Hopane Ratios Research Articles

Fate of hopane biomarkers during in-situ burning of crude oil — A laboratory-scale study

In-situ burning (ISB) is a remediation strategy that is used for managing oil spills. ISB generates heavy residues that can submerge and negatively impact benthic environments. To track the fate of toxic contaminants in ISB residues, a conservative hopane biomarker, such as C30-αβ hopane, is often used. Furthermore, diagnostic ratios of various hopanes are used for source oil identification. Use of these biomarkers assume that during ISB the quantity of C30-αβ hopane will be conserved, and the diagnostic ratios of various hopanes will be stable. The objective of this study is to test the validity of these two assumptions. We conducted laboratory-scale ISB experiments using a model oil prepared from commercial C30-αβ hopane standard, and a reference crude oil. Laboratory data collected under controlled burning conditions show that C30-αβ hopane will not be conserved; however, the diagnostic ratios of hopanes will still remain fairly stable.

Biological marker indicators of paleo-depositional environment for the Eocene formations around Zara-Bolucan region (central anatolia, Turkey)

Biological marker (biomarker) characteristics and paleo-depositional conditions of Eocene sediments around Zara-Bolucan northeast of Sivas Basin have been determined by means of detail organic geochemical studies such as gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis. In the investigated area, northeast section of the Sivas Basin, the Eocene Bozbel and Kozluca Formations consist of a organic matter rich sedimentary rocks deposited within trangressive-regressive cycles. During the evaluation of paleoenvironment characteristics with biomarker data, data related to n-alkanes, isoprenoids, terpenoids and steroids were used. Especially, the marine environment features indicated by C23 tricyclic/C30 hopane, the C26 /C25 tricyclic terpanes, homohopane/C30 hopane and diasterane/regular sterane data. The ratios of Pr/Ph, Pr/n-C17 and Ph/n-C18 indicate that there is marine organic matter predominantly precipitated under reducing environmental conditions. In the studied samples, the algal-bacterial organic matter contribution was interpreted to be due to the superiority of the C23 tricyclic terpene to C24 , as well as larger than the C24 tricyclic terpane. However, the gammacerane index suggests that there is initially a low salinity environment. The significant terrestrial organic matter contribution is indicated by the isoprenoid and biomarker ratios, dominance of higher n-alkanes such as n-C29 , n-C31 , CPI, TAR values, n-C19 /n-C31, relative abundance of C27-C28-C29 regular steranes. Suboxic to anoxic palo-depositional conditions are confired by the isoprenoid and n-alkane ratios and T m /T s , C 29 T s /C 29 hopane ratios.

Seasonal variability of anthropogenic indices of PAHs in sediment from the Kuala Selangor River, west coast Peninsular Malaysia.

Rapid increase in industrialization and urbanization in the west coast of Peninsular Malaysia has led to the intense release of petroleum and products of petroleum into the environment. Surface sediment samples were collected from the Selangor River in the west coast of Peninsular Malaysia during four climatic seasons and analyzed for PAHs and biomarkers (hopanes). Sediments were soxhlet extracted and further purified and fractionated through first and second step column chromatography. A gas chromatography-mass spectrometry (GC-MS) was used for analysis of PAHs and hopanes fractions. The average concentrations of total PAHs ranged from 219.7 to 672.3ngg-1dw. The highest concentrations of PAHs were detected at 964.7ngg-1dw in station S5 in the mouth of the Selangor River during the wet inter-monsoonal season. Both pyrogenic and petrogenic PAHs were detected in the sediments with a predominance of the former. The composition of hopanes was homogeneous showing that petroleum hydrocarbons share an identical source in the study area. Diagnostic ratios of hopanes indicated that some of the sediment samples carry the crankcase oil signature.

