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• Intramolecular C isotopes of propane were analyzed in overmature shale gases. • Unusual 13C depletion observed in the central C of all propane samples. • Inter- and intramolecular isotopes support C 2+ alkanes formed by C 1 polymerization. Abiotically synthesized hydrocarbons are significant in the fields of astrobiology, biosphere, energy, environment, and geology. Abiotic hydrocarbons have never been unambiguously proven to occur in common sediments partly because identifying these components in crustal settings is challenging as they share similar characteristics with hydrocarbons of biotic origin. In this study, we measured intramolecular carbon isotopes of 10 propane samples from Longmaxi shale in the Sichuan Basin, China, and found distinct 13C-depletion in the central position carbon with its δ13C decreased by −8.0 to −7.0‰ (Vienna Pee Dee Belemnite) with respect to terminal carbons. The δ13C value of central carbon (H 3 C–#CH 2 -CH 3) in propane is equal to the second methyl (H 3 C–#CH 3) in ethane possibly resulting from surface catalytic polymerization reactions of methane. Our novel findings indicate that the abiotic synthesis of C 2+ hydrocarbons by the surface catalyzed polymerization of methane occurs in commercial gas reservoirs and suggest that this process may be more common than previously thought in methane-rich scenarios on Earth and other planets. [ABSTRACT FROM AUTHOR]

Lakes Dianchi (DC) and Bosten (BST) were determined to be at different stages of eutrophication, by use of total organic carbon content, bulk carbon isotopic composition, bulk nitrogen isotopic composition, and bound saturated fatty acid (BSFA) concentrations in sediment cores. A rapid increase in the supply of organic matter (OM) to DC began after the 1950s, while the environment and trophic status of BST remained constant as indicated by characteristics of OM input to sediments. The BSFA ratios of nC14 + nC16 + nC18/nC24 + nC26 + nC28 increase upward from 7 to 13 in the DC core, which are significantly greater than those from BST (2 to 3). This result is consistent with algae or bacteria being the dominant contribution of the OM increase induced by eutrophication in DC. The positive shift of nC16 compound-specific δ C in the upper section might be an indicator of excess algal productivity, which was observed in the two lakes. The positive shifts of compound-specific δ C of other BSFAs were also observed in the upper section of the core only from DC. The observed trends of compound-specific δC of BSFA originated from different sources became more consistent, which reflected the intensified eutrophication had profoundly affected production and preservation of OM in DC. The results observed for BST indicated that accumulation of algae did not affect the entire aquatic ecosystem until now. [ABSTRACT FROM AUTHOR]

Abstract: In this study, headspace single-drop microextraction (HS-SDME) coupled with gas chromatography–isotope ratio mass spectrometry (GC–IRMS), was employed to determine compound specific carbon isotopic values (δ13C) of gasoline range hydrocarbons. The reproducibility of the method was found to be satisfactory. By comparison with the δ13C values of the twelve target compounds determined using direct injection of their n-C16 solution, no obvious isotopic fractionation was observed during the HS-SDME procedures. Some parameters that could affect the carbon isotopic fractionation, such as ionic strength of working solutions and inlet split ratio, were examined. The results also suggest that these factors had no significant effect on the carbon isotopic determination of gasoline range hydrocarbons. The application of HS-SDME to a crude oil sample proved that this method could be a promising tool for the determination of carbon isotopic values of gasoline range hydrocarbons in oils or aqueous samples. [Copyright &y& Elsevier]

Abstract: Combined with quantitative determination of concentration and isotopic composition of petroleum hydrocarbons, weathering simulation experiments on artificially mixed oils and their two end-member oils are performed for identification and quantification of mixed sources. The >C18 n-alkanes show no appreciable losses during a short-term weathering process. An approach based on distribution of long-chain n-alkanes (>C18) is suggested for estimating the contribution proportion of each source in mixed oils. Stable carbon isotope profile of individual n-alkanes is a powerful tool to differentiate sources of oil spills, but unavailable to accurately allocate each contribution due to a relatively large analytical error. [Copyright &y& Elsevier]

