Regular Steranes Research Articles

Biomarkers and Re–Os geochronology of solid bitumen in the Beiba Dome, northern Sichuan Basin, China: Implications for solid bitumen origin and petroleum system evolution

Solid bitumen is ubiquitous in the Sinian Dengying Formation dolomite, a crucial gas reservoir in the Sichuan Basin. We use biomarkers and Re–Os geochronology, coupled with burial-thermal evolution history to trace the source and formation timing of the solid bitumen, and to reconstruct the petroleum system evolution in the Dengying Formation of the Beiba Dome, northern Sichuan Basin. The bitumen reflectance (Rb) values of solid bitumen range from 1.92 to 3.95%, with the solid bitumen showing no fluorescence, indicating that the solid bitumen in the Dengying Formation dolomite has a high thermal maturity, which consequently belongs to pyrobitumen formed by thermal cracking of crude oil. The isoprenoids and terpanes indicate that the solid bitumen was sourced from a marine shale deposited under a reducing environment. The C27–C29 regular sterane distribution of the solid bitumen is consistent with the Lower Cambrian Guojiaba Formation (Є1g) organic-rich shale, indicating that the solid bitumen was derived from the Є1g shale, rather than dolomite or shale within the Dengying Formation. The burial-thermal evolution history indicates that initial oil generation began in the Late Cambrian to the Early Silurian. Since the Early Permian, the Є1g organic-rich shale re-entered the oil generation window, and formed a giant paleo-oil accumulation during the Late Triassic. Thermal cracking of crude oil occurred from the Late Triassic to the Early Cretaceous forming the extensive solid bitumen. The Re–Os isotopes of solid bitumen yield two similar isochron ages of 128.1 ± 7.3 Ma and 126.2 ± 8.0 Ma, which represents the timing of thermal cracking termination, i.e. the final timing of dry gas generation. This work highlights that the Re–Os geochronology has great potential to constrain the absolute timing of evolution of the petroleum system.

Organic petrology and geochemistry of Lower Cretaceous lacustrine sediments in the Chaoyang Basin (Liaoning Province, northeast China): Influence of volcanic ash on algal productivity and oil shale formation

Airfall volcanic ash deposited in the form of tuff layers is widely distributed in oil-shale bearing basins in western Liaoning Province, northeast China. In order to investigate the impacts of volcanic ash on algal productivity and the formation of organic-rich oil shales, the organic petrology and organic geochemistry of lacustrine sediments of the Lower Cretaceous Jiufotang Formation were investigated. Molecular geochemical data (the pristane/phytane ratio, the gammacerane index and the relative amount of β-carotane) indicate that they were formed in suboxic/anoxic depositional environments, with salinity fluctuations from freshwater to saline. Oil shales in the semi-deep to deepwater lake facies were mainly deposited under still and anoxic bottom water conditions, concomitant with water column salinity stratification. The relative abundances of diterpenoids and regular steranes indicate a limited terrigenous conifer plant contribution and a predominantly algal contribution to the organic matter. Blooms of algae contributing to high primary productivity, and anoxic depositional environments contributing to organic matter preservation, were key to organic-rich oil shale formation. Fish bones, ostracod shells, and alginite are present below the thick tuff layer. Alginite richness gradually increased from very low just above the tuff layer, to a higher concentration further above the tuff layer, indicating that during major volcanic eruptions, large quantities of volcanic ash led to water column poisoning, causing the sudden mass mortality of some aquatic organisms that were sensitive to turbid, acidified and toxic water. The increased proportion of brackish/saline-water dinoflagellates, which is corroborated by the high concentration of telalginite, abundant dinoflagellate-specific biomarkers (C31 4,22,23,24-tetramethylcholestanes), high values of the C27/C29 ααα-20R and C28/C29 ααα-20R sterane ratios, 4-methylsterane index, dinosterane index, and tetramethylsterane index, suggest that brackish/saline-water dinoflagellates show a positive response to volcanic ash input. Some specific brackish/saline-water dinoflagellates are present in organic-rich tuff laminae, indicating that during periods of minor volcanic eruptions or in intermissions between major eruptions, a minor volcanic ash input was more likely to result in rapid and efficient volcanic fertilisation effects and high-quality oil shale formation. Although the lacustrine oil shale system of the Jiufotang Formation shares some similar petrological and geochemical characteristics with the classical saline lacustrine oil shale system of the Green River Formation in the United States of America, the significant influence of airfall volcanic ash on algal productivity makes the formation mechanism of oil shales within the Jiufotang Formation strata quite different from that of oil shales within the Green River Formation.

