the presence of compounds with molecular weight 254 a.m.u. in aromatic fractions. However, their unequivocal identification has not been achieved due to a lack of reference substances. In geological samples, such m/z 254 compounds could potentially be represented by a number of structural isomers of binaphthyl, phenylphenanthrene, phenylanthracene and indenofluorene with each compound type possessing several positional isomers. In this work, all these m/z 254 compounds, with the exception of the tentatively recognised indenofluorenes, have been unequivocally identified in sedimentary rocks for the first time. Comparison of the mass spectra and the gas chromatography (GC) retention times of synthesised standards with the natural compounds in rocks shows that the major components of aromatic fractions are phenylphenanthrene isomers and, to a lesser extent, binaphthyls and 9-phenylanthracene. The elution sequence expressed as standard retention indices of all these m/z 254 isomers were determined by using high resolution capillary GC with three stationary phases: 5%, 35% and 50% (mole fraction) phenyl substituted methylpolysiloxane on HP-5MS, DB-35MS and DB-17MS columns, respectively. A survey of more than 350 sedimentary rock samples of varying origins and maturity (R_r 0.3–1.4%) reveals that relative abundances of the m/z 254 isomers depend on the maturity of the organic matter. The isomers initially appear at the onset of oil generation (R_r > 0.5%) exclusively in diagenetically/catagenetically oxidised samples containing varying proportions of Types II and III kerogen. Interestingly, all five possible positional phenylphenanthrene isomers (including the most sterically hindered isomer 4-phenylphenanthrene) are present from the beginning of the oil generation window. Such a distribution suggests that low-regioselectivity reactions are likely to be involved in the formation. Interaction, during maturation, of phenyl radicals from primary cracking with aromatic moieties of more resistant terrigenic components of kerogen in an oxidising diagenetic realm is postulated to be responsible for the neoformation of phenylated aromatics in geological samples. Up to the final stages of oil generation, the evolution of the phenyl PAH distributions presumably involve 1,2-phenyl shift reactions and cyclisation, if allowed by the molecular geometry. A near thermodynamically-controlled distribution, with only 3- and 2-phenylphenanthrene and minor 2,2′-binaphthyl remaining, is approached when vitrinite reflectance reaches 1.2% (R_r). The three compounds seem to be persistent beyond the oil window as is suggested by their presence in hydrothermal oil formed at T > 300 °C.     

A molecular and carbon isotope biogeochemical study of biomarkers and kerogen pyrolysates of the Kimmeridge Clay Facies: palaeoenvironmental implications

Structures and carbon isotopic compositions of biomarkers and kerogen pyrolysis products of a dolomite, a bituminous shale and an oil shale of the Kimmeridge Clay Formation (KCF) in Dorset were studied in order to gain insight into (i) the type and extent of water column anoxia and (ii) changes in the concentration and isotopic composition of dissolved inorganic carbon (DIC) in the palaeowater column. The samples studied fit into the curve of increasing δ¹³C of the kerogen (δ¹³C_TOC) with increasing TOC, reported by Huc et al. (1992). Their hypothesis, that the positive correlation between TOC and δ¹³C_TOC is the result of differing degrees of organic matter (OM) mineralisation in the water column, was tested by measuring the δ¹³C values of primary production markers. These δ¹³C values were found to differ on average by only 1‰ among the samples, implying that differences in the extent of OM mineralisation cannot fully account for the 3‰ difference in δ¹³C_TOC. The extractable OM in the oil shale differs from that in the other sediments due to both differences in maturity, and differences in the planktonic community. These differences, however, are not likely to have significantly influenced δ¹³C_TOC either. All three sediments contain abundant derivatives of isorenieratene, indicating that periodically euxinia was extending into the photic zone. The sediments are rich in organic sulfur, as revealed by the abundant sulfur compounds in the pyrolysates. The prominence of C₁-C₃ alkylated thiophenes over n-alkanes and n-alkenes is most pronounced in the pyrolysate of the sediment richest in TOC. This suggests that sulfurisation of OM may have played an important role in determining the TOC-δ¹³C_TOC relationship reported by Huc et al. (1992).