On-line gas chromatography–pyrolysis coupled to gas chromatography–isotope ratio mass spectrometry was used here for the position-specific isotope analysis (PSIA) of propane. First, based on the conversion rate of propane and its products, 800–840 °C was considered optimal for propane pyrolysis. The major pyrolytic fragments of propane included CH 4 , C 2 H 4 , C 3 H 6 , and C 2 H 6 . Subsequent isotope labeling experiments showed that CH 4 and C 2 H 6 were derived entirely from the terminal carbons, whereas C 2 H 4 and C 3 H 6 were derived from both terminal and central positions of propane. Therefore, the 13 C enrichment factor associated with the major reactions during the pyrolysis process and position-specific δ 13 C values of propane can be estimated from the amount and δ 13 C values of the pyrolytic fragments using isotope mass balance. The obtained enrichment factors depended on the pyrolysis temperature, which can be used to calculate position-specific δ 13 C values for propane measured with this system. The results suggest that a relatively accurate site-preference value for propane can be obtained by this method. Therefore, the combination of compound-specific isotope analysis and PSIA of propane will be a powerful tool to discriminate the different origins of gases. [ABSTRACT FROM AUTHOR]

Abstract: The distribution and compound-specific carbon isotope ratios of n-alkanes and fatty acids in a sediment core (63cm) collected from Lake Dianchi were examined to investigate organic matter sources in the eutrophic lake. Fatty acids included free and bound fatty acids. The carbon isotope compositions of individual n-alkanes and fatty acids from Lake Dianchi sediments were determined using gas chromatography/isotope ratio mass spectrometry (GC–IRMS). The δ13C values of individual n-alkanes (C16–C31) varied between −24.1‰ and −35.6‰, suggesting a dominance of 13C-depleted n-alkanes that originated from C3 plants and lacustrine algae. Fatty acids from the sediment extracts were analyzed for their abundances and carbon isotopic compositions. Molecular and isotopic evidence indicates that most of the short-chain fatty acids from Lake Dianchi sediment extracts are sourced from intense microbial recycling and resynthesis of organic matter. Long-chain free fatty acids are mainly derived from terrestrial sources. However, long-chain bound fatty acids are sourced from a combination of terrestrial organic matter, bacteria and algae, with the contribution from algal sources higher in the hypereutrophic stage. [Copyright &y& Elsevier]

• Intramolecular isotopes of 40 propane and 5 methane samples were analyzed. • Different intramolecular C isotopes of propane from coal and coal mudstone. • Extreme 13C depletion in central C of over mature propane at cooling setting. • Different effect on intramolecular isotopes by various post-generation processes. Evaluating the origin and fate of hydrocarbons holds significance in many fields, such as energy, geology, astrobiology, biosphere, and environment. However, challenges arise in many cases owing to the limitations of conventional methods. Intramolecular isotope analysis of propane is a new technique that offers the potential to provide insights into gas formation mechanisms; however, a comprehensive understanding of the method remains limited. Specifically, there is little knowledge about its ability to distinguish gases generated by source rocks deposited in similar sedimentary environments such as coal and coaly mudstone. Therefore, this study was undertaken to investigate position-specific carbon isotopes of 40 propane samples from 15 coal-type gas accumulations across four basins in China (Sichuan, Ordos, Qaidam, and Songliao); additionally, clumped isotopologues of methane samples were also analyzed. These coal systems cover a wide range of thermal maturity (from marginally mature to over-mature) at various age strata from the Permian, Triassic, and Jurassic periods, to the Cretaceous period. Our results revealed that propane generated from coal and coaly mudstone differed greatly in intramolecular isotope compositions despite having similar bulk δ13C 3 values. Propane generated from coaly mudstone at a wide range of maturity had relatively more stable δ13C values in both the central (δ13C cental) and terminal carbons (δ13C terminal), near the theory generation line of chain-alkane cracking in the plot of δ13C terminal vs. δ13C cental. However, propane generated from coal had higher stable δ13C terminal values and lower δ13C central values that progressively increased with maturity. Under cooling-down conditions, propane from over-mature natural gases became extremely 13C-depleted in both the terminal and central positions, with Δ13C central (δ13C central minus δ13C terminal) values being as low as –10‰, suggesting a partial origin of methane polymerization. Two propane samples from Cretaceous brown sandstone in the Songliao Basin exhibited an increase in both central and terminal carbons, suggesting that they may have surpassed chemical oxidation owing to high-valence Fe(Mn) oxides. These results indicate that intramolecular isotopes of propane can effectively differentiate dominant gas sources in a coal system with coal or coaly mudstone, as well as easily identify isotopic fractionation caused by post-generation processes, such as methane polymerization, mixing and chemical oxidation. [ABSTRACT FROM AUTHOR]