Evaluation of sedimentary features and biomarkers in the Paleogene Niubao formation in the Lunpola basin, Tibetan plateau: implications for the oil source rocks and exploration

The Paleogene Niubao formation is mainly exposed in the central part of the Lunpola basin in the hinterland of the Tibetan Plateau and has high potential for oil and gas exploration and development. The biomarkers of source rocks are systematically studied by selecting representative samples. The middle and upper Niubao formation contain rhythmically inter-bedded mudstone and sandstone that show periodic variations characteristic of the lake water environment. Additionally, the large volumes of muddy dolomite in the upper Niubao formation (UNF) are associated with high salinity in the lake environment under a hot and arid climate. The organic geochemical analysis shows that the middle Niubao formation (MNF) and UNF have high organic abundance (total organic carbon (TOC) contents of 1.48–8.96% in the MNF and 1.52–12.83% in the UNF) and type I–II kerogen. The analysis shows that the UNF was deposited in a persistent, anoxic, stratified and highly saline paleo-lake environment, which was beneficial to the preservation of organic matter. The high phytoalkane and γ-paraffin contents in the UNF indicate a strongly reducing and highly saline lacustrine depositional environment. The high pregnane contents and ascending sterane in the source rocks in the MNF indicate high maturity, and the distribution of n-alkanes and regular sterane indicates the input of some terrestrial organic matter. The high maturity indicates that the source rock in the MNF may represent the main hydrocarbon-generating layer in the basin.

Origin, biodegradation, and water washing of bitumen from the Mishraq Sulfur Mine, northern Iraq

Bitumens from Middle Miocene Fat'ha Formation at Mishraq Sulfur Mine were investigated to determine their sources and the extent to which they may have been diagenetically altered. For these purposes, fourteen bitumen samples were subjected to detailed biomarker analysis using gas chromatography-mass spectrometry (GC-MS). Stable carbon and deuterium isotopes were also measured on chromatographic fractions. These bitumens are present in cavities and fractures of limestone from the native sulfur productive series. Polar fractions accounts for about half of the bitumen composition. These bitumens are showing a partial removal of n-alkanes; whereas their terpanes and steranes are intact. In addition, dibenzothiophene (DBT) and phenanthrene (Phen.) concentrations and DBT/Phen. ratio are variable among the samples. Most deuterium isotope values of asphaltene and resin fractions are similar to those of non-biodegraded bitumen seeps from the Qaiyarah area. These criteria suggest that bitumens of Mishraq mine are slightly affected by biodegradation and they are water washed to different extents but are not affected by abiotic oxidation. The biodegradation of these bitumens is mainly due to sulfate-reducing (anaerobic) bacteria.Terpane and sterane ratios suggest that bitumens were generated from early-mid mature carbonate source rocks deposited under reducing conditions. They show high values of C29/C30 and C35S/C34S hopane (>1.0), low values of diasteranes/regular steranes, Ts/Tm, moretane/hopane, and low values of Pr/Ph for most samples. Regular sterane distributions show dominance of C29 over C27 steranes. Middle and Upper Jurassic Sargelu and Chia Gara formations are proposed as source rocks for these bitumens; but Chia Gara is most likely.

Geochemical characteristics and significance of the bitumen in Sinian reservoirs in Sichuan basin and its periphery

The bitumen of Doushantuo Formation distributed along fractures has been firstly discovered in the dolomite reservoirs of Dou-3 Member in Zigui region, Yichang, Hubei. Research has been conducted using multiple methods on the characteristics of the newly discovered bitumen of Doushantuo Formation, bitumen of Dengying Formation-Longwangmiao Formation in Anyue gas field in Sichuan Basin, and bitumen of Sinian and source rocks of Sinian-Cambrian in Northeast Sichuan. The results show:①The reflectivity of the bitumen of Dou-3 Member is about 2.8%; ② The carbon number of n-alkanes in the bitumen of Dou-3 Member is C13-C33, where nC17 is the main peak; the relative proportion of the regular steranes is C27≈C29>C28, and C27, C28 and C29ααα20R steranes are distributed in “L” shape; abundant tricyclic terpanes and pentacyclic triterpanes have been detected, and the abundance of gammacerane is high; Aromatics are mainly phenanthrene series compounds. ③ The heteroatomic compounds in the bitumen of Dou-3 Member are mainly O2 carboxylic acids, and the oxygen compounds in the bitumen of Dengying Formation in Anyue gas field are close to those of the source rocks of Doushantuo Formation. The research results are of vital guiding significance to the study of potential source rocks of Doushantuo Formation in Sichuan Basin and the exploration of oil and gas from the source rocks of Doushantuo Formation.

