天然气的同位素组成受源岩有机质类型、热演化程度和成藏后的次生变化等多种因素的影响，其单体烃同位素分布特征是区分各种因素影响程度的地球化学指纹。天然气单体烃同位素通常有随碳数增加而变重的趋势，而徐家围子深层天然气单体烃同位素倒转现象却比较普遍，高地温和细菌氧化对于造成该区同位素倒转的可能性不大，浅层油型气或无机气的混合作用没有充分的地质证据，同层有机质中不同类型气的混合和盖层微渗漏造成的蒸发分馏作用可能是导致徐家围子断陷深层天然气同位素倒转的主要原因。

e biodegradation – ranging from levels 2 to 5 on the [Peters, K.E., Moldowan, J.M., 1993. The BiomarkerGuide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs,NJ, p. 363] scale (abbreviated as _PM level_) -while the shallower Es1 column has undergone more severe biodegradation,ranging from PM level 5 to 8. Both columns show excellent vertical biodegradation gradients, with degree of biodegradationincreasing with increasing depth toward the oil–water contact (OWC). The compositional gradients in theoil columns imply mass transport control on degradation rates, with degradation occurring primarily at the OWC. Thediffusion of hydrocarbons to the OWC zone will be the ultimate control on the maximum degradation rate. The chemicalcomposition and physical properties of the reservoired oils, and the _degradation sequence_ of chemical componentsare determined by mixing of fresh oil with biodegraded oil.The PAH concentrations and molecular distributions in the reservoired oils from these biodegraded columns showsystematic changes with increasing degree of biodegradation. The C3+-alkylbenzenes are the first compounds to bedepleted in the aromatic fraction. Concentrations of the C0-5-alkylnaphthalenes and the C0-3-alkylphenanthrenesdecrease markedly during PM levels 3–5, while significant isomer variations occur at more advanced stages of biodegradation(>PM level 4). The degree of alkylation is a critical factor controlling the rate of biodegradation; in most cases the rate decreaseswith increasing number of alkyl substituents. However, we have observed that C3-naphthalenes concentrations decrease faster than those of C2-naphthalenes, and methylphenanthrenes concentrations decrease faster than that of phenanthrene.Demethylation of a substituted compound is inferred as a possible reaction in the biodegradation process.Differential degradation of specific alkylated isomers was observed in our sample set. The relative susceptibility ofthe individual dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, pentamethylnaphthalene, methylphenanthrene,dimethylphenanthrene and trimethylphenanthrene isomers to biodegradation was determined. The C20and C21 short side-chained triaromatic steroid hydrocarbons are degraded more readily than their C26-28 long sidechainedcounterparts. The C21-22-monoaromatic steroid hydrocarbons (MAS) appear to be more resistant to biodegradationthan the C27-29-MAS.Interestingly, the most thermally stable PAH isomers are more susceptible to biodegradation than less thermally stableisomers, suggesting that selectivity during biodegradation is not solely controlled by thermodynamic stability and thatsusceptibility to biodegradation may be related to stereochemical structure. Many commonly used aromatic hydrocarbonmaturity parameters are no longer valid after biodegradation toPMlevel 4 although some ratios change later than others.The distribution of PAHs coupled with knowledge of their biodegradation characteristics constitutes a useful probe forthe study of biodegradation processes and can provide insight into the mechanisms of biodegradation of reservoired oil.

The applicability of sulphur aromatic parameters, based on ratios of dibenzothiophene (DBT) and its alkylatedhomologues, as indicators of oil maturity has been investigated in 27 oils from the Damintun and Zhuanhua Depres-sions, Bohai Bay Basin. The oils were characterised by bulk composition and environmentally-sensitive aliphatic bio-markers and were derived from a wide range of lacustrine source depositional environments ranging from highlyreducing hypersaline (sulphur-rich oils) to terrestrial input-dominated freshwater (very high wax oils) with maturitiesranging from immature to probable mid-mature. Dibenzothiophene-based maturity parameters are not well-correlatedwith sterane isomerization ratios, and immature oils, in particular, could not be discriminated by these aromaticmaturity indicators. In contrast, DBT-based parameters are strongly-correlated with pristane to phytane and Ts/Tmratios. High methyldibenzothiophene ratios (MDR) are associated with fresh water source deposition whilst lowMDRs are found in oils of hypersaline source affnity. Unusually high relative concentrations of 2+3-MDBT wereobserved in some of the immature oils inferred to be sourced by lacustrine organic laminites. The data provide furtherevidence that thermal stress is not the sole factor controlling the distribution patterns of dibenzothiophenes in oils andthat source depositional environments can exert a dominant in

A detailed organic geochemical analysis of six oil samples from the Baiyinchagan depression in the Erlian basin, Northern China, wascarried out in order to evaluate their origin. The oils are reservoired at a very shallow depth (223-560 m subsurface) and their chemical andphysical properties vary greatly, ranging from normal to extremely heavy oil. The preservation of non-biodegraded oil in such a shallowreservoir is possibly related with palaeo-pasteurization of the reservoir before uplift. Maturity difference is not the primary control on thechemical and physical properties of the oils and there is considerable geochemical evidence to suggest the additional influence of inreservoir/post-accumulation processes such as biodegradation, water-washing and (possibly) evaporation. Whereas some oils appear to beless affected, others are moderately biodegraded up to level 4 on the Peters and Moldowan (1993) scale, with sterane distributions largelyunaffected and 25-norhopanes undetected. Contrary to classical biodegradation, the unusual heavy oil shows little evidence of biodegradationfrom aliphatic components. Water-washing is suggested to be the primary process leading to its formation since the severe alteration ofsoluble aromatic hydrocarbons is observed. In addition, since the oils have been uplifted significantly after accumulation, evaporation and/orleakage to modify oil compositions cannot be ruled out.

Partition coefficient difference of benzocarbazoleisomers between oil, water and mineral phase makesthem a useful indicator to quantify petroleum migrationdistance. Because of their nitrogen-heteroatom andannelated aromatic cycles they are generally regarded asbeing more resistant and the effects of biodegradation ontheir concentrations and distributions have not previouslybeen investigated. Reservoir extracts from three wells locatedin the Leng 43 block of the Liaohe Basin were analyzedto investigate their occurrence and the effect of biodegradation.Both hydrocarbon biomarkers and benzocarbazoleisomers show systematical changes with the increase extentof biodegradation in study columns. Carbazole compoundsmay be biodegraded in a similar way to that observed inaliphatic and aromatic hydrocarbons. The distance from oilwater contact is a primary control factor for biodegradation.The concentrations of benzocarbazole isomers show a slightincrease in the upper part of the columns then a sharp decreasetowards oil water contact (OWC). Among three isomersbenzo[a]carbazole seems more susceptible to biodegradationthan other two isomers and benzo[b]carbazole hashigher ability to resist bacterial attack. Benzo[b]carbazole/benzo- [a]carbazole ratios can sensitively indicate thedegree of biodegradation and the benzocarbazole index (BCratio) cannot be directly used as a migration indicator inbiodegraded oils.