• n -C 18 and squalane yield simple precursors for modeling gas generation. • Two isotopic fractionation patterns are observed in n - and iso -propyl generative models. • Free radical reactions are the main pathway during the main stage of propane generation. Investigating the mechanisms that determine the bulk and position-specific carbon isotopic distributions of propane in natural gas will help elucidate its formation and evolution. We used octadecane and squalane as model compounds that give simple precursors for gas production in gold-tube isothermal pyrolysis experiments to study the variations in intermolecular and intramolecular distributions of isotopes in the generated gaseous hydrocarbons. The δ13C values of the products and inverses of their carbon numbers (1/ n where n = C 1 –C 5) showed a negative linear relationship versus maturity, indicating that they formed by homolytic cleavage of C–C bonds and that the precursors showed homogeneous distributions of carbon isotopes. However, the significant difference in the kinetic isotopic effects (KIEs) of the gas generated by cracking of two model compounds under the same experimental conditions is not readily explained by single homolytic bond cleavage. The results indicate that besides the KIE of C–C bond cleavage, the bulk and position-specific carbon isotopic compositions of propane are related to the chemical and isotopic structures of the precursors and the propane transformation ratio. Based on the sites of C–C bond cleavage, two isotopic fractionation patterns (i.e., normal propyl and isopropyl models) may explain the bulk and position-specific carbon isotopic distributions of the generated propane. According to the propyl model, propane originates from (normal) propyl structures (CH 3 CH 2 CH 2 *) in the precursor via C–C bond cleavage at a terminal site of a propyl group, while the isopropyl model involves propane derived from isopropyl structures (CH 3 CH*CH 3) in the precursor, with C–C bond cleavage at the central site. Simulations show that these distributions for propane cracked from octadecane closely follow the propyl model, whereas propane generated from squalane showed mixed contributions from both models, and its bulk and position-specific carbon isotopic distributions depend on the proportions of propyl and isopropyl structures in the precursors. Variations of propane's position-specific carbon isotopic distributions during the main stage of propane generation indicate that free radical reactions are the main pathway for thermogenic propane formation, resulting in similar KIEs for propane terminal and central carbons. Therefore, the position-specific carbon isotopic distribution of propane can provide evidence of its formation mechanism and shows potential for revealing the origin and evolution of natural gas. [ABSTRACT FROM AUTHOR]

• An online hydrocarbon gas concentration system is used for the intermolecular and intramolecular isotope analysis of low-abundance C 2+ gaseous hydrocarbons. • The system allows marked enrichment of low-abundance C 2+ gaseous hydrocarbons and results in no obvious isotopic fractionation. • The system coupled to GC–IRMS or GC–Py–GC–IRMS can be used for accurate and precise determination of the C–H isotopic compositions of low-content C 2+ gaseous hydrocarbons and the position-specific c isotopic compositions of low-content propane in highly mature natural and shale gases. The stable isotopic composition of natural gas can be used to identify its origin and source. However, low concentrations of gaseous hydrocarbons in high-mature natural and shale gases hinder accurate determination of their compound- and position-specific isotopic compositions. In this study, an online C 2+ hydrocarbon gas concentration system combined with gas chromatography–isotope ratio mass spectrometry (GC–IRMS) or gas chromatography–pyrolysis–gas chromatography–isotope ratio mass spectrometry (GC–Py–GC–IRMS) was developed to determine compound- and position-specific isotopic compositions of low-abundance gaseous hydrocarbons. The lower limit of the gas concentration required for isotope ratio determination using the online concentration system is 0.001% (0.003%) for compound-specific carbon (hydrogen) isotopes and 0.005% for position-specific carbon isotopes and is thus applicable to most natural gas samples. The online concentration technique does not cause significant isotopic fractionation effects, and the combination with GC–IRMS and GC–Py–GC–IRMS can accurately and precisely determine the compound-specific δ13C and δD values of low-content C 2+ gaseous hydrocarbons and the position-specific δ13C values (δ13C a , δ13C b , and SP values) of propane in low-content propane samples, respectively. The application of our method to two natural gas samples from the Ordos and Sichuan basins further confirms that the online concentration method allows simple and rapid determination of the compound- and position-specific isotopic compositions of low-abundance gaseous hydrocarbons. [ABSTRACT FROM AUTHOR]

