Sources of fluids and gases expelled at cold seeps offshore Georgia, eastern Black Sea

Four seep sites located within an ∼20 km² area offshore Georgia (Batumi seep area, Pechori Mound, Iberia Mound, and Colkheti Seep) show characteristic differences with respect to element concentrations, and oxygen, hydrogen, strontium, and chlorine isotope signatures in pore waters, as well as impregnation of sediments with petroleum and hydrocarbon potential. All seep sites have active gas seepage, near surface authigenic carbonates and gas hydrates. Cokheti Seep, Iberia Mound, and Pechori Mound are characterized by oil-stained sediments and gas seepage decoupled from deep fluid advection and bottom water intrusion induced by gas bubble release. Pechori Mound is further characterized by deep fluid advection of lower salinity pore fluids. The Pechori Mound pore fluids are altered by mineral/water reactions at elevated temperatures (between 60 and 110 °C) indicated by heavier oxygen and lighter chlorine isotope values, distinct Li and B enrichment, and K depletion. Strontium isotope ratios indicate that fluids originate from late Oligocene strata. This finding is supported by the occurrence of hydrocarbon impregnations within the sediments. Furthermore, light hydrocarbons and high molecular weight impregnates indicate a predominant thermogenic origin for the gas and oil at Pechori Mound, Iberia Mound, and Colkheti Seep. C₁₅₊ hydrocarbons at the oil seeps are allochtonous, whereas those at the Batumi seep area are autochthonous. The presence of oleanane, an angiosperm biomarker, suggests that the hydrocarbon source rocks belong to the Maikopian Formation. In summary, all investigated seep sites show a high hydrocarbon potential and hydrocarbons of Iberia Mound, Colkheti Seep, and Pechori Mound are predominantly of thermogenic origin. However, only at the latter seep site advection of deep pore fluids is indicated.     

Bacterial degradation of crude oil: Comparison of field and experimental data

It has been reported that the composition of crude oils in the subsurface may be altered by bacterial action to the extent that oil correlations (Winters and Williams, 1969) and the value of the crude (Evans et al., 1971) are severely affected. Experimental documentation of these effects is provided by this study.A crude oil was degraded in a 21-day laboratory experiment by a culture of four aerobic bacteria isolated from an oil-contaminated soil. The progress of the experiment was measured by the changes induced in the chemical composition of the oil fraction boiling above 270°C. These changes were similar to the variations in composition found in the MC5 oils of Saskatchewan, Canada.Normal paraffins through to at least nC₃₄ were severely depleted although the attack was temporarily blocked at nC₂₅ (Jobson et al., 1972). The position of this blockage is a function of the isolate employed. The isoprenoids, pristane and phytane, were metabolised after the disappearance of the n-paraffins. Lower-ring naphthenes and aromatics were attacked at the same time as the lighter normal paraffins and before the heavier ones.The more condensed cyclic hydrocarbons were apparently unaffected. Additional non-hydrocarbon NSO, and particularly asphaltene (both defined under section “Methods”), compounds were formed by the metabolism of the hydrocarbons.The residual oil after attack was heavier by approximately 30° API than the initial crude oil.     

The influence of marine and terrestrial source material on the composition of petroleum

This paper consists of two interrelated parts. In the first part, the influence of the composition of sediment organic matter on crude oil composition is discussed. The second part deals with the origin of normal paraffins in petroleum.Source beds with abundant terrestrial plant matter generate heavy hydrocarbons rich in five-ring naphthenes. Unless such source beds are exposed to a high temperature for a prolonged time, the oils released are also rich in five-ring naphthenes. Such oils are rare; thus far the only examples found are some Eocene Wilcox oils from the Texas Gulf Coast and some Eocene Green River oils from the Uinta Basin, Utah. Normally, oil source beds are not rich in terrestrial plant matter and the five-ring naphthene content of the source bed hydrocarbons, as well as that of the produced oils, is low.The n-paraffins generated by oil source beds rich in terrestrial plant matter are characterized by abnormally low (C₂₁ + C₂₂)/(C₂₈ + C₂₉) ratios of 0.6–1.2. In oils of dominantly marine origin, this ratio is in the range 1.5–5.0. The ratio of marine to terrestrial organic matter in source beds appears to influence both the naphthene composition and the n-paraffin composition of the generated oils.Evidence is presented that petroleum n-parainns originate from slow thermal cracking of fatty acids contained in fats and waxes. Reaction equations are discussed which explain the major geochemical observations, including the difference in carbon-number distribution of the assumed parental fatty acids and of their descendant n-paraffins. In normal oils, which originate mostly from fat rich marine organic matter, the n-paraffin concentration tapers off above C₂₀. The molecular weight range of the fatty acids of plant waxes is considerably higher than that of fats. If plant waxes contribute strongly to the oil source material, the molecular weight distribution of the petroleum n-paraffins formed is abnormal and high carbon numbers in the C₂₄-C₃₂ range dominate.     