Sedimentology and geochemistry of Cretaceous source rocks from the Tiancao Sag, Yin'e Basin, North China: Implications for the enrichment mechanism of organic matters in small lacustrine rift basins

Petrography and compositions of organic matters of Cretaceous source rocks from the Tiancao Sag, the Yin'e Basin, Northern China, have been presented in order to determine the Cretaceous sedimentary environment and processes that enriched organic matters. The mudstones of the Bayangobi Formation have medium–high total organic carbon (TOC) (0.27–2.59%) and S1 + S2 (0.30–12.23 mg HC/g rock) values. Vertically, three sedimentary units (2460–2600 m; 2640–2860 m; and 3020–3050 m) present good hydrocarbon potentials. The macerals, the C27-C29 regular sterane distribution, and the isotopic and element geochemistry of the kerogen prove that the organic matter has a mixed origin of plankton/algae and terrestrial plants. The vitrinite reflectance, Tmax, and biomarker parameters collectively suggest that the source rocks have reached the mature stage. The mudstones were deposited in a reducing brackish-saline environment, and the salinity-sensitive proxies indicate a gradual evolution of the sedimentary conditions from low to high salinity. The positive correlation between the hydrogen index value and TOC content and the negative correlation between Pr/Ph ratio and TOC content suggest that the primary productivity and preservation in reducing water collectively contributed to organic matter enrichment of the Bayangobi Formation. The relationship between sedimentation rate and TOC content suggests that a sedimentation rate of 60–70 m/Myr is conducive to the formation of effective source rocks. Therefore, the organic matter enrichment of Bayangobi Formation was controlled by moderate-high paleoproductivity, reducing conditions, and an appropriate sedimentation rate. This paper proposes a “supply-preservation-dilution” model for the source rocks in the Tiaocao Sag, Yin’e Basin.

HIGHER LANDPLANT –DERIVED BIOMARKERS IN LIGHT OILS AND CONDENSATES FROM THE COAL‐BEARING EOCENE PINGHU FORMATION, XIHU SAG, EAST CHINA SEA SHELF BASIN

The Cenozoic Xihu Sag in the East China Sea Shelf Basin contains large reserves of coals together with liquid petroleum derived from coal‐associated sediments. However, the origin of the petroleum is not well understood. In this study, biomarker assemblages in a suite of recently discovered light oils and condensates from the Paleogene succession in the western margin of in the Xihu Sag were investigated using gas chromatography – mass spectrometry. The objectives were to investigate the samples' thermal maturity and the depositional environment of the precursor source rocks which generated the oils. The light oils are believed to have been derived from coaly source rocks in the Eocene Pinghu Formation.Assessment of thermal maturity based on CPI, pristane/n‐C17 ratio and isomerisation ratios of C29 steranes and C31 homohopanes suggest that the hydrocarbons have a relatively low maturity in the early to mid oil generation window. The distribution of isoprenoids relative to n‐alkanes, the high pristane/phytane ratios (5.1–10.7), the almost complete absence of gammacerane and C33+ homohopanes, and the low dibenzothiophene/phenanthrene ratios indicate that the source rocks of the hydrocarbons were deposited in a relatively oxic and sulphate‐poor fluvio‐deltaic environment which was favourable for coal measure development.Abnormally abundant gymnosperm‐derived diterpanes including labdane, 19‐norisopimarane, fichtelite, rimuane, pimarane, isopimarane, 17‐nortetracyclic diterpene, phyllocladanes and abietane were detected in the samples analysed. 16a(H)‐Phyllocladane was identified unambiguously and kauranes were confirmed to be absent. In addition, three 19‐norisopimarane isomers, 13β(H)‐atisane, and 20‐normethylatisane were tentatively identified in the studied samples. The distributions of n‐alkanes, isoprenoids and regular steranes, the presence of 4β(H)‐eudesmane and oleanane, high Pr/Ph ratios and the abundant diterpanes together suggest that the hydrocarbons were derived from a coaly source rock. Gymnosperms of the conifer families Cupressaceae (especially the former Taxodiaceae) and Pinaceae are interpreted to be the major source of the diterpanes and to have made a significant contribution to the coaly source rock. However, the low abundance of oleanane relative to diterpanes may underestimate the contribution from angiosperms relative to gymnosperms. This could be due to differential preservation and alteration of the di‐ and triterpenoid precursors during diagenesis and the occurrence of non‐specific precursors in higher land plants.