Oil-based drilling mud (OBM) contamination, commonly occurring in cuttings during petroleum exploration, has had been a problem seriously hindered source rock identification and evaluation. To eliminate the effect of contamination on "free" bitumen extracted directly from source rock, "enclosed" and "bound" bitumens were released from the mineral matrix by demineralization and from the kerogen structure using an improved catalytic hydropyrolysis technique, respectively. Geochemical and carbon isotopic compositions of the three types of bitumen were used to reconstruct the geochemical characteristics of original organic matter in their source rocks. Results indicate that "free" and "enclosed" bitumens have similar biomarker distributions and n -alkane δ13C values in non-contaminated cuttings, but the former is more susceptible to OBM contamination. "Enclosed" bitumen can thus be used to characterize the organic geochemistry of source rocks contaminated by OBM. The carbon isotopic compositions of individual n -alkanes in "free" and "enclosed" bitumens differ from those in "bound" bitumen, likely because of the effects of diagenesis and other secondary alteration. "Bound" bitumen is more likely to preserve the molecular carbon isotopic characteristics of original organic matter in source rocks, while the "free" and "enclosed" bitumens more represent the generation products. Based on biomarker characteristics and carbon isotopic compositions of "bound" hydrocarbons, Wenchang (WC) Formation (Fm.) source rocks in the Zhu I Depression of the Pearl River Mouth Basin (PRMB) can be divided into three types in normal medium–deep (WC–I), shore–shallow (WC-II), and special medium–deep (with different algal blooms growth rates; WC-III) lacustrine source rocks. The special medium–deep lacustrine source rocks of Wenchang Fm. are characterized by high ratio of C 30 4-methylsteranes to C 29 steranes (4-Me/C 29 > 0.66) and heavy δ13C values (δ13C kerogen < −26.1‰), which are obviously different from those of normal medium–deep lacustrine source rocks. The combination of "free", "enclosed", and "bound" hydrocarbons geochemical and isotopic compositions thus provides a comprehensive understanding of source rocks. • Geochemical feature of free, enclosed and bound bitumens in cuttings was compared. • Enclosed and bound bitumens were less affected by oil-based drilling mud. • Bound bitumen may record information of original organic matter in source rock. • Three types of Wenchang Fm. source rocks in Zhu I Depression were identified. [ABSTRACT FROM AUTHOR]

• Pyrolysis experiments on four organic matter types simulate propane evolution. • Propane intramolecular isotope fractionation shows variations with mechanism. • Rayleigh model indicates the heterogeneity of the isotopic structure of oil. • Intramolecular isotopes of propane predict the origin and maturity of natural gas. Position-specific isotope analysis (PSIA) of propane provides novel insights into formation mechanisms and thermal maturity of natural gas. The evolution of thermogenic propane in natural gas was simulated with pyrolysis experiments using types I, II A , III kerogen and an oil. PSIA of propane involved on-line pyrolysis with gas chromatography–isotope ratio mass spectrometry to explore evolution characteristics and fractionation mechanisms of propane position-specific δ13C values. Results indicate the formation of propane can be considered in three stages. Stage I (<1.3% EasyRo) involves propane production mainly from primary kerogen cracking, with its precursors having homogeneous isotopic distributions and with position-specific 13C variation patterns depending on the kerogen molecular structure. In Stage II (1.3% EasyRo to maturity of peak propane yield), propane is derived mainly from secondary cracking of bitumen/oil, resulting in 13C enrichment of terminal (δ13C a) and central (δ13C b) C atoms with maturity and a strong linear relationship between δ13C a and δ13C b. A suggested new pattern is explained by a Rayleigh model for the evolution of propane position-specific δ13C values in this stage. In Stage III (maturity greater than the peak propane yield), propane destruction mainly controls its amount and isotopic signature. Propane site-preference (SP) value (δ13C a –δ13C b) can be used to determine its formation mechanism and maturity, with positive and negative values respectively indicating primary kerogen cracking and secondary oil/bitumen cracking. Increasing maturity causes decreasing values. The application of propane PSIA to the Tarim Basin gas samples demonstrates its potential for determining the mechanism of formation and maturity of natural gas. [ABSTRACT FROM AUTHOR]