A natural oil seep in the floodplain of the Amga River (Siberian Platform)

A natural oil seep has been revealed in the floodplain of the middle stream of the Amga River, in zone of the exposure of Middle Cambrian sediments. A distinctive feature of saturated hydrocarbons of this oil is the absence of 12- and 13-methylalkanes, biomarkers that are present in oils of the Vendian–Cambrian deposits of the Nepa–Botuobiyan petroliferous province. In this feature the studied seep oil is similar to the Middle Cambrian oil from hydrogeologic wells (1-P and 1-T) drilled earlier downstream of the Amga River.     

Hydrocarbons in Mississippi Fan and intraslope basin sediments

Aliphatic and aromatic hydrocarbons in sediments of the middle and lower Mississippi Fan and two intraslope basins in the Gulf of Mexico are derived from terrestrial organic matter and thermogenic, mature hydrocarbons. The terrestrial hydrocarbon component consists primarily of terrigenous, plant biowaxes (n-alkanes with 21 to 33 carbons). The occurrence of thermogenic hydrocarbons in immature near-surface sediments, their molecular distributions and concentration variations with depth suggest that the majority of these mature hydrocarbons have migrated from a source much deeper in the sediment column. A portion of the thermogenic hydrocarbons may be derived from recycled material and includes phenanthrene, methyl phenanthrenes, chrysene and benzopyrenes. The migrated, thermogenic hydrocarbons include normal and isoprenoid alkanes with less than 21 carbons, naphthalene, methyl naphthalenes, ethyl naphthalenes and other aromatics of similar volatility (i.e., biphenyl, acenaphthene and fluorene). Triterpane, sterane and aromatized sterane distributions suggest that the thermogenic hydrocarbons at both sites have a common source and are overprinted with immature sediment hydrocarbons. The biomarker distributions and carbon isotopic compositions of the thermogenic hydrocarbons are atypical for petroleum produced in the Gulf of Mexico. Molecular distributions of the hydrocarbons are constant, regardless of the present depth of occurrence, suggesting that they have migrated in a separate phase. The upward migration of hydrocarbons from deeper sources is a wide-spread phenomenon in the Gulf of Mexico with several documented cases of massive seepage (visible oil) as well as the more diffuse permeation of Pleistocene sediments of the Mississippi Fan and two intraslopes reported here.     

高分子量烃油气化探方法的模拟实验研究——以黄海沉积物为例

传统上认为大分子烃类很难通过微渗漏方式逸散到地表,但已有研究表明高分子量烃类也可以逸散到现代沉积物中。本文基于黄海现代沉积物与典型原油地球化学特征的不同,将二者进行正交配比,系统研究不同配比产物的组成特征。结果表明:随着配比实验中原油比例的增大,正构烷烃和部分芳烃的色谱指纹呈现规律性变化,其正构烷烃奇偶优势逐渐消失,烷基芳烃丰度随之增加;三环萜烷、藿烷、规则甾烷等化合物的绝对浓度,以及二苯并噻吩/菲等的比值也呈现规律性变化,其中三环帖烷、C_(24)四环萜烷/C_(26)三环萜烷和三环萜烷/藿烷三者的变化明显且平稳,其数值范围均在0~3.0,适合用于渗逸图版。将研究区采集的未知样品与配比产物的组成特征进行对比,在排除外源污染的情况下可定性判识该研究区是否存在地下油气藏;将样品的相关参数投到图版上,有望进一步定量判识样品中渗入原油的比例。该方法可以作为常规油气化探的补充,在油气藏评价方面提供诸多信息,甚至在环境污染监控等领域有望获得推广。     

Effect of waterwashing on light ends compositional heterogeneity

A number of experiments have been carried out to study in the laboratory the effects of waterwashing on light ends composition. The experimental data were then compared with the results of a simple numerical model. Both approaches confirm the fast removal of the light aromatics by the flushing water and the very slow alteration of the C₁₀₊ compounds. The experimental results also indicate that the normal paraffins are removed faster than the cycloalkanes which is not in agreement with their respective solubility data. the simulations show that, over geological time, compounds such as benzene and toluene should be completely absent in an oil column which is not the case. Therefore, it is likely that in nature oil-water interactions are not as effective as one could imagine, which has implications for oil migration, as well as for the understanding of hydrodynamism phenomena in oil pools.     

