塔里木盆地台盆区原油甾、萜烷浓度与热稳定性

对塔里木盆地台盆区未遭受生物降解的138个原油样品进行甾、萜烷定量分析,展示原油甾、萜烷浓度随成熟度变化特征。C_(23)三环萜烷/(C_(23)三环萜烷+C_(30)藿烷)值与甾、萜烷浓度图版显示各类甾、萜烷具有不同的热稳定性。依据热稳定性的差异将甾、萜烷分为3类:第1类包括Tm、C_(29)藿烷和C_(30)藿烷等五环三萜烷,随成熟度增高,这类化合物浓度降低较快,具有相对较低的热稳定性;第2类既包括Ts、C_(29)Ts和C_(30)重排藿烷等五环三萜烷,也包括C_(27)和C_(29)规则甾烷各异构体,随成熟度增高,这类化合物浓度降低较慢,具有中等热稳定性;第3类包括C27重排甾烷、C_(21)甾烷和C_(23)三环萜烷,随成熟度增高,这类化合物浓度先增高、后降低,降低速率低于前2类化合物,表明其热稳定性相对较高。在常用的甾、萜烷成熟度指标中,Ts/(Ts+Tm)、C_(29)Ts/(C_(29)Ts+C_(29)藿烷)和C_(30)重排藿烷/(C_(30)重排藿烷+C_(30)藿烷)是基于第1类和第2类化合物之间的热稳定性差异;C_(27)重排甾烷/(C_(27)重排甾烷+C_(27)规则甾烷)和C_(21)/(C_(21)+∑C_(29))甾烷是基于第2类与第3类化合物之间的热稳定性差异;C_(23)三环萜烷/(C_(23)三环萜烷+C_(30)藿烷)则是基于第1类与第3类化合物之间的热稳定性差异,具有更好的应用效果。

塔里木盆地寒武系烃源岩分布的重新认识及其意义

塔里木盆地台盆区海相烃源岩的分布是制约油气勘探的主控因素,在构造—古地理背景分析基础上,通过典型井地球化学分析结合测井标定与地震追踪,重新厘定寒武系主力烃源岩分布。结果表明,东部烃源岩主要分布在中-下寒武统泥页岩中,西部烃源岩主要分布在下寒武统泥页岩中,中-上寒武统—下奥陶统碳酸盐岩缺乏有效烃源岩,单井纵向厚度标定东部低于100 m、西部低于40 m,远低于早期预测厚度。寒武系烃源岩并非满盆分布,主要分布在满东凹陷、满西台内洼,并发现塔西南南缘可能发育大面积陆缘斜坡烃源岩,而巴楚—麦盖提、塘古—塔中、塔北等基底古隆起区缺乏烃源岩。受控前寒武纪古隆起与寒武纪弱伸展背景影响,寒武系发育4种构造背景下的烃源岩:主要为台间盆、台内洼烃源岩,以及可能存在被动陆缘斜坡、板块边缘裂谷烃源岩。

柯坪褶皱冲断带构造变形特征解析

柯坪褶皱冲断带位于塔里木盆地西北缘,勘探程度低,地表、地下构造变形复杂,构造模型存在多解性.本文通过对最新地震资料的解释,结合地质露头和钻井资料,建立起该地区新的构造模型.柯坪褶皱冲断带构造变形具有东西分段、南北分带的特征.东西向具有“跷跷板式”的沉积特征.中部皮羌段中寒武统盐层厚度大,主要发育盐下冲断构造和盐上反向滑脱构造;东、西两段主要发育基底卷入构造.石炭纪柯坪褶皱冲断带存在一期古隆起构造.晚石炭世一早二叠世沉积的康克林组具有填平补齐的特征.皮羌段发育两期断裂.早期为海西期断裂,晚期为喜山期断裂.石炭纪古隆起受控于早期断裂.喜山期形成的盐上滑脱构造对下伏潜在的古油藏具有封闭和保存作用.