• Deans-switch is used to avoid complex preparation and losses of trace samples. • Baseline separation of crocetane and phytane is achieved (R >1.5) by 2D-GC technique. • First accurate determination of δ13C of crocetane in natural samples by 2D-GC-IRMS. Compound-specific isotope analysis of crocetane in sediments can be used to determine its origin. However, the co-elution of crocetane with phytane and complex substrates hinders the accurate determination of its isotopic composition. A heart-cutting two-dimensional gas chromatography–isotope ratio mass spectrometry (2D-GC–IRMS) method was developed to determine δ13C values of crocetane. Full chromatographic separation of crocetane was achieved with a 30 m CycloSil-B chiral capillary first column and a 60 m Chiraldex B-pH capillary second column (two series-connected 30 m Chiraldex B-pH capillary columns). Analyses of standard mixtures confirmed that the new method has satisfactory accuracy (deviations from the authentic values <0.5‰) and precision (RSDs <5%) for carbon isotope analyses of crocetane and phytane. The application of the method to two sediment samples indicates that baseline resolution is achieved (R >1.5) for crocetane and phytane, with 13C-depleted crocetane (δ13C values of −119.5‰ and −111.5‰ in the two natural samples, respectively) being an indicator of anaerobic oxidation of methane in cold seep areas. The new 2D-GC–IRMS method allows simple and efficient determination of the compound-specific carbon isotopic compositions of crocetane. [ABSTRACT FROM AUTHOR]

Petroleum exploration has found that the Baiyun deep-water area of the Pearl River Mouth Basin (PRMB), China, holds abundant oil and gas resources with great prospects. However, conditions restricting offshore drilling in this area present significant challenges to the investigation of source rocks. Core samples are lacking, and any cuttings obtained are generally highly contaminated by drilling mud. As a result, the identification and evaluation of hydrocarbon sources in deep-water areas would be greatly aided by new techniques capable of reliably obtaining the geochemical characteristics of potential source rocks. This study combines catalytic hydropyrolysis (HyPy) and gas chromatography–isotope ratio mass spectrometry (GC–IRMS) to determine the carbon isotope composition of individual n -alkanes covalently bound in kerogen framework. Furthermore, it reconstructs the source and depositional environment of sedimentary organic matter in potential source rocks from the Baiyun deep-water area of the PRMB. The results indicate that the optimized degassing (i.e., degassing under high vacuum at 250 °C for 24 h) can effectively eliminate hydrocarbon contaminants in the kerogen. Determination by GC–IRMS of bound n -alkanes released from the degassed kerogen by HyPy can provide the molecular carbon isotope characteristics of the original organic matter in cuttings contaminated by drilling mud. The results improve the understanding of potential hydrocarbon source rocks in the Baiyun deep-water area of the PRMB. • Optimized off-line degassing process could eliminate contaminants in kerogen. • Bound n -alkanes released from kerogen maintain δ13C values of their precursors. • δ13C of n -alkanes for source rocks in the Baiyun Sag are firstly identified. • Catalytic hydropyrolysis combined with GC–IRMS can regain original geochemistry. [ABSTRACT FROM AUTHOR]

Source rock samples (i.e., drill cuttings) of the Wenchang, Enping, and Zhuhai formations from the Baiyun sag in the deep-water area of the Pearl River Mouth Basin (PRMB) in the South China Sea were collected and subjected to organic geochemical analysis. The three sets of potential source rocks were further divided into six types, based on biomarkers and carbon isotopic compositions: shallow lacustrine and semideep–deep lacustrine source rocks of Wenchang Formation, shallow lacustrine and marine transgression-related source rocks of Enping Formation, and littoral and neritic source rocks of Zhuhai Formation. The Wenchang Formation semideep–deep lacustrine source rocks developed in the Baiyun Sag are characterized by low ratio of C 30 4-methylsteranes to C 29 steranes (4-Me/C 29 < 0.25) and relatively 13C-depleted isotopic compositions (δ13C kerogen < −27.5‰), which are obviously different from those from shallow-water area in the PRMB. Rock–Eval analysis shows the Wenchang Formation develops good source rocks, which are better than those in the Enping Formation. The Zhuhai Formation does not contain effective source rocks because of its low maturity. According to the distribution of bicadinanes and oleanane, it is speculated that shallow lacustrine source rocks of Wenchang Formation in the northern Baiyun Sag have similar high bicadinanes to C 30 hopane ((W + T)/C 30 H) and low oleanane to C 30 hopane (OL/C 30 H) ratios to those of the Enping Formation, indicating their possible contribution to the discovered hydrocarbon accumulations. In summary, hydrocarbons generated by the Wenchang Formation source rocks should become the next key exploration target in the Baiyun Sag. • Geochemical data obtained from the extracted source rocks are more reliable. • Wenchang semideep–deep lacustrine source rock in Baiyun Sag is newly identified. • Sediment supply direction controls distributions of (W + T) and OL in source rocks. • Hydrocarbons derived from Wenchang Formation are the next key exploration target. [ABSTRACT FROM AUTHOR]