rease slightly during theearly stages of biodegradation and then sharply decrease after level 4, when preferential depletion of alkylated carbazolescompared to benzocarbazoles and dibenzocarbazoles or naphthocarbazoles is observed. The susceptibility of alkylated carbazoleisomers to biodegradation decreases with increasing carbon number of the alkyl substituents. Furthermore, nitrogen(N–H) shielded or partially shielded compounds substituted in the 1 and/or 8 positions seem more susceptible to biodegradationthan their nitrogen-exposed counterparts. For example, biodegradation resulted in the preferential removal of 1-methylcarbazole relative to the other methylcarbazole isomers and among the dimethylcarbazoles; 1,8-dimethylcarbazoleseems more susceptible to biodegradation than other isomers, even though some exceptions do occur in our sample set.The benzocarbazole ratio [Nature 383 (1996) 593] decreased with increasing degree of biodegradation. Benzo[b]carbazolehas the highest apparent ability to resist biodegradation among the benzocarbazole isomers.

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.

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 suite of petroleum reservoir extracts from the Liaohe basin, NE China, was analyzed to investigate the effects andcontrols of biodegradation on petroleum composition.Systematic changes, seen as marked gradients in petroleum bulkcomposition, component concentrations and molecular indicators, have been observed in two biodegraded oil columns.In the Es3 column (1700-1850 m), biodegradation has removed substantial amounts of normal and isoprenoid alkanes.More severe biodegradation occurs in the shallower Es1 column (1500-1650m), with both steranes and hopanes beingpartially removed.Temperature has an overall control on biodegradation in the two columns, a lower reservoir temperatureleading to a higher degree of biodegradation.However, observed variation in the degree of biodegradation inthe Es3 column at different sites is controlled by water leg size, with a higher level of degradation being associated witha thicker water leg.The supply of nutrients from the water leg is thought to have a significant impact upon the degreeof biodegradation.In addition to water leg size/nutrient supply, the compositional gradients also are controlled by therelative rate of mixing of fresh oil charge.Gradients can only be conserved in biodegraded petroleum columns ifthe rate of diffusive mixing is similar to or lower than the rate at which the hydrocarbons are removed.A biodegradationmodel is proposed that couples geochemical and geological factors to provide a coherent approach forassessing the impact of degradation on petroleum.Our geochemical study indicates that biodegradation occurs mainlywithin a narrow region at or near the base of the oil column.The stable compositional gradients are produced after aninitial induction period involving diffusive transport of alkanes to the oil-water contact (OWC) and diffusion of biodegradationproducts such as 25-norhopanes and other metabolites away from the reaction site.The mixing of continuously-charged oil with 'residential' biodegraded oil by diffusion may be considered as one of the most importantfactors controlling the biodegradation process.The conceptual model illustrates all of these processes and opens thepossibility of model-driven prediction of oil properties and 'sweetspot' location in reservoirs.

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。