Oil-source rock correlation and quantitative assessment of Ordovician mixed oils in the Tazhong Uplift, Tarim Basin

The origin of the marine oils in the Tarim Basin has long been a disputed topic. A total of 58 DST （drill stem test） crude oil and 8 rock samples were investigated using a comprehensive geochemical method to characterize and identify the origin of the Ordovician oils in the Tazhong Uplift, Tarim Basin, northwest China. Detailed oil–oil and oil–source rock correlations show that the majority of the oils have typical biomarker characteristics of the Middle-Upper Ordovician （O2＋3） source rock and the related crude oil. These characteristics include a distinct ＂V-shaped＂ relative abundance of C27, C28 and C29 regular steranes, low abundance of dinosterane, 24-norcholestanes, triaromatic dinosteroids and gammacerane. Only a few oils display typical biomarker characteristics indicating the Cambrian–Lower Ordovician （∈-O1） genetic affinity, such as linear or anti ＂L＂ shape distribution of C27, C28 and C29 regular sterane, with relatively high concentrations of dinosterane, 24-norcholestanes, triaromatic dinosteroids and gammacerane. It appears that most of the Ordovician oils in the Tazhong Uplift were derived from the O2＋3 intervals, as suggested by previous studies. However, the compound specific n-alkane stable carbon isotope data indicate that the Ordovician oils are mixtures from both the ∈-O1 and O2＋3 source rocks rather than from the O2＋3 strata alone. It was calculated that the proportion of the∈-O1 genetic affinity oils mixed is about 10.8%-74.1%, with an increasing trend with increasing burial depth. This new oil-mixing model is critical for understanding hydrocarbon generation and accumulation mechanisms in the region, and may have important implications for further hydrocarbon exploration in the Tarim Basin.

Comparison between several multi-parameter seismic inversion methods in identifying plutonic igneous rocks

With the objective of establishing the necessary conditions for 3-D seismic data from a Permian plutonic oilfield in western China, we compared the technology of several multi-parameter seismic inversion methods in identifying igneous rocks. The most often used inversion methods are Constrained Sparse Spike Inversion (CSSI), Artificial Neural Network Inversion (ANN) and GR Pseudo-impedance Inversion. Through the application of a variety of inversion methods with log curves correction, we obtained relatively high-resolution impedance and velocity sections, effectively identifying the lithology of Permian igneous rocks and inferred lateral variation in the lithology of igneous rocks. By means of a comprehensive comparative study, we arrived at the following conclusions: the CSSI inversion has good waveform continuity, and the ANN inversion has lower resolution than the CSSI inversion. The inversion results show that multi-parameter seismic inversion methods are an effective solution to the identification of igneous rocks.

Origin of the Silurian Crude Oils and Reservoir Formation Characteristics in the Tazhong Uplift

The Silurian stratum in the Tazhong uplift is an important horizon for exploration because it preserves some features of the hydrocarbons produced from multi-stage tectonic evolution. For this reason, the study of the origin of the Silurian oils and their formation characteristics constitutes a major part in revealing the mechanisms for the composite hydrocarbon accumulation zone in the Tazhong area. Geochemical investigations indicate that the physical properties of the Silurian oils in Tazhong vary with belts and blocks, i.e., heavy oils are distributed in the TZ47-15 well-block in the North Slope while normal and light oils in the No. I fault belt and the TZ16 well-block, which means that the oil properties are controlled by structural patterns. Most biomarkers in the Silurian oils are similar to that of the Mid-Upper Ordovician source rocks, suggesting a good genetic relationship. However, the compound specific isotope of n-alkanes in the oils and the chemical components of the hydrocarbons in fluid inclusions indicate that these oils are mixed oils derived from both the Mid- Upper Ordovician and the Cambrian-Lower Ordovician source rocks. Most Silurian oils have a record of secondary alterations like earlier biodegradation, including the occurrence of ＂UCM＂ humps in the total ion current （TIC） chromatogram of saturated and aromatic hydrocarbons and 25-norhopane in saturated hydrocarbons of the crude oils, and regular changes in the abundances of light and heavy components from the structural low to the structural high. The fact that the Silurian oils are enriched in chain alkanes, e.g., n-alkanes and 25-norhopane, suggests that they were mixed oils of the earlier degraded oils with the later normal oils. It is suggested that the Silurian oils experienced at least three episodes of petroleum charging according to the composition and distribution as well as the maturity of reservoir crude oils and the oils in fluid inclusions. The migration and accumulation models of these oils in the TZ47-15 well-blocks, the No. I fault belt and the TZ16 well-block are different from but related to each other. The investigation of the origin of the mixed oils and the hydrocarbon migration and accumulation mechanisms in different charging periods is of great significance to petroleum exploration in this area.

