A series of Palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the TarimBasin, NW China have been examined to delineate oil families. Crude oils from the Mesozoic foreland Kuqa Depressionwere also analyzed for comparison. The genetic family was assessed using bulk hydrocarbon isotopic composition,alkylation pattern of polycyclic aromatic hydrocarbons (PAHs) and biomarker distributions. Detailed oil–oil correlationsuggests two distinct families occur in the cratonic region. Despite the diversity of the reservoir rocks and somevariability in oil physical properties, the Tabei and Tazhong oils are classified, in general, into a single family, witha possible indication for secondary subdivision. These oils are characterized by low pristane/phytane (Pr/Ph) ratio, highdibenzothiophene/phenanthrene (DBT/P) ratio, light bulk isotopic composition, high degree of PAH alkylation, highproportion of C23 and C24 tricyclic terpanes and low abundance of C24 tetracyclic terpane and triaromatic dinosteroids.This family is inferred to have a Middle–Upper Ordovician source rock. The Tadong family, distinctly different fromthe Tabei and Tazhong families, is characterized by relatively high Pr/Ph ratio, low DBT/P ratio, slightly heavier bulkisotopic compositions and low degree of PAH alkylation. It also has a unique biomarker signature, with a high proportionof triaromatic dinosteroids, possibly indicating Cambrian–Lower Ordovician source rocks.The oils from the Kuqa Depression are easily distinguished from the other two families due to their terrigenous origin.The major characteristics of the Kuqa oils are high Pr/Ph ratio, very low DBT/P ratio, heavy bulk isotopic composition,low PAH alkylation degree, high proportion of C20 and C21 tricyclic terpanes and C24 tetracylic terpane, andabundance of triaromatic dinosteranes and 4-methyl-24-ethyltriaromatic cholestanes. These characteristics are consistentwith terrestrial organic source rocks deposited under sub-oxic conditions.

A detailed organic geochemical study; utilising petrography, biomarker hydrocarbon analysis and high temperatureGC analysis of extractable wax hydrocarbon constituents was performed on four marginally oil window-mature sourcerocks from the Shahejie Formation (Eocene), Damintun depression in eastern China. The main maceral components inthe source rocks were vitrinite, liptinite and exinite, with vitrinite being more abundant (>50 vol.%) in organic-leansamples whose TOC contents were between 1 and 2 wt.%. Large differences in pristane/phytane ratios suggested thatthe organic-rich samples were deposited in a less oxic depositional environment than that for the organic-lean rocks.The distribution of extractable wax hydrocarbons, determined by high temperature GC, showed a marked differencebetween these two sample types. The organic-rich samples contained high molecular weight hydrocarbons (HMWHCs)dominated by macrocrystalline n-alkanes (n-C23–n-C37, typically maximising at n-C29), while the organic-lean samplescontained lower amounts of extractable wax hydrocarbons but were relatively rich in microcrystalline components (>n-C35). In all source rocks (Es3 and Es4), a noticeable odd-over-even predominance (OEP) of n-alkane chain lengths(up to n-C65) was evident, consistent with a direct biological origin for the long n-alkyl chains. They were mostprobably formed during diagenesis from decarboxylation of predominantly even-carbon-numbered aliphatic acidsoriginating from higher plant or lacustrine algal sources and/or were directly biosynthesised in hydrocarbon form. Atleast two other homologous series of branched/cyclic HMWHCs were observed, one of which was confirmed as a seriesof branched alkanes (probably methyl-branched). The carbon number distribution patterns of HMWHCs may be primarilycontrolled by thermal maturity and biogenic source input as well as being influenced by diagenetic reactionsgoverned by depositional environmental conditions, as shown previously [Carlson, R.M.K., Teerman, S.C., Moldowan,J.M., Jacobson, S.R., Chan, E.I., Dorrough, K.S., Seetoo, W.C., Mertani, B., 1993. High temperature gas chromatographyof high wax oils. In: Indonesian Petroleum Association, 22nd Annual Convention Proceedings. Jakarta, Indonesian,pp. 483–507. Carlson, R.M.K., Jacobsen, S.R., Moldowan, J.M., Chan E.I., 1994. Potential application of hightemperature gas chromatography to Middle Eastern petroleum exploration and production. In: Al-Husseini, M.I.(Ed.), Geo'94, Vol 1., Selected Middle East Papers from The Middle East Petroleum Geoscience Conference, 1994; Gulf PetroLink. Manama, Bahrain, pp. 258–267]. Our study indicates for the first time that Es3 source rocks as well asEs4 facies contain HMWHCs. The distributions of extractable wax hydrocarbons suggest that both Es4 and Es3members may potentially serve as important parent source rocks for generating waxy petroleum in this region.

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

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.

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.