Effect of Salinity on Source Rock Formation and Its Control on the Oil Content in Shales in the Hetaoyuan Formation from the Biyang Depression, Nanxiang Basin, Central China

A total of 75 shale samples from the Biyang Depression were analyzed with Rock-Eval, total organic carbon (TOC), organic maceral, and gas chromatography–mass spectrometry (GC–MS) techniques to reveal the effect of salinity on source rock formation and its control on the oil content. On the basis of the diversities in salinity and redox conditions reflected by the gammacerane/αβ C30 hopane (G/H) ratio, the pristine/phytane (Pr/Ph) ratio, and the extended tricyclic terpane ratio [ETR = (C28 + C29) tricyclic terpane/Ts)], three types (T1, T2, and T3) of shales were identified, which deposited in the brackish condition (G/H ratio 0.6), respectively. The comparisons among T1, T2, and T3 shales revealed that the salinity had a significant effect on the paleoproductivity of the lacustrine system. Most algae thrive in semi-saline conditions (T2) but are restrained in saline conditions (T3), so that moderate salinity con...

Families of reservoired crude oils from the Cangdong Sag, Bohai Bay Basin, China

The organic geochemistry of 35 crude oils from six oil fields across the Cangdong Sag, Bohai Bay Basin, was characterized and an oil family classification was carried out for the first time. Four oil families were classified based on 18 parameters related to source and depositional environment using hierarchical cluster analysis. The families have broadly similar geochemical characteristics which suggest oils derived from lacustrine source rocks deposited in a reducing environment. Family A has relatively abundant C27 steranes and C30 4α-methyl-24-ethylcholestanes, high Pr/Ph, C22/C21 tricyclic terpane and C31R/C30 hopane ratios, and low ETR, C26/C25 tricyclic terpane and C35S/C34S homohopane ratios. Family A shows higher thermal maturity than families C and D, but lower than family B. The most distinct feature for Family B is the high gammacerane index; they also show slightly higher C22/C21 tricyclic terpane, C31R/C30 hopane and C29/C30 hopane, lower dibenzothiophene/phenanthrene, and higher maturity than the other families. Families C and D oils are low maturity and they are more similar to each other than families A and B based on most biomarker ratios. However, family C can be distinguished from family D by slightly higher C27/C29 steranes and C30 4Me/C29 steranes C30 4α-methyl-24-ethylcholestanes/C29 steranes, and lower C29 steranes/C30 hopane ratios. A preliminary oil–source rock correlation suggests that the oil families originated from various intervals having different organofacies within the second member of the Kongdian Formation. Family A is more widespread than the other families, supporting extensive distribution of the source rock. Family B was only found in the southern part of Kongdong slope, which suggests limited geographical distribution of the related source rock. Families C and D show similar geographical distributions and dominate Wangguantun, Shenjiapu and Shenvsi fields, suggesting that the source kitchens are near these regions.

Biomarkers stratigraphy of Irati Formation (Lower Permian) in the southern portion of Paraná Basin (Brazil)

This study covers the application of biomarkers as stratigraphic markers in a Lower Permian marine section from the Paraná Basin, southern Brazil. The biomarker variations reveal changes in sedimentary environment that were not detected by sedimentological and paleontological methods and will contribute to high resolution stratigraphy. The Irati Formation contains two lithostratigraphic units: (1) the lower unit (Taquaral Member) is composed of shales, deposited in a marine epicontinental to restricted environment and (2) the upper unit (Assistência Member) which consists of carbonates interbedded with shales and organic-rich shale intervals deposited in a restricted marine environment. The biomarker differences observed in the Taquaral Member can be explained by two different types of organic matter. The lower and upper parts are characterized by a high relative abundance of high molecular weight n-alkanes, C27 (17α) trisnorhopane, C31 αβR homohopane, high hopanes/steranes ratio, and low hydrogen index values. This facies contains greater relative amount of terrestrial organic matter. The low hopanes/steranes ratio, together with the high hydrogen index values and amorphous organic matter relative proportion observed in the middle part of the Taquaral Member indicate a more marine organic matter contribution. In the Assistência Member based on gammacerane/C30 αβ hopane ratio, C28ααα (20R) methylcholestane/C29ααα (20R) methylcholestane ratio, total organic carbon, hydrogen index values, palynofacies and sedimentary structures it was possible to distinguish changes in salinity and redox conditions. The bituminous shales intervals were deposited in an anoxic environment (high total organic carbon, high hydrogen index values, high preserved amorphous organic matter content) with lower salinity (lower gammacerane/C30 αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) ethylcholestane ratios) on the other hand, the limestones intervals were deposited in an aerobic environment (low total organic carbon, low hydrogen index values, high oxidized organic matter content) with higher salinity (higher gammacerane/C30 αβ hopane and C28ααα (20R) methylcholestane/C29ααα (20R) methylcholestane ratios).