The geochemical characteristics and origin of crude oils in the Kekeya Oilfield,Xinjiang,China

GC/MS and GC/MS/MS techniques were employed to describe the characteristics of biomarker assem-blages in two sets of hydrocarbon source rocks,Jurassic and Permian,in southwestern Tarim,and the parameters for the classification of the two sets of hydrocarbon source rocks have been established. It is found that diahopane and C30-unknown terpane are abundant in Permian samples,the contents of diahopane in Jurassic samples are relatively low,and terpenoids have been detected in Jurassic samples but not in Permian source rock samples. Kekeya crude oils are abundant in diahopane and C30-unknown terpane. The results of fine oil-rock correlation indicated that Kekeya crude oils were derived mainly from the Permian hydrocarbon source rocks. However,a small amount of diterpenoid was detected in the crude oils,indicating that the Jurassic hydrocarbon source rocks also made a certain contribution to Kekeya crude oils.

Re-Examination of the Oil and Gas Origins in the Kekeya Gas Condensate Field,Northwest China——A Case Study of Hydrocarbon-Source Correlation Using Sophisticated Geochemical Methods

This work discussed the origins, alteration and accumulation processes of the oil and gas in the Kekeya gas condensate field based on molecular compositions, stable carbon isotopes, light hydrocarbons, diamondoid hydrocarbons and biomarker fingerprints. A comparison study is also made between the geochemical characteristics of the Kekeya hydrocarbons and typical marine and terrigenous hydrocarbons of the Tarim Basin. Natural gas from the Kekeya gas condensate field is derived from Middle–Lower Jurassic coal measures while the condensates are derived from Carboniferous–Permian marine source rocks with a higher maturity. In the study area, both natural gas and condensates have experienced severe water washing. A large amount of methane was dissolved into the water, resulting in a decrease in the dryness coefficient. Water washing also makes the carbon isotopic compositions of the natural gas more negative and partially reverse. Considering that the gas maturities are higher than once expected, gas generation intensity in the study area should be much stronger and the gas related to the Jurassic coal measures could promise a greater prospecting potential. As a result of evaporative fractionation, the Kekeya condensates are enriched in saturates and lack aromatics. Evaporative fractionation disguises the original terrigenous characteristics of the light hydrocarbons associated with the natural gas, making it appear marinesourced. Thus, alteration processes should be fully taken into consideration when gas–source correlations are carried out based on light hydrocarbons. With the condensates discovered in the study area all being ＂migration phase＂, the pre-salt Cretaceous and Jurassic reservoirs may promise great exploration potential for the ＂residual phase＂ hydrocarbons. This research not only is of significance for oil and gas exploration in the southwest Tarim Basin, but also sheds light on the oil/gas-source correlations in general.

Kinetic Parameters of Methane Generated from Source Rocks in the Kuqa Depression of Tarim Basin and Their Application

In a thermal simulation experiment of gold tubes of closed-system, calculating with the KINETICS and GOR-ISOTOPE KINETICS software, kinetic parameters of gas generation and methane carbon isotopic fractionation from Triassic-Jurassic hydrocarbon source rocks in the Kuqa depression of Tarim Basin are obtained. The activation energies of methane generated from Jurassic coal, Jurassic mudstone and Triassic mudstone in the Kuqa Depression are 197-268 kJ/mol, 180-260 kJ/mol and 214-289 kJ/mol, respectively, and their frequency factors are 5.265×10^13 s^-1, 9.761×10^11 s^-1 and 2.270×10^14 s^-1. This reflects their differences of hydrocarbon generation behaviors. The kinetic parameters of methane carbon isotopic fractionation are also different in Jurassic coal, Jurassic mudstone and Triassic mudstone, whose average activation energies are 228 kJ/mol, 205 kJ/mol and 231 kJ/mol, respectively. Combined with the geological background, the origin of natural gas in the Yinan-2 gas pool is discussed, and an accumulation model of natural gas is thus established. The Yinan- 2 gas is primarily derived from Jurassic coal-bearing source rocks in the Yangxia Sag. Main gas accumulation time is 5-0 Ma and the corresponding Ro is in the range from 1.25 %-1.95 %. The loss rate of natural gas is 25 %-30 %.