Molecular marker and carbon isotope geochemistry of ordovician oils in the Tuofutai Bulge, northern Tarim Basin: Implications for oil maturity, source characteristics and filling directions

The Tuofutai Bulge in the Tabei Uplift of Tarim Basin, western China, has become a hot spot for petroleum exploration in recent years. Fifteen Ordovician oil samples from 15 wells in the Tuofutai Bulge were analyzed for biomarkers and carbon isotopes using gas chromatography–mass spectrometry and stable carbon isotope analysis to constrain its depositional environment, thermal maturity, source rocks and the possible petroleum charging directions. The samples were characterized by abundant terpenoids, relatively high contents of C29 regular steranes and dibenzothiophene, relatively low ratios for Pr/Ph and C22/C21 tricyclic terpanes, and high ratios of C35/C34 homohopanes. Results suggest that the source rocks for the oils were deposited in a reducing marine carbonate and marl environment. Cambrian and Ordovician source rocks contribute to the oils, and the Middle to Upper Ordovician source rocks exert a greater contribution. n-alkanes and abundant 25-norhopanes in the saturated fractions suggest that the oil is a mixture of early biodegraded oil and late normal crude oil charged at later time. Data from methyldiamantane parameters suggest that the crude oil is in a high maturity stage. The tracing of oil charging suggests that preferential migration directions are primarily from south to north, connected with oil migration along the Shenbei No.1 Fault from the Shuntuoguole Lower Uplift to the south of Tabei Uplift.

Method for identifying effective carbonate source rocks: a case study from Middle\u2013Upper Ordovician in Tarim Basin, China

Hydrocarbon expulsion occurs only when pore fluid pressure due to hydrocarbon generation in source rock exceeds the force against migration in the adjacent carrier beds. Taking the Middle–Upper Ordovician carbonate source rock of Tarim Basin in China as an example, this paper proposes a method that identifies effective carbonate source rock based on the principles of mass balance. Data from the Well YW2 indicate that the Middle Ordovician Yijianfang Formation contains effective carbonate source rocks with low present-day TOC. Geological and geochemical analysis suggests that the hydrocarbons in the carbonate interval are likely self-generated and retained. Regular steranes from GC–MS analysis of oil extracts in this interval display similar features to those of the crude oil samples in Tabei area, indicating that the crude oil probably was migrated from the effective source rocks. By applying to other wells in the basin, the identified effective carbonate source rocks and non-source rock carbonates can be effectively identified and consistent with the actual exploration results, validating the method. Considering the contribution from the identified effective source rocks with low present-day TOC (TOCpd) is considered, the long-standing puzzle between the proved 3P oil reserves and estimated resources in the basin can be reasonably explained.

Geochemical evaluation of Khami Group oils in the South Dezful Embayment, Iran

An integrated geochemical study, including GC, GC–MS and stable carbon isotope analyses, was conducted on a suite of oil samples from the Khami Group reservoirs to provide new insights into the Upper Jurassic–Lower Cretaceous petroleum system in the South Dezful Embayment. Possible source rocks were also characterized using Rock–Eval pyrolysis to address the likely potential source rocks. The oil samples representing four major reservoirs (Surmeh, Fahliyan, Gadvan and Dariyan) constitute a single genetic oil family according to bulk and biomarker parameters. High API (> 38°), highly saturated hydrocarbons (Sat > 65%), very low asphaltene content (Asp < 2%) and raised saturates/aromatics ratio (Sat/Aro > 2) are the bulk diagnostic characteristics of the studied oils implying more dominant terrigenous nature of the precursor organic matter. The predominance of C29 regular steranes (~ 40%), presence of C29Ts as well as very low gammacerane (< 10%) and moderate C35/C34 homohopane (< 1) are consistent with the mixed marine–terrigenous dysoxic organic matter input. The oils are assumed to be originated from Early Cretaceous source rocks at the peak of the oil generation window in a kitchen area located to the south of the studied region (i.e., the Binak–Borazjan Trough). The hydrocarbons were migrated from this kitchen to the structurally shallow-seated reservoirs in the center and west of the Kharg-Mish local paleo-high. The Khami Group reservoirs are not effectively sealed by the thin Hith anhydrites, and the Kazhdumi Formation finally trapped the migrated hydrocarbons. This study improves our knowledge regarding one of the active petroleum systems in the South Dezful Embayment, enhancing petroleum exploration success by navigating further drillings into the more prosperous targets.

Understanding the geobiology of the terminal Ediacaran Khatyspyt Lagerstätte (Arctic Siberia, Russia).