Organic petrology and geochemistry of mudstones from the lower Shahejie Formation in the Tanggu area of eastern China: evidence for the presence of an ancient saline lake

Mudstones in the lower Shahejie Formation, in the Tanggu area of the Huanghua Depression, have been found to contain analcime and ankerite. Hydrothermal sedimentation has been invoked to explain the origin of these two minerals, raising the question of whether hydrothermal activity occurred at a sufficient scale to significantly raise the salinity of the depositional environment. We applied a suite of organic petrological and geochemical methods to directly address this question. Maceral composition, kerogen type, and the distribution of n-alkanes, hopanes, and steranes indicate that the organic matter contained in these mudstones and dolomitic mudstones is mainly derived from algae and bacteria. The dominant acritarch genera, C31R/C30 hopane ratio, gammacerane index, Pr/Ph ratio, and the relationship between Pr/n-C17 and Ph/n-C18 suggest that the mudstones and dolomitic mudstones were deposited in an anoxic, saline lacustrine environment. Tmax, biomarker maturity indices, the thermal alteration index (TAI) and acritarch alteration index (AAI), and vitrinite reflectance all indicate that the organic matter is at an immature to early mature stage. The estimated maximum paleotemperature is close to the present-day burial temperature, and much lower than the homogenization temperature of the analcime veins. Combined with the absence of unresolved complex mixtures on the n-alkane pattern, this suggests that hydrothermal activity had a negligible impact on the salinity and alkalinity of the depositional lake.

Variations in abundance and distribution of methyltrimethyltridecylchromans (MTTCs) in sediments from saline lacustrine settings in Cenozoic lacustrine basins, China

This study examines the abundance and distribution of MTTCs and biomarker compositions in samples of MTTC-rich sediments obtained from Paleogene strata in the Bohai Bay, Jianghan and Western Qaidam Basins in China. Biomarker signatures, including high C35/C34 hopane ratios, high gammacerane index values, low pristane/phytane (Pr/Ph) ratios and the presence of aryl isoprenoids, indicate saline lacustrine settings and photic zone euxinia (PZE). The distributions of MTTCs indicate significant palaeosalinity differences in the depositional environments of the three sets of samples obtained from the Bohai Bay, Jianghan and Western Qaidam Basins. These sets of samples were deposited under normal to mesosaline, meso- to hypersaline and hypersaline environments, respectively. The δ-MTTC% values are positively correlated with the values of the gammacerane index, but are negatively correlated with the Pr/Ph ratios. This result demonstrates that MTTCs do not form in the upper or lower parts of stratified water columns, but may form in the euxinic portions of photic zones. Therefore, MTTC parameters can only be used to reconstruct palaeosalinity in the euxinic portions of photic zones and are not applicable to the upper or lower parts of stratified water columns. This paper infers that variations in palaeosalinity in the euxinic portions of photic zones likely play a key role in controlling the distribution and abundance of MTTCs.

Organic geochemical characteristics and generating potential of source rocks from the Lower-Middle Jurassic coal-bearing strata in the East Junggar Basin, NW China

