Seven crude oils of known source and maturity, representing a natural sequence of increasing degree of biodegradation, were collected from reservoirs in the Liaohe Basin, NE China, in an effort to determine the magnitude and direction of isotopic shift of carbon and hydrogen in individual n-alkanes during microbial degradation. The results show that biodegradation has little effect on the carbon isotopic composition of the whole oil. However, a sequential loss of n-alkanes leads to 13C depletion of the bulk residual saturate fraction. The stable carbon isotopic compositions of aromatics and macromolecular organic matter (resins and asphaltenes) follow a pattern, with an overall trend towards 13C enrichment of 0.8-1.7&in the residues. The stable carbon and hydrogen isotope values of individual n-alkanes demonstrate that they follow different trends during biodegradation. No significant carbon isotopic fractionation occurs for n-alkanes during slight to moderate biodegradation. However, there is a general increase of up to 4&in the d13C values of low molecular weight n-alkanes (C15-C18) during heavy biodegradation. In contrast, no isotopic fractionation occurs for higher molecular weight n-alkanes (PC19). The conservative character of 13C concentration for high molecular weight n-alkanes during biodegradation indicates that these compounds are effective tracers for source identification of heavily biodegraded oils. The hydrogen isotope compositions of n-alkanes show a significant fractionation as moderate biodegradation proceeds, resulting in an enrichment inDof up to -35 & upon heavy biodegradation. The dynamic fractionation in the stable carbon and hydrogen isotopes of n-alkanes accompanying biodegradation opens the possibility of quantitative estimation of the extent of biodegradation in subsurface petroleum reservoirs by introducing an isotopic kinetic modelling approach.

We report a convenient paper layer chromatographic method involving urea inclusion for the separation of straight chain and branched/cyclic alkanes in saturated fractions from sedimentary organic matter. Compared with the wellknown 5 A molecular sieving and traditional urea adduction methods, urea paper layer chromatography is more effective, simple and economic.

Isotopic compositions of carbon-bound hydrogen in individual n-alkanes from sev-eral typical crude oil samples from Lunnan Oilfield, Tarim Basin, NW China, were firstly measured using newly developed gas chromatography-thermal conversion-isotope ratio mass spectrometry. The similar range of δ D of individual n-alkanes of crude oils among reservoirs of different geo-logical times reflects that hydrocarbons are all derived from the same marine depositional envi-ronment. Compared to the theoretic value (−150‰) and the reported δ D values (nC13-nC27, -160‰--90‰) of individual n-alkanes for Ordovician-sourced crude oils in the Canadian Wil-liston Basin, the hydrogen isotopic composition of individual n-alkanes in crude oils from Lunnan Oilfield is characterized by heavy hydrogen isotopic values (nC12-nC27, -120‰--60‰). In terms of the factors that control the fractionation of hydrogen isotopes, relatively saline deposi-tional environment and higher thermal maturation were attributed to the heavy δ D values of in-dividual n-alkanes in crude oils from Lunnan Oilfield.

Using molecular geochemical data from infrared spectrometer and pyrolysis gas chromatography-mass spectrometry, this paper investigates the petroleum generation characteristics of Jurassic coal measures from Kuche depression, Tarim Basin, NW China. The results showed that the Jurassic coaly rocks with medium maturity (Ro%: 0.8-1.1) were enriched in gas-prone functionalities (-CH3) and low molecular weight pyrolysates (<nC21), indicating that the coaly rocks from Kuche depression were gas/condensate prone at the stages of middle to high maturation, and it was further supported by the oil/source correlation from well Yinan 2 in this region.

Hydrocarbons, include heavy oils, normal oils, waxy oils and light oils, condensate oils and gases were all found in Lunnan Oilfield, the occurrences of hydrocarbons with complicated physical properties indicates a complicated distribution of reservoirs. By the drilling data, the distribution pattern had been found that, in plan view, the density of crude oils in the western part is heavier than that of oils in the eastern part in Lunnan region, namely the crude oils were mainly found in the western part while the natural gases were mainly found in the eastern, which shows that an obvious differentiation in the east and west part was presented in the Lunnan region. Furthermore, in vertical view, the light hydrocarbons were mainly found in the Carboniferous reservoir while the heavier hydrocarbons were mainly found in the Ordovician and Triassic reservoirs. By analysis of the semi-logarithm distribution of n-alkanes, biomarker ratios and stable carbon isotopic compositions of crude oils, we have found that two oil families and discontinuous distribution were presented in hydrocarbons in Lunnan Oilfield. This kind of discontinuous distribution of geochemical parameters was not from continuous fractionation in migration process. The biodegraded heavy oil reservoirs in western part belong to the lower matured oil family, while the light or condensate oils reservoirs belong to higher matured oil family. Thus, a mixed filling of hydrocarbons was occurred in the middle part in Lunnan Oilfield and resulted in the occurrence of waxy oil. The mixed filling model may help to explain the distribution pattern of hydrocarbon reservoirs in Lunnan Oilfield, especially can lead to a relatively consistent explanation for all kinds of geochemical data, such as physical properties, pyrrolic nitrogen compounds, biomarker parameters and stable carbon isotopic compositions.