Origin of Crude Oil in the Lunnan Region,Tarim Basin

The oil source of the Tarim Basin has been controversial over a long time. This study characterizes the crude oil and investigates the oil sources in the Lunnan region, Tarim Basin by adopting compound specific isotopes of n-alkanes and biomarkers approaches. Although the crude oil has a good correlation with the Middle-Upper Ordovician （02＋3） source rocks and a poor correlation with the Cambrian-Lower Ordovician （C-O1） based on biomarkers, the ~3C data of n-alkanes of the Lunnan oils show an intermediate value between C-O1 and 02＋3 genetic affinity oils, which suggests that the Lunnan oils are actually of an extensively mixed source. A quantification of oil mixing was performed and the results show that the contribution of the Cambrian-Lower Ordovician source rocks ranges from 11% to 70% （averaging 36%）, slightly less than that of the Tazhong uplift. It is suggested that the inconsistency between the biomarkers and od3C in determining the oil sources in the Lunnan Region results from multiple petroleum charge episodes with different chemical components in one or more episode（s） and different sources. The widespread marine mixed-source oil in the basin indicates that significant petroleum potential in deep horizons is possible. To unravel hydrocarbons accumulation mechanisms for the Lunnan oils is crucial to further petroleum exploration and exploitation in the region.

The generation kinetics of natural gases in the Kela-2 gas field from the Kuqa Depression, Tarim Basin, northwestern China

The Kela-2 gas field, found in the Kuqa Depression of the Tarim Basin, northwestern China, is a large-sized dry gas field (C1 /C1-5 =0.992 0.999) and characterized by ultra-high pressure (pressure factor up to 2.0 2.2). The pyrolysis experiment was carried out under isothermal gold-tube closed system, with samples collected from the Jurassic coal, Jurassic mudstone and Triassic mudstone in the Kuqa Depression. The result of gas yield showed that the Middle and Lower Jurassic source rocks have higher gas generation potential than the Triassic source rocks. The kinetic modeling of gas generation and methane carbon isotope fractionation suggested that the Kela-2 gases belong to the products of high-over mature stages and were mainly derived from the Middle and Lower Jurassic coal-bearing strata. The Triassic source rocks made a minor contribution to the Kela-2 gases. The Kela-2 gases chiefly generated from coal-bearing source rocks with R o values from 1.3% to 2.5%, and thus primarily accumulated after 5 Ma.

Molecular fossils and oil-source rock correlations in Tarim Basin, NW China

The distribution of 'molecular fossils' （bio-markers） of steroid compounds in extracts from some spe-cific geologic age in the Tarim Basin have been analyzed andare used as the fingerprints for the oil-source rock correla-tion. Having been affected by maturation, migration, phasefractionation and biodegradation, not any molecular fossilsrelated to source and environment can be used as the finger-prints for oil-source rock correlation. Some special bio-markers widely existed in the extracts from Cambrian andOrdovician rocks in the Tarim Basin and showed obviousdifference in each stratum, including dinosteranes (C30),4-methyl-24-ethyl-cholestanes (C30) and their aromatizedsteroids, C24-norcholestanes and C28 steranes originated fromdinoflagellates and diatom. Few oils such as the heavy oildrilled in the Cambrian reservoir from Tadong 2 well （TD2）correlated well with the extracts from the Cambrian. Theamazing similarity of the relative contents of these com-pounds between the marine oils produced in Tazhong andTabei uplifts and the extracts from the Upper Ordoviciansuggests that the Middle-Upper Ordovician is the very likelymain source for the marine oils.

Migration and accumulation of natural gas in Kela-2 gas field

With the guidance of petroleum system theory,the dynamic filling history of natural gas in the Kela-2 gasfield is analyzed by using a large suite of oil and gas geo-chemistry evidence in combination with the tectonic evolu-tion history and reservoir evolution history. It concludes thatthe Kela-2 gas field was formed by capturing the gas gener-ated during the main gas generation period, while the latekerogen cracking gas contributed a little to the gas field. Itsuggests that the gas generated during the main gas genera-tion accumulated in the early-formed wide-gentle anticline,which is the necessary condition for natural gas to re-migrateand enrich late to form the large-scale gas reservoir. Thenewest research shows that the filling history of gas in theDabei-1, Yinan-2, Tuziluoke and Dina-2 gas fields was re-lated with the natural gas accumulation in the early wide-gentle anticline as well as late re-migration and enrichmentof natural gas.