Abstract The Junggar Basin located in NW China is a large, intracontinental superimposed basin. Samples were collected from the Lower-Middle Jurassic coal-bearing strata of the eastern Junggar Basin and include mudstones, carbargilites and coal. The origin of organic matter, thermal maturity, paleoenvironment of sediment deposition and hydrocarbon potential of drill core samples was investigated. Organic geochemical analysis performed on 65 source rock samples indicates a wide range of TOC values from 0.81 wt % to 81.06 wt %. This study combined sedimentology, biomarker, organic petrography, and stable carbon isotopic composition of kerogen and extracts analysis demonstrating the organic matter of the source rocks is derived from terrestrial higher plants with a minor contribution of aquatic algae and microbes. Rock-Eval data, elemental composition of kerogen and odd over even normal alkane distribution reveal immaturity to low maturity with a humic - type organic matter. The source rocks were deposited in a freshwater oxic to sub-oxic environment indicated by high Pr/Ph ratios, a predominance of C29 steranes, low tricyclic terpanes and a low gammacerane index. High dia/regular C29 sterane ratios and moderate to high C29/C30 hopane ratios found in most samples are related to the presence of clay minerals. Based on the following parameters: H/C vs. O/C, TOC, S1+S2, hydrogen index, saturated hydrocarbons, aromatic hydrocarbons and Chloroform A, we concluded that the samples collected from organic-rich coal-bearing strata may yield not only gas, but also have the capability to generate and release liquid hydrocarbons.

Organic Geochemistry and Depositional Environment of the Oltu Gemstone (Coal) in the Erzurum Area, NE Anatolia, Turkey

The Oltu Gemstone is located in the north of Oltu town (Erzurum–NE Turkey) city as a low rank coal. The Oltu Gemstone occurs as lenticular forms with thickness not exceeding centimeter size and lateral continuity of a few meters within the Liassic–Lower Malm Olurdere Formation consisting chiefly of claystone, sandstone, and volcanics. Coals that are operated as Oltu Gemstone are represented by very high TOC (67.39–78.56% wt), high hydrogen index (HI) values (314–379 mg HC/g TOC) and very low oxygen index (OI) values (1–3 mg CO2/g TOC). Low Pr/Ph ratios indicate that coals were prevented from oxidation and deposited under anoxic conditions. In Oltu Gemstone samples, C29 dominates over C27 and C28 steranes. In general, high (C19 + C20)/C23 tricyclic terpane, low Ts/(Ts + Tm), diasterane/sterane, and C31R/C30 hopane ratios were recorded. C29 MA steroids dominate with respect to others, and the C29/(C28 + C29) MA ratio is mostly high. DBT/P ratio of Oltu Gemstone samples has low values. Tmax values of Oltu Ge...

Paleoenvironmental conditions of organic-rich Upper Permian Dalong Formation shale in the Sichuan Basin, southwestern China

Organic geochemical and trace element analysis of a suite of 33 samples collected from the Shangsi section of the Upper Permian Dalong Formation, located near Guangyuan in the northwestern Sichuan Basin, have been used to better understand the marine depositional environment of its shale lithofacies. A 15 m-thick black shale interval near the middle of the formation is enriched in total organic carbon (TOC = 1.1–14.4%, mean = 6.4%). The n-alkane, terpane and sterane biomarker distributions of the shale suggest that its organic matter is of predominantly marine algal origin. Pr/Ph, Pr/n-C17, Ph/n-C18 and gammacerane/C30 hopane ratios demonstrate that reducing conditions prevailed during its deposition. This interpretation is supported by the redox-sensitive abundance of Mo and the trace element redox ratios Ni/Co, V/Cr, U/Th and V/(V + Ni), all of which indicate deposition in a euxinic/anoxic setting. The Mo-TOC relationship in the Dalong shales plots mid-way between those of sediments in the present-day Saanich Inlet and Cariaco Basin, suggesting a moderately restricted depocentre. However, the δ13Corg values of these shales reveal that their organic matter is isotopically much lighter than that found in modern euxinic sediments, implying extremely anoxic bottom waters and a slow-down of oceanic circulation during the Late Permian.