The Khatyspyt Lagerstätte (~544Ma, Russia) provides a valuable window into late Ediacaran Avalon-type ecosystems with rangeomorphs, arboreomorphs, and mega-algae. Here, we tackle the geobiology of this Lagerstätte by the combined analysis of paleontological features, sedimentary facies, and lipid biomarkers. The Khatyspyt Formation was deposited in carbonate ramp environments. Organic matter (0.12-2.22 wt.% TOC) displays characteristic Ediacaran biomarker features (e.g., eukaryotic steranes dominated by the C29 stigmastane). Some samples contain a putative 2-methylgammacerane that was likely sourced by ciliates and/or bacteria. 24-isopropylcholestane and 26-methylstigmastane are consistently scarce (≤0.4% and ≤0.2% of ∑C27-30 regular steranes, respectively). Thus, Avalon-type organisms occupied different niches than organisms capable of directly synthesizing C30 sterane precursors among their major lipids. Relative abundances of eukaryotic steranes and bacterial hopanes (sterane/hopane ratios=0.07-0.30) demonstrate oligotrophic and bacterially dominated marine environments, similar to findings from other successions with Ediacara-type fossils. Ediacara-type fossils occur in facies characterized by microbial mats and biomarkers indicative for a stratified marine environment with normal-moderate salinities (moderate-high gammacerane index of 2.3-5.7; low C35 homohopane index of 0.1-0.2). Mega-algae, in contrast, are abundant in facies that almost entirely consist of allochthonous event layers. Biomarkers in these samples indicate a non-stratified marine environment and normal salinities (low gammacerane index of 0.6-0.8; low C35 homohopane index of 0.1). Vertical burrowers occur in similar facies but with biomarker evidence for stratification in the water column or around the seafloor (high gammacerane index of 5.6). Thus, the distribution of macro-organisms and burrowers was controlled by various, dynamically changing environmental factors. It appears likely that dynamic settings like the Khatyspyt Lagerstätte provided metabolic challenges for sustenance and growth which primed eukaryotic organisms to cope with changing environmental habitats, allowing for a later diversification and expansion of complex macroscopic life in the marine realm.

Geochemical characteristics and depositional environments of the Narimba Formation source rock, Bass Basin, Australia

This research aims to conduct source rock characterization on the Narimba Formation in the Bass Basin, Australia, which is made of mostly sandstone, shale and coal. The geochemical characteristics and depositional environments have been investigated through a variety of data such as rock–eval pyrolysis, TOC, organic petrography and biomarkers. Total organic carbon (TOC) values indicated good to excellent organic richness with values ranging from 1.1 to 79.2%. Kerogen typing of the examined samples from the Narimba Formation indicates that the formation contains organic matter capable of generating kerogen Type-III, Type-II-III and Type-II which is gas prone, oil–gas prone and oil prone, respectively. Pyrolysis maturity parameters (Tmax, PI), in combination with vitrinite reflectance and some biomarkers, all confirm that all samples are at early mature to mature and are in the oil and wet gas windows. The biomarkers data (the isoprenoids (Pr/Ph), CPI, isoprenoids/n-alkanes distribution (Pr/nC17 and Ph/nC18), in addition to the regular sterane biomarkers (C27, C28 and C29) are mainly used to evaluate the paleodepositional environment, maturity and biodegradation. It has been interpreted that the Narimba Formation was found to be deposited in non-marine (oxygen-rich) depositional environment with a dominance of terrestrial plant sources. All the analyzed samples show clear indication to be considered at the early mature to mature oil window with some indication of biodegradation.

Geochemical characterization and origin of crude oils in the Oriente basin, Ecuador, South America

The Cretaceous oil reservoir located in eastern slope belt of the Oriente basin accounts for most of the discoveries in last decade in Ecuador, which makes the Oriente basin one of the most important oil exploration targets in South America. Crude oil samples (n = 31) in 31 wells together with mudstone and limestone source rocks (n = 18) from Cretaceous Napo Formation in 16 wells were analyzed to evaluate the geochemical characteristics of the oil reservoir oil.Geochemical analyses included measurements of total organic carbon (TOC) content, Rock-Eval Pyrolysis, gas chromatography (GC), and gas chromatography-mass-spectrometry (GC-MS).All the oil samples have pre-uimodal n-alkane distribution dominated by lower molecular weight components (peak at C15–C17) with a small peak at C25–C30. The carbon preference index (CPI) and odd-over-even predominance (OEP) values range between 1.0 and 1.34, and 1.0–1.21 respectively; Pr/Ph ratios range from 0.67 to 1.16; high Pr/n-C17 (0.80–3.47) and Ph/n-C18 ratios (0.82–7.86) values and dominate C27 regular steranes, all these biomarkers show that the oil derived from marine algal material. Furthermore, all oils have similar Terpanes and Steranes distribution shape, peak at C23TT, and low at C22TT (TT = tricyclic terpanes), relative low C22/C21, C24/C23 and C26/C25 tricyclic terpane ratios, higher C27R (R = regular steranes) content than C29R and C28R. These biomarkers demonstrate that all the oils are from same genetic family although they came from different zones. There are two kinds of source rocks in the Oriente basin, shale and argillaceous bioclastic limestone of Napo Formation. These rocks are good-excellent mature according to TOC and Rock-Eval assessment. The two source rocks show obvious different Terpanes and Steranes distribution shape in mz191 and mz217 maps. The Oil-Source correlation analysis demonstrates that the crude oils have similar Terpane and Steranes distribution shape and biomarker value, especially the C27R–C28R–C29R ratio, with argillaceous bioclastic limestone of Napo Formation. This new geochemical data suggests that the argillaceous bioclastic limestone is the major source rock in the Oriente basin. Thus, distribution of this stratigraphic unit should be the key point for future exploration.