Aryl isoprenoids have been detected for the first time in crude oils from Paleozoic petroleum systems in the Tabei uplift, Tarim Basin, northwestern China. The principal compounds possess the 1-alkyl-2,3,6-trimethyl substitution pattern characteristic of diaromatic carotenoids found in the Chlorobiaceae family of photosynthetic sulfur bacteria, with a predominance of C13-C23 homologues in these samples. Flash pyrolysates of the asphaltene fractions isolated from crude oils show an unusual abundance of 1,2,3,4-tetramethylbenzene, an indicator for a significant contribution of diaromatic carotenoids to the source kerogen of these oils. The wide distribution of these compounds in crude oils reveals that penetration of the photic zone by sulfidic waters during deposition enabled prolific growth of photosynthetic green sulphur bacteria (Chlorobiaceae). This suggests that the source rocks for the Paleozoic petroleum systems in the Tabei uplift were likely deposited under euxinic conditions with sulfate and sulfide-rich water bodies, which doesn’t support previously published conclusions of a Middle-Upper Ordovician source that marks the slope facies at the margins of structural uplifts.

Unusually abundant long-chain monomethylalkanes (MMAs) (C25-C36) and their short-chain homologues (C14-C22) have been tentatively identified in a Late Triassic oil sand sample from the Lunnan oilfield, Tarim Basin, NW China. Molecular distributions and stable carbon isotopic compositions of these compounds have been determined to investigate their bio-and/or geosynthetic sources. The results show that the most abundant isomers of short-chain MMAs were 4-methyl (m/z 70), 5-methyl (m/z 84), 6-methyl (m/z 98) and 7-methyl (m/z 112) alkanes, whereas in longchain MMAs, there is a clear predominance of the 9-methyl and 10-methyl isomers. Combined with the compoundspecific stable carbon isotopic analysis, it is concluded that short-chain MMAs and long-chain MMAs must have different sources. The similar isotopic compositions of MMAs and n-alkanes in this sample suggest that they may share the same sources. The short-chain MMAs, as previously suggested, are probably related to a cyanobacterial origin, while the long-chain MMAs may be mainly associated with microorganisms or heterotrophic bacteria, rather than cyanobacteria.

To date, questions remain as to how to differentiate between coal-and interbedded shale-sourced oils in coal measures. Based on the hypothesis that 13C value of sedimentary organic matter is controlled by the depositional environment and productivities of biota grown in such depositional setting, this note examines compound specific carbon isotopic profiles from coal and interbedded shale extracts using the gas chromatography-isotope ratio mass spectrometry technique (GC/IRMS). The results show that compound specific carbon isotope values from coal extracts are enriched in 13C relative to that of interbedded shale extracts, and provide useful information in characterizing coal-and interbedded shale-sourced oils in coal measures. An example for its successful application from the Turpan Basin, Northwestern China is presented.

Molecular geochemical methods have shown that it is often diﾁcult to diﾀerentiate between coal-and interbedded shale-sourced oils, even though coals and interbedded shales may exhibit considerable organic inux variation (e.g. land plant vs algal organic matter) due to the changes of depositional setting. However, compound-specirc stable carbon isotopic compositions are sensitive to the source input variations. Typically, specirc molecules are more depleted in 13C with increasing content of aqueous biota. This hypothesis is examined and exemplired by comparing the stable carbon isotopic ratios of n-alkanes from source rock extracts and related oils of the Turpan basin, north-western China. Stable carbon isotopic values of n-alkanes extracted from coals and interbedded shales show that d13C values of n-alkanes with less than 20 carbon atoms vary only slightly. However, there are dramatic changes in the isotopic compositions of higher molecular weight n-alkanes. Furthermore, n-alkanes from coal extracts are enriched in 13C relative to that of interbedded shales with excursions up to 2-3%. This comparison enables the dio erentiation of coal-and interbedded shale-sourced oils, and provides information useful in assessing the hydrocarbon system of a basin.