Geochemical composition of oils in the Dunga Field, western Kazakhstan: Evidence for a lacustrine source and a complex filling history

The onshore Mangyshlak Basin, located in western Kazakhstan along the eastern coastline of the Caspian Sea, is a prolific hydrocarbon province and home to numerous oil and gas fields. The primary reservoirs are Jurassic clastic sandstones, but the Dunga Field contains hydrocarbons in Lower Cretaceous (Valanginian, Barremian, Aptian, Albian) clastic and carbonate reservoirs. The general perception is that these petroleum accumulations were charged from marine source rocks of likely Middle Triassic age. A detailed organic geochemical investigation of oils from the Cretaceous reservoirs of Dunga Field suggests that all were generated from the same lacustrine source rock facies (organofacies C) in mature kitchens in the offshore Mangyshlak Basin, followed by a complex filling history. The oils are characterised by relatively high wax content, low sulfur content (mainly < 0.2 wt%), typical lacustrine tricyclic terpane ratios (high T26/T25, high T24/T23, low T22/T21) and low H31R/H30 hopane ratios. Maturity of the source rock that generated the oils would tentatively correspond to a VR of ∼0.9–1.1 %Ro. The oils are non-biodegraded, but some minor water washing might have occurred. Diamondoid data suggest that the Barremian reservoir contains a mixed oil pool composed of the non-cracked lacustrine oil and another charge of oil that is cracked from 79% to 83% and likely also to be slightly fractionated. This requires a complex filling history of the Dunga Field involving two hydrocarbon charges, likely from two different fetch areas. One charge of non-cracked oil migrated into all four reservoirs whereas another charge was ‘hoteled’ and cracked before remigration to the Barremian reservoir only and mixed with the non-cracked oil.

Hydrocarbon generation potential of a Lower Permian sedimentary succession (Mount Agony Formation): Southern Sydney Basin, New South Wales, Southeast Australia

This research presents a comprehensive approach on the hydrocarbon generation potential applied for the first time on the Lower Permian Mount Agony Formation that is exposed in the southern Sydney Basin, southeast Australia. Thirty mudstone samples were collected and analyzed using different organic geochemical methods such as, Rock-Eval VI-TOC, bitumens extraction, and GC–MS analysis. Attributes such as, quantity, quality and maturity of the organic matter were utilized to estimate the regional source rock potential. The obtained data indicate that the examined samples have poor to good, dominantly gas-prone source-rock potential, and contain a mixed in origin organic matter. The slight predominance of terrestrial organic material is indicated by the low odd to even predominance values, slight predominance of higher n-alkanes over short-chain n-alkanes, OEP25–33, CPI25–33, ACL25–33, and TAR values. Some availability of marine organic material is evidenced by the predominance of intermediate chain n-alkanes and the low n-C19/n-C31 and n-C24+/n-C24– ratios. The analyzed sediments are thermally mature, as suggested by the Tmax, PI, OEP 25–33 and CPI25–33 values, the Pr/n-C17, Ph/n-C18, C30-moretane/C30-hopane, Ts/(Ts+Tm) and C32S/(S+R) ratios, as well as by the C30-moretane/C30-hopane versus Tm/Ts plot. A mudstone-rich, siliciclastic source rock is implied by the C29/C30 hopane ratios and by the Ts/(Ts+Tm) versus 23tricyclic/hopane cross-plot. This research offers original organic geochemical data of a sedimentary succession that was not so far regarded as source rock. This study adds to the knowledge available for the types of potential reserves in the Southern Sydney Basin, which are mostly focused on coal and coal-seam gas resources.

Organic geochemical characteristics and paleodepositional conditions of an Upper Carboniferous mud-rich succession (Yagon Siltstone): Myall Trough, southeast Australia