Heating rate dependency of petroleum-forming reactions: implications for compositional kinetic predictions

The generation of bulk petroleum, liquid and gaseous hydrocarbons from the Duvernay Formation was simulated by heating immature kerogens in a closed system (MSSV pyrolysis) at four different heating rates (0.013, 0.1, 0.7 and 5.0 K/min). Using the established parallel reaction kinetic model, temperature and compositional predictions were tested to be suitable for geological conditions by comparing the laboratory results with natural changes in source bitumens and reservoir oil maturity sequences from the Duvernay Formation. In the case of bulk liquid and gaseous hydrocarbons, the above kinetic calculations can be considered valid because their maximum yields are independent of laboratory heating rates. In contrast, the contents of paraffins, aromatics and sulfur compounds show a pronounced heating rate dependence. Extrapolated to geological heating rates, the compositional predictions are consistent with the bulk composition of natural products in the Duvernay-petroleum system showing an increase of paraffinicity and hydrogen content. In contrast to that, the “hump” decreases with decreasing heating rate, a trend which is confirmed by the low amounts of unresolved compounds in natural high maturity products. Because of these heating-rate dependent compositional changes, geological predictions of natural molecular composition by the commonly used kinetic models are not suitable.     

Oil seepage and carbonate formation: A case study from the southern Gulf of Mexico

Oil seeps from the southern Gulf of Mexico can be regarded as natural laboratories where the effect of crude oil seepage on chemosynthesis‐based communities and carbonate precipitation can be studied. During R/V Meteor cruise 114 the seep sites UNAM (Universidad Nacional Autónoma de México) Ridge, Mictlan Knoll and Tsanyao Yang Knoll (Bay of Campeche, southern Gulf of Mexico) were investigated and sampled for authigenic carbonate deposits containing large amounts of liquid oil and solid asphalt. The δ¹³C values of individual carbonate phases including: (i) microcrystalline matrix aragonite and calcite; (ii) grey, cryptocrystalline to microcrystalline aragonite; and (iii) clear, fibrous aragonite cement, are between ?30‰ and ?20‰, agreeing with oil as the primary carbon source. Raman spectra reveal that residual heavy oils from all sites are immature and most likely originate from the same reservoir. Geochemical batch modelling using the software code PHREEQC demonstrates how sulphate‐driven oxidation of oil‐derived low‐molecular to high‐molecular weight hydrocarbons affects carbonate saturation state, and shows that the oxidation state of carbon in hydrocarbon compounds and oxidation rates of hydrocarbons control carbonate saturation and precipitation at oil seeps. Phase‐specific trace and rare earth element contents of microcrystalline aragonite and calcite, grey cryptocrystalline aragonite and clear aragonite were determined, revealing enrichment in light rare earth elements for grey aragonite. By comparing trace element patterns of carbonates with those of associated oils, it becomes apparent that liquid hydrocarbons constitute an additional source of trace metals to sedimentary pore waters. This work not only demonstrates that the microbial degradation of oil at seeps may result in the precipitation of carbonate minerals, it also elucidates that trace metal inventories of seep carbonates archive diagnostic elemental patterns, which can be assigned to the presence of heavy hydrocarbons in interstitial pore waters.     

近海油气藏渗漏烃的检测方法综述

主要总结了海域油气藏渗漏烃的航空遥感和地球化学检测方法.遥感测量可以从宏观上圈定海面油膜的分布范围,是海上油气藏勘探前期常用的廉价方法;海底沉积物取芯测量和海水嗅探器现场分析是海域油气藏渗漏烃地球化学检测的主要手段,海水样品的脱气处理是海水介质中渗漏烃地球化学检测的关键环节;海水和海底沉积物取样设备与脱气技术的不断完善,为海域油气藏渗漏烃的检测提供了基本保障;目前,底层水采样分析与海底沉积物取芯分析已成为一种有效且相对经济的海域油气藏渗漏烃检测技术组合,与一些地球物理方法结合应用,可有效地圈定海底下方渗漏源的位置.     