Paleolimnological environments and the formation of high quality source rocks in the Bohai Bay Basin: An integrated geochemical study of biomarkers, stable carbon and oxygen isotopes, and trace elements

Fourteen Paleogene rock samples from the Bohai Bay Basin, including samples from the third member of the Oligocene Dongying Formation (Ed3) and the third member of the Eocene Shahejie Formation (Es3), underwent total organic carbon (TOC) analysis, Rock-Eval pyrolysis, stable carbon (δ13Ccarb) and oxygen isotope (δ18Ocarb) analysis of carbonates, stable carbon isotope analysis of organic matter (δ13Com), biomarker and trace element compositional analysis to reconstruct paleolimnological environments and investigate the factors that control the occurrence of high quality source rocks. The Rock-Eval and TOC results revealed that the source rocks meet the standard for good to excellent source rocks. The biomarker parameters and trace elements suggest that warm and humid paleolimnological conditions prevailed during the deposition of Es3. In addition, heavier values of δ13Com and light values of δ18Ocarb indicate the existence of a hydrologically open paleolake with high primary productivity, which is supported by relatively high 4-methyl sterane index values and high barium/calcium (Ba/Ca) ratios. Ed3 rocks formed under less reducing and more acidic conditions, which is supported by high pristane/phytane, C27 diasteranes/C27 regular steranes and kaolinite/illite ratios and low strontium/barium (Sr/Ba) and vanadium/chromium (V/Cr) ratios. Moreover, Ed3 is characterized by a relatively high input of terrestrial organic matter under humid but cooler settings. A long-term positive shift of δ18Ocarb revealed by the Ed3 samples is associated with changes in oxygen isotopic composition of inflow water (higher precipitation) and temperature (colder). The conditions of both high productivity and enhanced preservation controlled the distribution of good source rocks in the Bohai Bay Basin (BBB). Es3 is more likely linked with high primary productivity, which resulted in an overall positive shift in δ13Com, whereas Ed3 may be more related to favourable preservation conditions associated with relatively high sedimentation rates.

Pore structure characteristics and hydrocarbon generation potential of middle Jurassic lacustrine source rocks in the Yuka depression, Qaidam Basin, NW China: Implications from petrographic and organic geochemical analyses

The Yuka depression is one of the most important energy production bases in the Qaindam Basin. Although shales in the 7th member of the Dameigou Formation (J2d7) are key members of Middle Jurassic Petroleum System, organic geochemistry and petrography of different rock facies have not been fully analyzed. In this study we evaluate hydrocarbon generation potential of the J2d7 by applying integrated petrographic, geochemical and mineralogical analyses. In addition, pore structure is characterized using field emission scanning electron microscopy (SEM), CO2 and N2 physisorption. Results indicate that mudstone, carbonaceous-mudstone and siltstone have total organic carbon (TOC) values of 2.64%, 5.61% and 0.48%, respectively, as well as variations in the hydrogen index (HI). Kerogen types are mainly Type Ⅲ, with maturity varying from marginally mature to mature. Notably, excess methane adsorption capacities were higher in the carbonaceous-mudstone sample (4.66 cm3/g) than in the mudstone sample (1.78 cm3/g). Primary minerals in the mudstone were quartz and clay; kaolinite, illite and illite/smectite (I/S) were the dominant clay minerals. Clay minerals and TOC content, as well as positive correlations between illite, I/S and mesopore SSA, were factors controlling pore structure development; in contrast, kaolinite contents were negatively correlated with SSA. Liptinite is the dominant maceral composition and a high content of C29 regular sterane in the carbonaceous-mudstones suggests primary higher-plant input, associated with a low gammacerane index (0.28–0.32) and a high pristane/phytane (Pr/Ph) ratio (3.02–3.96). Petrographic and geochemical results indicate an alternating depositional environment across suboxic semi-saline to oxic fresh water changes. Higher-plants were predominant and a small quantity of algae were preserved through in-situ accumulation and migration during shale development; an oxic water column with carbonaceous-mudstone was also present. Although organic matter was marginally mature to mature, the abundance of liptinite (average of 63%) was derived from higher-plants, possibly being the primary materials producing gaseous hydrocarbons.