This research evaluates the organic geochemical characteristics of a Carboniferous mud-rich sedimentary succession (Yagon Siltstone) that outcrops in the Myall Trough, a generally regarded fore-arc basin from southeast Australia. Twenty eight mudstone (28) samples were tested by using standard organic geochemistry methods (Rock-Eval VI-TOC, bitumen extraction and fractionation), as well as GC-MS analysis of the hydrocarbons fraction to define the origin, type and maturation level of organic material, and the environment conditions during deposition. The data indicate that the studied samples have poor to fair, gas-prone source rock potential. A marine environment of deposition, with organic material of mixed aquatic and terrigenous origin is envisaged. The marine setting is indicated by the C26/C25 tricyclic terpanes and C23 tricyclic/C30 hopane ratios and the C31R homohopane/C30 hopane versus Pr/Ph cross plot. The greater contribution of terrestrial organic matter is indicated by the Rock-Eval VI data, predominance of higher n-alkanes over short-chain n-alkanes, n-C19/n-C31 and n-C24+/n-C24- ratios, OEP25-33, CPI25-33, ACL25-33, and TAR values, relative abundance of C27-C28-C29 regular steranes and the C27/(C27 + C29) and C27/C29 versus Pr/Ph cross plots, and C24 tetracyclic/C26 tricyclic terpane versus C26/C25 and C23/C22 tricyclic terpane cross plots. Suboxic to anoxic bottom water conditions are supported by the Tm/Ts, C29Ts/C29 hopane ratios and the pristane/n-Cl7 versus phytane/n-Cl8 cross plot. The samples studied are mature and have experienced high temperature during burial (over 100 °C), as assessed from Tmax, OEP25-33, and CPI25-33 values, and the cross plots of 20S/(20S + 20R) versus ββ/(ββ + αα) and C30-moretane/hopane versus Tm/Ts.

Impacts of source input and secondary alteration on the extended tricyclic terpane ratio: A case study from Palaeozoic sourced oils and condensates in the Tarim Basin, NW China

A suite of oils and condensates from Cambrian-Ordovician sources in the cratonic region of the Tarim Basin, NW China has been characterized by gas chromatography–mass spectrometry with a focus on factors affecting the extended tricyclic terpane ratio [ETR=(C28+C29)TT/Ts]. These oils and condensates show ETR values ranging from 0.3 to 12.4, suggesting variable source contributions deposited under different environmental conditions. Coupled with tri- and tetracyclic terpane distributions, ETR can be used as an organofacies discriminator. Most of the samples have high ETR values (>2.0), a low relative abundance of the C19 tricyclic terpane (C19TT) and the C24 tetracyclic terpane (C24TeT) characteristic of a clastic marine depositional environment, possibly with upwelling conditions. A specific organofacies for a few samples is characterized by low ETR values (<2.0), high C19TT/C23TT (>0.8) and C24TeT/C26TT (>1.0) ratios reflecting deposition under dysoxic conditions. However, various other alteration processes such as thermal maturity, thermochemical sulfate reduction (TSR), biodegradation and in-reservoir mixing probably have a significant impact on this parameter, which makes understanding the meaning of the ETR complicated. The effect of thermal maturity on ETR is minimal in the oil generation window, but the ETR values decrease dramatically at the post oil generation stage. TSR seems to preferentially lead to the loss of the C28TT and C29TT before the 18α(H)-22,29,30-trisnorneohopane (Ts) and thus the ETR value decreases as TSR progresses. While the dibenzothiophene/phenanthrene ratio can help to differentiate TSR alteration from thermal cracking processes, the ETR is no longer a valid geochemical parameter when these two processes occur. ETR also increases with biodegradation. The C28 and C29TT are more resistant to biodegradation than Ts. Supporting evidence can be derived from a positive correlation between ETR and C29 25-norhopane to C30 hopane ratio. In-reservoir mixing is another important alteration process which normally results in lower ETR values after a fresh charge is mixed with a biodegraded charge in a reservoir. Evaporative fractionation may also have an impact on ETR values. Some unusually high ETR values might be attributed to evaporative fractionation. The ETR values from our case study suggests that the ETR parameter is much more complicated than just being an age-diagnostic parameter. Maturation and secondary alteration processes need to be fully accounted for before the ETR can be properly applied to interpretations of depositional environment conditions such as upwelling.