Hydrous pyrolysis of Monterey asphaltenes

Hydrous pyrolysis of asphaltenes has been tested as a method to reconstruct the chemical composition of biodegraded oils and oil seeps. The asphaltenes of three oils (a nondegraded oil, a biodegraded oil, and a biodegraded oil seep) from the Monterey Formation were studied. Results show that the aliphatic fraction generated by hydrous pyrolysis is very similar in chemical composition to the non-degraded oil. This makes the method very useful in correlation studies of biodegraded and nondegraded oils. It also allows to roughly estimate the maturity of the source of the biodegraded oil or oil seep.     

An examination by GC×GC-TOFMS of organic molecules present inhighly degraded oils emerging from Caribbean terrestrial seeps of Cretaceous age

For our ancestors, oil seeps were both a fascination and a resource but as the planet's reserves of high quality low density oil becomes increasingly depleted, so there is now a renewed interest in heavier,biodegraded oils such as those encountered in terrestrial seeps. One such seep is Pitch Lake in the Caribbean island of Trinidad, which is the largest natural deposit of asphalt in the world. At the northern end of the Caribbean, oil emerges along a tectonic contact on the island on Cuba. The sources of the oils from these seeps are relatively recent and both are subject to intense weathering due to the tropical conditions. When analysed by gas chromatography(GC) both oils appear as unresolved complex mixtures(UCM) and show a very high degree of biodegradation thus presenting an analytical challenge. In this case study, these two Caribbean seep oils were analysed by comprehensive two dimensional GC with time of flight mass spectrometry(GC×GC-TOFMS) to expose many thousands of the individual compounds that comprise the UCM. The high chromatographic resolution of the GC×GC-TOFMS produced good quality mass spectra allowing many compounds including molecular fossil ‘biomarkers' to be identified. Compound classes included diamondoid hydrocarbons, demethylated hopanes and secohopanes, mono-and tri-aromatic steroids. D-ring aromatised structures of the 8,14-seco-hopanes,including demethylated forms were present in both oils but further demethylation, probably at position C-25 during biodegradation, was only observed in the Pitch Lake oil. Many polycyclic aromatic hydrocarbons(PAHs) were absent although the fungal-derived pentacyclic PAH perylene was present in both oils. The presence of the angiosperm biomarker lupane in the Pitch Lake oil constrained the age to the Late Cretaceous. The higher degree of biodegradation observed in the Cuban oil was likely due to relatively slow anaerobic processes whereas oil within Pitch Lake was probably subject to additional more rapid aerobic metabolism within the lake.     

Fractionated aromatic petroleums and the generation of gas-condensates

Based on experimental and observational evidence, a mode of origin involving evaporative fractionation is proposed for a class of petroleums enriched in light aromatic and naphthenic hydrocarbons (benzene, toluene, meta-xylene, para-xylene, methylcyclopentane, cyclohexane, and methylcyclohexane). Progressive gas loss from gas-saturated oil is suggested as the causal mechanism, with simultaneous loss of light ends in gaseous solution, and accompanying fractionation. These processes were simulated experimentally.Residual oils exhibit the following changes in their remaining light hydrocarbons, <C₉: (1) increase in aromaticity (in aromatic hydrocarbons relative to normal alkanes of molecular weight); (2) increase in “normality” (in unbranched alkanes and naphthenes relative to branched isomers), and (3) decrease in paraffinicity (in paraffins relative to naphthenes). Retrogressive changes in maturity indicators take place leading to spurious evidence of immaturity in residual oils and the derived evaporative condensates.The phenomena occur in many basins, and are a key to understanding major aspects of petroleum variability. On the basis of aromaticity and paraffinicity relationships, evaporative gas-condensates are distinguishable from those generated by thermal cracking. Unfractionated thermal gas-condensates are rare. Evaporative condensates are the daughter products of oils which have suffered evaporative fractionation.     