Pyrolysis of oil at high temperatures:Gas potentials,chemical and carbon isotopic signatures

Although the gas cracked from oil has been believed to be one of the important sources in highly matured marine basins, there are still some debates on its resource potentials and chemical and isotopic compositions. In this study a Cambrian-sourced marine oil sample from the Silurian reservoir of well TZ62 in the central Tarim basin was pyrolyzed using sealed gold tubes with two different pyrolysis schemes: continuous pyrolysis in a closed system and stepwise semi-open pyrolysis. The results show that the maximum weight yield of C1-5 gases occurs at EasyRo=2.3% and the residual gas poten-tial after this maturity is only 43.4 mL/g, about 12% of the yield of 361 mL/g at EasyRo=2.3%. Combined with the results of kinetic modeling, the main stage of gas generation from oil cracking is believed within the EasyRo=1.6%―2.3%. The increase in the volume yield of C1-5 gases at EasyRo>2.3% in a closed system is mainly related to the re-cracking of previously formed C2-5 wet gases, not the direct cracking of oil. The stepwise pyrolysis experiments show that the gas from the cracking of residual oil at EasyRo>2.3% is characterized by very high dryness index (higher than 92%) and heavy methane carbon isotopes ranging from ?28.7‰ to ?26.7‰, which is quite different from the gases from the con-tinuous pyrolysis in a closed system. The kinetic modeling of methane carbon isotope fractionation shows that the carbon isotopes of methane within the main stage of gas generation (EasyRo<2.3%) are far lighter than the carbon isotopes of the precursor oils under a geological heating rate of 2 ℃ /Ma. The above observations and results provide some new clues to the accurate recognition and objective resource evaluation of oil cracking gas in highly mature marine basins.

塔里木盆地塔西南坳陷柯东构造带甫沙4井原油来源及充注过程

甫沙4井位于塔里木盆地塔西南坳陷昆仑山前冲断带的柯东构造带上,北部和东部分别发育有柯克亚和柯东1井油气田。为研究甫沙4井原油来源与充注过程,对原油样品和连续抽提后的含油砂样各组分(游离态、束缚态、包裹体)进行GC、GC-MS和GC-ⅠRMS分析,与柯克亚凝析油气田油样进行油—油对比。结果表明:甫沙4井晚期充注原油组分具有C_(29-32)重排藿烷、重排甾烷和Ts相对含量高,C_(27-29)甾烷ααα 20R分布呈反“L”型,以及正构烷烃单体碳同位素值较低等特征,与柯克亚凝析油气田来源于二叠系普司格组(P_(2-3)p)烃源岩的主体原油(Ⅰ类)地球化学特征一致。而早期充注的原油组分具有重排藿烷、重排甾烷和Ts相对含量较低,C_(27-29)甾烷ααα 20R分布呈“V”型,以及正构烷烃单体碳同位素值较高等特征,与柯克亚凝析油气田来源于中—下侏罗统湖相泥岩的Ⅱ类原油地球化学特征一致。甫沙4井经历3个阶段成藏过程:(1)在上新世,二叠系烃源岩于生油晚期阶段生成的Ⅰ类原油运移至柯克亚构造带或柯东构造带深部形成油藏;(2)在更新世早期,侏罗系烃源岩于生油早—中期生成的Ⅱ类原油运移至甫沙4井白垩系储层;(3)在第四纪,强烈的构造作用使深部Ⅱ类原油沿断裂调整进入甫沙4井白垩系储层。最终造成甫沙4井白垩系储层Ⅱ类原油先充注,Ⅰ类原油后充注的特殊现象。

Oil cracking to gases: Kinetic modeling and geological significance

从 Tarim 盆的 LN14 的一件三叠纪的油样品是用在在 50 MPa 的恒压下面的 200-620 deg C 的封上的金试管的 pyrolyzed。气体并且剩余可溶的烃被分析。结果证明到气体的石油的裂化能被划分成二个不同阶段：从液体油的全部的 C1-5 气体的主要产生由 C2-5 烃和 secondary 或到甲烷的 C2-5 气体的进一步的裂开的优势描绘了导致气体的进步的干的 andcarbon 富有的事。基于试验性的数据，运动参数为主要产生和第二等的石油的裂化裂化气被决定并且外推到地质的条件预言热稳定性和原油的裂开的程度。最后，为原油的热破坏的一个进化模型被建议，它到油裂化气的迁居和累积的含意被讨论。