The effects of igneous dike intrusion on organic geochemistry of black shale and its implications: Late Jurassic Jhuran Formation, India

This study documents the effects of igneous dike intrusion on organic geochemical parameters of a shallow-marine originated, Late Jurassic black shale (av. TOC=2.2wt%) in the Kutch rift basin in western India. The dike intrusion forms extremely to mildly altered, meter-scale zones around them. The primary wrinkled micro-texture of the shale is lost completely, and organic matter is charcoalized in the extremely altered zone recording a Rock-Eval Tmax>458°C. Immature type-III kerogen dominating the unaltered shale records abundant coniferal biomarkers such as retene, ent-beyerane, phyllocladane, ent-kaurane, abietane, labdane and norisopimarane, besides n-alkanes, hopanes and steranes. The transition from the unaltered (Rock-Eval Tmax<432°C) to the altered zone involves a decrease in carbon preference index (CPI), pristane/n-C17, phytane/n-C18, C30 hopane/C29 ααα-sterane and gradual increase in C29 diasterane/sterane, trisnorneohopane/trisnorhopane (Ts/Tm), Ts/C30 hopane, C29/C30 hopane and C29 20S/(20S+20R) sterane ratios. Although most saturate biomarkers disappear in the extremely altered zone, it records a high abundance of thermogenic polycyclic aromatic hydrocarbons (PAHs), commonly found in palaeo-fire deposits. Besides the PAH ratios, methylphenanthrene index-1, pristane/phytane, CPI and C29/C30 hopane ratios are found to be influenced significantly by thermal maturation in the extremely altered zone. The fluoranthene/(fluoranthene+pyrene) ratio correctly distinguishes different intensities of combustion products, while other PAH ratios behave differently at the immediate contact of the dike. Pr/Ph ratio may correctly interpret the redox condition up to a Rock-Eval Tmax~520°C, while it is ineffective at high maturity. The results of the investigation are significant for the assessment of stability of biomarkers in highly matured source rocks, recording potential redox-sensitive biomarkers in highly matured shales and detection of palaeo-fire deposits in ancient sedimentary succession.

Organic geochemistry of the Lower Cambrian Sinyaya Formation (northern slope of the Aldan anteclise)

We investigated organic matter (OM) of the Lower Cambrian Sinyaya Formation in the southeast of the Siberian Platform. The studied collection of rocks was divided into groups according to the contents of organic carbon, bitumens, and sulfur and lithologic characteristics. The content and distribution of saturated biomarkers were examined. Lanostanes C30, norlanostanes C29, 28,30-bisnorhopanes, and 2α- and 3β-methylhopanes have been identified in the bitumens. Relationships between the content of organic carbon and the distribution of hopanes, hopane ratios, and 2α-methylhopane index have been established. The conditions of sedimentation, diagenesis, and catagenesis of OM and the generation potential of the rocks have been estimated. It is shown that lanostanes, 28,30-bisnorhopanes, and methylhopanes can be used as biomarkers of the source rocks of the Sinyaya Formation and thus can help to determine the source of bitumens on the northern slope of the Aldan anteclise of the Siberian Platform.

The petroleum system of the Tenere Basin: oil geochemistry from the SH-1 wildcat well in eastern Niger

The SH-1 wildcat well, firstly encountering oil, proved the existence of a working petroleum system in the Tenere Basin, but the origin of oil and its generation history is not fully understood. In this regard, one crude oil and nine Upper Cretaceous source rock samples from the SH-1 wildcat well in the Tenere Basin were characterized by a variety of biomarker parameters and stable carbon isotopic composition. The oil from the Tenere Basin displays a low pristine/phytane (Pr/Ph) ratio, low C 29 /C 30 and C 35 /C 34 hopane ratios, low C 22 /C 21 and high C 24 /C 23 tricyclic terpane ratios, and high gammacerane index (gammacerane/C 30 hopane), suggesting shale source rocks deposited under an anoxic and relatively high-salinity environment. Relatively high C 29 sterane concentrations, a low value of the regular steranes/17 α (H)-hopanes ratio and a canonical variable from stable carbon isotopic composition larger than 0.47 indicate that the oil is mainly derived from terrestrial higher plants. Both aromatic maturity parameters and sterane isomerization parameters suggest that the oil was generated from an early mature source rock. Correlation between oils from the Tenere Basin and those from the Termit Basin confirms that these oils belong to the same genetic family. The oil–source rock correlation suggests that the oil from the Tenere Basin originated from the Upper Cretaceous marine source rocks, and the Yogou Formation can be considered as effective source potential.

