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.