The generation and accumulation of natural gas from Yinan 2 gas pool in Kuqa Depression

By using the methods of hydrocarbon generation kinetics and carbon isotope kinetics, combined with geological background of natural gas pool formation, the generation and accumulation of natural gas from Yinan 2 gas pool was studied in Kuqa Depression of the Tarim Basin. Natural gas of Yinan 2 gas pool is mainly derived from Middle and Lower Jurassic coal-bearing source rocks, and generally belongs to long time-accumulated gas. It is suggested that Yinan 2 gas is chiefly accumulated in the last 5 Ma, its Ro ranges from 1.25% to 1.95%, and the loss rate of natural gas is about 25%-30%. This work not only complements and reduces the deficiency of formation model of natural gas pools which traditionally depends on the matching relationships between source rock, reservoir, cap rock, and trap, but also is a useful reference in the study of other gas pools.

A discussion on the upper limit of maturity for gas generation by marine kerogens and the utmost of gas generative potential:Taking the study on the Tarim Basin as an example

The experimental data yielded by Rock-Eval pyrolysis, kerogen atomic H/C, Py-GC and gold-tube sealing thermal simulation on the marine Cambrian-Ordovician source rock from the Tarim Basin revealed that the upper limit of maturity for natural gas generation or the "deadline of gas generation" for marine types I and II kerogens is equal to 3.0% of vitrinite reflectance (Ro); while the "deadline of gas generation" for type III kerogens typically like coals is as high as 10% Ro. Thus, different organic matter has obviously different utmost maturities for gas generation. The mass-balance calculation by kerogen elements showed that when Ro>1.5%, the utmost amount of gas generation for the marine type II kerogen is less than 185 m3/t TOC, accounting for less than 30% of its total hydrocarbon generative potential; when Ro>2.0%, it becomes 110 m3/t TOC, less than 20% of the total hydrocarbon generative potential. The amount of the gas generative potential obtained by Rock-Eval is only around one tenth of the calculated value by the mass balance of kerogen elements at the same thermal evolutionary stage, while those by Py-GC and gold-tube sealing simulation are intervenient between the above two. The utmost of gas generative potential at the over 1.3% Ro stage is around 60―90 m3/t TOC, therefore, the amount of gas generation obtained by Rock-Eval is the minimum of gas generative potential, while that by the mass-balance calculation of kerogen elements is the maximum that the actual amount of gas generation should not exceed.

Constraints on the diversity of crude oil types in the Lunnan Oilfield,Tarim Basin,NW China

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

Carbon isotopic compositions of 1,2,3,4-tetramethylbenzene in marine oil asphaltenes from the Tarim Basin:Evidence for the source formed in a strongly reducing environment

Although 1-alkyl-2,3,6-trimethylbenzenes and a high relative amount of 1,2,3,4-tetramethylbenzene have been detected in marine oils and oil asphaltenes from Tabei uplift in the Tarim Basin, their bio-logical sources are not determined. This paper deals with the molecular characteristics of typical ma-rine oil asphaltenes from Tabei and Tazhong uplift in the Tarim Basin and the stable carbon isotopic signatures of individual compounds in the pyrolysates of these asphaltenes using flash pyrolysis-gas chromatograph-mass spectrometer (PY-GC-MS) and gas chromatograph-stable isotope ratio mass spectrometer (GC-C-IRMS), respectively. Relatively abundant 1,2,3,4-tetramethylbenzene is detected in the pyrolysates of these marine oil asphaltenes from the Tarim Basin. δ 13C values of 1,2,3,4-tetrame-thylbenzene in the pyrolysates of oil asphaltenes vary from-19.6‰ to-24.0‰, while those of n-alkanes in the pyrolysates show a range from-33.2‰ to-35.1‰. The 1,2,3,4-tetramethylbenzene in the pyro-lysates of oil asphaltenes proves to be significantly enriched in 13C relative to n-alkanes in the pyro-lysates and oil asphaltenes by 10.8‰―15.2‰ and 8.4‰―13.4‰, respectively. This result indicates a contribution from photosynthetic green sulfur bacteria Chlorobiaceae to relatively abundant 1,2,3,4-tetramethylbenzene in marine oil asphaltenes from the Tarim Basin. Hence, it can be speculated that the source of most marine oil asphaltenes from the Tarim Basin was formed in a strongly reducing water body enriched in H2S under euxinic conditions.