GC-MS vs. GC-MS-MS analysis of pentacyclic terpanes in crude oils from Libya and Serbia - a comparison of two methods

The values of parameters calculated from distribution and abundance of the selected pentacyclic terpanes in crude oils from Libya and Serbia, which were originally derived from gas chromatography-mass spectrometry (GC?MS) were compared with results of quantification based on gas chromatography? ?mass spectrometry-mass spectrometry (GC?MS-MS). The parameters analyzed are the most often used terpane source and maturity parameters, which were applied to a large sample set of 70 oils, originating from five oil fields. The aim of the paper was to investigate to which extent the measurements of the selected parameters by these two instrumental techniques agree and to determine the influence of differences between parameter values on geochemical interpretation. For that purpose two methods, concordance correlation coefficient and mean-difference plot were used. The obtained results indicate that calculation of C2718?(H)-22,29,30-trisnorneohopane/(C2718?(H)-22,29,30- -trisnorneohopane + C2717?(H)-22,29,30-trisnorhopane), C2918?(H)-30-norneohopane/ C2917?(H)21?(H)-30-norhopane and C2917?(H)21?(H)-30-norhopane/ C3017?(H)21?(H)-hopane ratios either by GC?MS or GC?MS-MS do not significantly influence interpretation. On the other hand, the determination of C3017?(H)21?(H)-moretane/C3017?(H)21?(H)-hopane ratio, gammacerane index and oleanane index by GC?MS vs. GC-MS-MS could notably affect interpretation. These differences can be explained by the co-elution and the peak overlapping in GC?MS but also by better separation, higher precision and better selectivity of the GC?MS-MS. Deviation of the almost all studied parameters from the line of equality was similar for the oils from the same oil field but some differences were observed for the oils from different oil fields. Therefore, when GC?MS-MS results are to be used in organic geochemical interpretations, a regional calibration of GC?MS vs. GC-MS-MS relationship for each petroleum system is highly recommended.

Depositional environments and maturity evaluated by biomarker analyses of sediments deposited across the Cenomanian–Turonian boundary in the Yezo Group, Tomamae area, Hokkaido, Japan

AbstractBiomarker analyses for evaluating maturity of organic matter and depositional environments such as redox conditions, were performed in sediments across the Cenomanian–Turonian boundary (CTB) in the Saku Formation of the Yezo Group distributed along the Shumarinai‐gawa River and the Omagari‐zawa River, both in the Tomamae area, Hokkaido, Japan. Maturity indicators using steranes and hopanes, show that organic matter in sediments from the Shumarinai‐gawa and Omagari‐zawa sections are of lower maturity than those from the Hakkin‐gawa section (Oyubari area). Moreover, the ββ hopane ratios clearly show that the maturity of the Shumarinai‐gawa samples is lower than that of the Omagari‐zawa samples. These variations in the maturity of organic matter presumably reflect the difference in their burial histories. The results for the pristane/phytane (Pr/Ph) ratios suggest that the Shumarinai‐gawa samples were deposited under dysoxic to anoxic environments across the CTB, while the depositional environments of the Omagari‐zawa samples were relatively oxic. By another paleoredox indicator using C35 homohopanoids including a homohopene index (HHenI), higher values are observed in the Shumarinai‐gawa section, particularly in the horizons of the preceding period and an early stage of the first negative shift phase and the latest oceanic anoxic event 2 (OAE2) interval. These results suggest that the Shumarinai‐gawa samples record dysoxic to anoxic environments across the CTB. In contrast, the signals for the C35 homohopanoid index values show a relatively oxic condition in the Omagari‐zawa section. The trends of stratigraphic variations in redox conditions are different from those in the OAE2 interval in the proto‐Atlantic and Tethys regions as reported previously. Hence, the redox variations in the Tomamae area were basically related to a local environmental setting rather than global anoxia. However, the prominent anoxic emphasis observed in the HHenI profile of the Shumarinai‐gawa section can be a distinctive, and possibly global, event in the North‐West Pacific just before the OAE2.