Bound hydrocarbons and structure of pyrobitumen rapidly formed by asphaltene cracking: Implications for oil–source correlation

The pyrobitumen in conventional petroleum reservoirs is a thermally altered product of ancient oil pools, which may provide valuable source information using its molecular and isotopic signatures. Pyrobitumen formation is often associated with thermal alteration of asphaltene or polar-rich components rather than that of oil as a whole. Although asphaltenes are useful in source correlation of altered oils, the geochemical characteristics of asphaltene-derived pyrobitumen are poorly understood. In this study, artificial pyrobitumen formation through oil asphaltene cracking was performed at different thermal simulation (i.e., pyrolysis) temperatures. Systematic variations in the amounts and distributions of extractable and bound hydrocarbons, released by catalytic hydropyrolysis of the artificially produced pyrobitumen were studied. Pyrobitumen production yield was high at pyrolysis temperatures corresponding to post oil peak maturities, with a maximum yield of ∼70 wt% of asphaltene being reached in the early condensate–wet-gas stage. The molecular structure of pyrobitumen varied only slightly over the high maturity range (EasyRo 1.64–2.51%). Compared with the parent asphaltene, the pyrobitumen had a low biomarker (e.g., regular steranes and terpanes) content, and even the carbon isotopic values of the more stable bound n-alkanes were strongly altered. Thermal cracking of asphaltene alone, rather than whole oil, accelerates cross-linking of aromatic units, and cleavage or condensation of molecules bound in the pyrobitumen. These molecular changes suggest that care is needed when using the geochemical characteristics of bound hydrocarbons in natural pyrobitumen for source determinations. Nevertheless, the carbon isotopic ratios of bulk pyrobitumen hydropyrolysate were similar to those of bulk oil or pyrobitumen, even at very high maturities, suggesting these may be reliable indices for source tracing. A comparison of carbon isotopic compositions between pyrobitumen-bound and solvent-extractable n-alkanes could be useful in determining whether they have the same origin. Furthermore, the compositional and isotopic characteristics of bound hydrocarbons in pyrobitumen may provide information on the stage of maturity under geological conditions. The investigation of free and bound molecules may thus elucidate pyrobitumen genesis as related to its source, thermal maturity, and possible later hydrocarbon charging.

Biomarker distributions and depositional environments of continental source rocks in Sichuan Basin, SW China

In order to study the distributions of the biomarker of the continental source rocks in the Sichuan Basin, 71 source rock samples were collected from the Upper Triassic-Lower Jurassic strata in different regions. The n-alkanes, isoprenoids, terpane, sterane, sesquiterpenes, caranes and aromatics in the extracts were analyzed in detail. GC-MS analysis has been conducted to analyze the biomarker of the continental source rocks. The results of GC-MS analysis indicate that the Upper Triassic source rocks are high in the content of extended tricyclic terpanes, pristane, phytane, gammacerane, C28 regular sterane and carotene. However, they are low in content of rearranged compounds. The ratio of Pr/Ph is less than 1, with the characteristics of tricyclic terpane C21 &gt; C23. The Lower Jurassic source rocks are extremely low in content (even zero) of extended tricyclic terpanes, pristane, phytane, gammacerane, C28 regular sterane and carotene, and high in content of rearranged compounds. The ratio of Pr/Ph is more than 1, with tricyclic terpane C21 &gt; C23. These characteristics are still preserved after maturation. Moreover, during the sedimentation of the source rocks of T3x2–T3x3 members, the supply of continental plants was low (TAR &lt; 1, with regular sterane C27 &gt; C29, 1-MP/9-MP &lt; 1). The source rocks of T3x5 member were low in salinity (slightly low content of gammacerane and carotene), being different significantly from the other Upper Triassic source rocks. In addition, during the sedimentation of the source rocks of J1dn Member, the supply of continental plants was also low (regular sterane C27 &gt; C29, 1-MP/9-MP &lt; 1), being quite different from that of J1l member. Through analysis of the difference in biomarkers, it is indicated that the sedimentary environment had changed from anoxic and brackish water during the Late Triassic to oxygen-rich and freshwater during the Early Jurassic in the Sichuan Basin. During this process, the types of organic matters had changed for several times.