天然气运移的气体组分的地球化学示踪

本文通过我国不同含油气盆地典型地区岩石酸解气、罐顶气和天然气中化学组分分析,结合天然气的形成和盆地的地质演化,研究了天然气运移时组分的变化。结果表明,天然气通过地层孔隙系统运移时,组分会发生明显分馏,表现在甲烷相对重烃、异构丁烷相对正构丁烷的优先迁移。酸解气、罐顶气和成藏天然气中C₁/C₂+、iC₄/nC₄及总烃/非烃等比值,是天然气运移示踪的有效指标。     

The effect of minor to moderate biodegradation on C5 to C9 hydrocarbons in crude oils

A suite of 18 oils from the Barrow Island oilfield, Australia, and a non-biodegraded reference oil have been analysed compositionally in order to detail the effect of minor to moderate biodegradation on C₅ to C₉ hydrocarbons. Carbon isotopic data for individual low molecular weight hydrocarbons were also obtained for six of the oils. The Barrow Island oils came from different production wells, reservoir horizons, and compartments, but have a common source (the Upper Jurassic Dingo Claystone Formation), with some organo-facies differences. Hydrocarbon ratios based on hopanes, steranes, alkylnaphthalenes and alkylphenanthrenes indicate thermal maturities of about 0.8% R_c for most of the oils. The co-occurrence in all the oils of relatively high amounts of 25-norhopanes with C₅ to C₉ hydrocarbons, aromatic hydrocarbons and cyclic alkanes implies that the oils are the result of multiple charging, with a heavily biodegraded charge being overprinted by fresher and more pristine oil. The later oil charge was itself variably biodegraded, leading to significant compositional variations across the oilfield, which help delineate compartmentalisation. Biodegradation resulted in strong depletion of n-alkanes (>95%) from most of the oils. Benzene and toluene were partially or completely removed from the Barrow Island oils by water washing. However, hydrocarbons with lower water solubility were either not affected by water washing, or water washing had only a minor effect. There are three main controls on the susceptibility to biodegradation of cyclic, branched and aromatic low molecular weight hydrocarbons: carbon skeleton, degree of alkylation, and position of alkylation. Firstly, ring preference ratios at C₆ and C₇ show that isoalkanes are retained preferentially relative to alkylcyclohexanes, and to some extent alkylcyclopentanes. Dimethylpentanes are substantially more resistant to biodegradation than most dimethylcyclopentanes, but methylhexanes are depleted faster than methylpentanes and dimethylcyclopentanes. For C₈ and C₉ hydrocarbons, alkylcyclohexanes are more resistant to biodegradation than linear alkanes. Secondly, there is a trend of lower susceptibility to biodegradation with greater alkyl substitution for isoalkanes, alkylcyclohexanes, alkylcyclopentanes and alkylbenzenes. Thirdly, the position of alkylation has a strong control, with adjacent methyl groups reducing the susceptibility of an isomer to biodegradation. 1,2,3-Trimethylbenzene is the most resistant of the C₃ alkylbenzene isomers during moderate biodegradation. 2-Methylalkanes are the most susceptible branched alkanes to biodegradation, 3-methylalkanes are the most resistant and 4-methylalkanes have intermediate resistance. Therefore, terminal methyl groups are more prone to bacterial attack compared to mid-chain isomers, and C₃ carbon chains are more readily utilised than C₂ carbon chains. 1,1-Dimethylcyclopentane and 1,1-dimethylcyclohexane are the most resistant of the alkylcyclohexanes and alkylcyclopentanes to biodegradation. The straight-chained and branched C₅–C₉ alkanes are isotopically light (depleted in ¹³C) relative to cycloalkanes and aromatic hydrocarbons. The effects of biodegradation consistently lead to enrichment in ¹³C for each remaining hydrocarbon, due to preferential removal of ¹²C. Differences in the rates of biodegradation of low molecular weight hydrocarbons shown by compositional data are also reflected in the level of enrichment in ¹³C. The carbon isotopic effects of biodegradation show a decreasing level of isotopic enrichments in ¹³C with increasing molecular weight. This suggests that the kinetic isotope effect associated with biodegradation is site-specific and often related to a terminal carbon, where its impact on the isotopic composition becomes progressively ‘diluted’ with increasing carbon number.     