Geochemical characteristics of Middle-Upper Ordovician source rocks in the Kalpin outcrop profiles and marine oil-source correlation, Tarim Basin, NW China

The Middle-Upper Ordovician source rocks developed in six outcrop profiles in the Kalpin area, Tarim Basin, NW China were analyzed using an organic geochemical screening method. Most TOC values of source rocks of Salgan Formation of Middle-Upper Ordovician developed in the Dawangou, Kamatikan, and Tonggusibulong outcrop profiles are greater than 1.0%, whereas the TOC value of the source rocks of Yingan Formation of Upper Ordovician only developed in the Dawangou outcrop ranged from 0.14% to 1.29%, with an average value lower than that of the Salgan Formation source rocks. Combined with the analysis results of source rocks of Yuertusi Formation of Lower Cambrian developed in the Xiaoerbulake, Sugaite and West Yuti outcrop profiles, the source rocks of Yuertusi, Salgan and Yingan Formations showed similar molecular geochemical characteristics, with low Pr/Ph value, C23 greater than C21 tricyclic terpane, high gamacerane and C28 sterane content. The similarity in molecular compounds of Cambrian to Ordovician source rocks in the Kalpin area may be a result of the high maturity. In the relative distribution of C27–C29 regular steranes, the marine oils with C28 sterane lower than 25% showed great difference with that of the source rocks of Lower Cambrian Yuertusi Formation. In the distribution of carbon isotopic curves, the source rocks of Lower Cambrian Yuertusi Formation have lighter carbon isotopes, compared with that of the Middle-Upper source rocks, especially in kerogen carbon isotopes, Lower Cambrian Yuertusi Formation lighter 5‰ than that of Middle-Upper Ordovician source rocks. Overall, the carbon isotopes fractionation of saturate, bitumen, aromatic fraction, resin, asphaltene, and kerogen is small in source rocks from Middle-Upper Ordovician Salgan and Yingan Formations in the Kalpin area. It is suggested that the carbon isotope of kerogen of source rocks can be used as a parameter for oil-source correlation excluding the TSR of the oil reservoirs.

Organic geochemical characteristics and hydrocarbon generation potential of Jurassic shales and mudstones in Qixiang Co area southern Qiangtang Basin, Tibet

The Qixiang Co area is located in the South Qiangtang Depression Zone. Jurassic shales and mudstones with large thicknesses are widely developed in this area. The Jurassic was a potential exploration horizon in the Qiangtang Basin; however, few detailed analyses have been conducted on the geochemical characteristics of mudstones and shales in this area. In this paper, the hydrocarbon generation potential and sedimentary environment of shales and mudstones are analyzed using organic geochemical data. The results show that the abundance of organic matter is low, as indicated by the low TOC value (averaging 0.70%) and hydrocarbon potential (S1+S2) (0.13 mg/g). The main macerals of kerogen are exinites with an average value of 82%, and they are mainly type II2 kerogen. A summary of the results is presented: the thermal maturity of organic matter is high; the average Ro value is more than 1.2%; n-alkanes are dominated by a high carbon number and have no obvious odd-even advantage; the average value of Pr/Ph is 0.74; the distribution of C23 triterpenoids satisfies C20< C21< C23; the content of the gammacerane index is moderate (0.21) the variation range is small; and the C27 content in regular steranes is dominant. It can be concluded that the organic matter precursors in this area were a mix of higher plants and marine phytoplankton. Organic matter was deposited under a reductive environment with stable water and slightly high salinity. The hydrocarbon generation potential of the Jurassic shale and mudstone in this area is poor, which is unfavorable to oil and gas exploration.

Palaeoenvironmental reconstruction and evidence of marine influence in Permian coal-bearing sequence from Lalmatia Coal mine (Rajmahal Basin), Jharkhand, India: A multi-proxy approach

Lalmatia coal-bearing sequences belong to the Barakar Formation (Permian) of Lower Gondwana. The well-developed coal-bearing sequences (approx. 52 m) exposed along the mine section have been studied to understand the depositional environment. Along with this, a cuticle study was performed to understand the palaeoclimate with the help of the stomatal index. The distribution of n–alkanes (n–C15 to n–C31) suggests major higher plant and algal dominant source input, although variations are seen in the relative input. Steranes identified include C29 regular sterane and 4-Methylsteranes. The Pr/Ph ratio varied from 0.43 to 4.26. Warm temperate to subtropical palaeoclimate with fresh to brackish water bodies, and reducing oxygen-poor environment were inferred from the Rb/Sr, Sr/Cu, Th/U, Sr/Ba, V/(V+Ni). Mean δ13C value measured is −23.0‰ ± 0.60‰ for coal and −22.6‰ for shale. These values are well correlated with typical δ13C values of the Permian coal and shale. The stomatal index of Glossopteris leaves (280 million years) was taken into account, and it showed a value of 10.7 that assumes a higher level of atmospheric CO2 during the Barakar Formation (Artinskian age). The result also shows that Permian taxon Glossopteris can be used for CO2–proxy during Permian age. The geochemical evidence suggests that towards the middle part (middle coal seam) of the studied section, the depositional milieu was under a marine-influenced marginal condition. The geochemical studies and carbon isotopic ratios unequivocally suggest that fluctuating climatic conditions existed during the deposition of the Lalmatia coal-bearing sequence.