试论轻烃形成过程中过渡金属的催化作用

轻烃和天然气是由富含干酪根矿物基质上过渡金属的催化作用形成，而不是干酪根和重质烃的热催化作用所致。催化作用是通过不同碳数闭作用，碳－碳及碳－金属键的断裂作用来实现的，它在轻烃形成的几个重要转化反应中起重作用，本文围绕着过渡金属的催化机理和轻烃的催化成因等基本问题展开经，目的在于不断探讨油气成因的新理论。     

A preliminary evaluation model for reservoir hydrocarbon generating potential established based on dissolved hydrocarbons in oilfield water

OILFIELDWATER,WHICHEXISTSCONCOMITANTLYWITH OILORGASINSUBSURFACERESERVOIRS,ISAKINDOFIMPOR TANTCOMPONENTOFRESERVOIRFLUIDANDALSOISADRIVING FORCEANDCARRIERFORHYDROCARBONMIGRATION,ACCUMULA TIONANDHYDROCARBONRESERVOIRFORMATION.OILFIELDWA TER,HYDROCARBONANDRESER…     

Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China: Part 3. Thermal cracking of liquid hydrocarbons and gas washing as the major mechanisms for deep gas condensate accumulations

The Lunnan Lower Uplift in the Tarim Basin, NW China contains a composite petroleum system with mainly biodegraded heavy oil in the west, normal oil in the center and gas condensate in the east. Twenty-three gas samples and 37 oil samples taken from three major hydrocarbon bearing intervals in the Lunnan lower bulge were analyzed for their stable carbon isotopes and molecular biomarkers. On the basis of their isotopic compositions, biomarkers, especially diamondoids, and integrating the physical properties of the hydrocarbons including densities, GOR and PVT relationships, it has been concluded that the recently discovered deep (6500 m) eastern Lungu giant Ordovician gas condensate pool with an estimated reserve of 723 million bbl oil equivalent is a secondary hydrocarbon accumulation derived from the mixing of an early formed oil and a late formed gas. The extremely dry gases with a gas dryness of >0.98 were derived from thermal cracking of crude oils and charged to an existing oil reservoir causing extensive gas washing and secondary alteration. Compared with most of the hydrocarbons in the Lunnan area, the gas from the eastern Lungu gas condensate pool has a heavier carbon isotopic composition, higher dryness and a higher maturity level. The unique physical, chemical and isotopic compositions of the gas condensate are believed to be a direct result of gas flushing of an early formed eastern Lungu oil pool by a late formed dry gas from oil cracking. The eastern Lungu gas condensate pool is presently characterized by containing “dry gas, heavy oil, abundant aromatics and high wax”.     

含油流体包裹体：地球化学分析与地质应用

利用含油流体包裹体可以获得与原油和源区常规分析同等质量的可靠地球化学数据。细致而又小心谨慎地对待各测试步骤(如样品清洗、背景空白等)是成功进行包裹体油气成分分析的基础。从技术上来说，每一分析步骤都具有挑战性，但如果我们能按步骤循序渐进，就不仅能够分析那些含有大量石油包裹体的样品(如当今或古油藏样品)，而且可以测试含极少量石油包裹体的样品(如迁移路径或极古老岩石样品)。包裹体中可被测试的碳氢化合物多种多样，包括低分子量的碳氢化合物、n-链烷、类异戊二烯、生物标志物、芳香族碳氢化合物等。流体包裹体内石油成分分析在地质上有广泛应用，比如可以更好地重建储集区石油重注史、确定盆地中以前未知的活性源岩。在储集区内由生物降解造成的石油再造和(或)水洗作用经常被抹去，流体包襄体分析则可以解释储集区复杂成油阶段，当然更可以去除钻孔泥浆添加剂或其他污染物的影响。此外，也可以获知地球早期生物圈碳氢化合物的组成及多样性，以及在勘探区或盆地进行二次迁移路径填图。     

济阳拗陷特殊生物降解油的初步研究

济阳拗陷草古100平19井奥陶系储层原油,饱和烃馏分均已损失殆尽,仅残余藿烷系列化合物;同时,芳烃馏分化合物也已损失殆尽,仅残余三芳甾烷系列化合物。上述芳烃化合物俱已损失殆尽的蚀变特征,在相关文献中的报道较为少见。原油中不受水洗作用影响的正构烷烃和不受生物降解作用影响的有机硫化合物(苯并噻吩和二苯并噻吩)均已损失殆尽,显然暗示了原油蚀变是遭受水洗作用和生物降解作用共同作用的结果。原油芳烃馏分的“UCM”鼓包幅度明显大于饱和烃馏分的“UCM”鼓包,暗示了芳烃化合物的降解速率平行甚至超过饱和烃的降解速率。原油中低分子量芳烃化合物的快速降解,暗示了油藏可能是在氧化条件下而非在还原条件下发生蚀变的。