Domestic 10X-type and 13X-type molecular sieves were used for the adsorption and desorption experiments of steranes and hopanes, and their stable carbon isotopic fractionation by the two kinds of molecular sieves during separation process was discussed in detail. These two kinds of molecular sieves have various adsorption effects on different types of compounds. They both have the weakest adsorption capacities for 5α, 14α, 17α-20S steranes, 5α, 14β, 17β-20R steranes and 5α, 14β, 17β-20S steranes, followed by 5α, 14α, 17α-20R steranes, β-carotane, and gammacerane. The 13X-type molecular sieve has stronger adsorption capacity for hopanes than that of the 10X-type molecular sieve, which can be used for the separation of hopanes. The 10X-type molecular sieve has stronger adsorption capacity for steranes than that of the 13X-type molecular sieve. By controlling the amount of eluent, different configuration of steranes can be further separated. The stable carbon isotopes of steranes and hopanes showed that no carbon isotopic fractionation occurred during separation process and the repeatability was very good, indicating that domestic 10X-type and 13X-type molecular sieves can be used for steranes and hopanes separation.

Both molecular clocks and the fossil record indicate that eukaryotic algae evolved by the late Paleoproterozoic Era (2500 to 1600 million years ago, Ma). However, the biomarker record of steranes does not reveal their presence until the early Tonian Period (1000 to 720 Ma), suggesting that eukaryotic algae were insignificant ecosystem members for some one billion years after they evolved. Here, we conducted high-temperature pyrolysis and fixed-bed catalytic hydropyrolysis of kerogen from the ca. 1400 Ma Xiamaling Formation, liberating a host of lipid molecules, including C-27 to C-29 steranes. With sterane to hopane ratios ranging up to 0.17, our results indicate the presence of significant populations of both red and green algae at 1400 Ma, some 600 million years earlier than previously recognized.

运用全二维气相色谱/飞行时间质谱(GC×GC/TOFMS)结合全二维气相色谱/氢火焰离子检测(GC×GC/FID)实现了对准噶尔盆地乌夏地区不同生物降解程度原油饱和烃复杂混合物成分的分析研究,提升了对生物降解原油组成及成因机理认识.结果显示:①乌夏地区生物降解原油中不可识别混合物主要由烷基取代环状化合物及其同系物组成,这些环状化合物主要以六元环为基本单元,包括单环烷烃(类胡萝卜烷)、双环烷烃(十氢化萘,脱-A,B环-甾烷)、三环烷烃(菲满类、三环萜烷、断甾烷)和金刚烷类、四环烷烃(甾烷、断藿烷、四环萜烷)和五环烷烃(藿烷、25-降藿烷)等六大类化合物;②不同生物降解原油中饱和烃总量变化不大,饱和烃中不同族系化合物随着生物降解作用增强呈现此消彼长的变化规律,在重度生物降解阶段,双环烷烃存在大量同系物及同分异构体,可能是微生物作用新生成的化合物,并且由于自身较强的抗生物降解能力,使双环烷烃成为饱和烃的优势成分,占饱和烃总量的50％左右,而多环烃类化合物含量随着生物降解程度逐渐增加,主要是由抗降解能力相对较强的高环数化合物的逐渐富集形成.

在用全二维气相色谱—飞行时间质谱定性石油样品中的金刚烷系列化合物基础上,建立了一套包括前处理方法在内的用全二维气相色谱—氢火焰离子化检测器定量分析石油样品中金刚烷类化合物的新方法.该方法利用全二维气相色谱正交分离的特点,使得常规的金刚烷系列化合物在色谱条件下就能很好地分离,实现了该类化合物的色谱定量分析.为减少前处理过程中低碳数金刚烷类化合物的挥发损失,新方法中对现有的石油样品饱和烃馏分的分离方法进行改进,建立了用小柱法分离收集饱和烃前馏分的方法,具有所需样品量少、分析时间短、实验耗材少的特点,金刚烷回收率满足色谱定量分析的需要.与传统的色谱—质谱定量方法相比,全二维色谱定量方法分离度高,重复性好,对内标物质要求低,7次实验的相对标准偏差小于5％,满足复杂体系的分析要求.

This paper investigates demethylation as a possible mechanism leading to isotopic reversals of shale gas during the thermal degradation using a model compound representative of methylated monoaromatic hydrocarbons presented in over-mature organic matter. The model compound used is 1,3,5-Trimethylbenzene (TB), which was subjected to isothermal pyrolysis experiments at three temperatures of 400 degrees C, 425 degrees C and 450 degrees C holding from 24 to 480 h at 300 bar. During the process of TB pyrolysis, heavy hydrocarbon gas was generated in addition to methane and hydrogen. Some aromatic compounds-bearing ethyl and propyl were also detected in the liquid products. The series of carbon isotope of hydrocarbon gas shows a reversal feature (i.e. delta C-13(1) > delta C-13(2)) at TB conversion below 80%. The reversal feature is replaced by normal isotopic distribution when the TB conversion is above 80%. According to the yields variation of different products during TB cracking, heavy hydrocarbon gas appears to have generated via two pathways, (1) combination among the CH3 radicals from demethylation and (2) cleavage of long aliphatic chains formed by combination between CH3 radicals and methyl linked on aromatic rings during TB cracking. The reversal of carbon isotope between CH4 and C2H6 was caused by carbon isotopic fractionation during the heavy hydrocarbon gas generation. The TB cracking experiments indicates that demethylation is one of the factors causing the isotopic reversals of shale gas generated at over maturity. The change of carbon isotope from reversal to normal distribution is attributed to the cracking of C2H6 generated at higher thermal dynamics. It is deduced that the phenomenon of isotopic reversal of shale gas would disappear and be replaced by normal isotopic distribution with further decreasing of natural gas wetness.

This paper examines the geochemical characteristics of pyrolysates from macerals such as the alginite, sporinite, cutinite, collotelinite and collodetrinite isolated from Jurassic coals and investigates their relative contributions to oil potential in respect to the Jurassic coal measures in the Turpan and other basins of northwest China. The alginite pyrolysate has the lightest carbon isotopic composition, followed by the sporinite and cutinite, while the collotelinite and collodetrinite pyrolysates show comparatively heavy carbon isotopic compositions. Pyrolysates from the alginite, sporinite and cutinite demonstrate much lower Pr/Ph ratios than those from the collotelinite and collodetrinite. Regarding the distributions of regular biomarkers such as steranes and hopanes, the alginite pyrolysate is comparatively rich in C-27 steranes, followed by pyrolysates from the sporinite and cutinite, whereas pyrolysates from the collotelinite and collodetrinite contain little or undetectable C-27 steranes. Furthermore, the pyrolysates from different kinds of macerals have remarkably different gammacerane concentrations. The gammacerane concentrations in pyrolysates from the alginite and cutinite are higher than that from the sporinite, though these macerals were separated from the same coal, whereas the collotelinite and collodetrinite pyrolysates have almost no gammacerane. The experimental results suggest that liptinite content in one given coal (e.g. alginite, sporinite and cutinite) controls the biomarker distributions in the coal-derived oils. Mass balance calculations of carbon isotopic compositions from the maceral pyrolysates show that the carbon isotopic composition of liquid hydrocarbons generated by organic matter in mudstone is very similar to that of most of the oils in the Jurassic coalbearing basins of northwest China, while liquids from coal are much heavier than that of the oils. This indicates that the mudstone is likely to be the major source rocks of the oils, and coals likely to be the subordinate source rocks. Mass balance calculation based on the maceral contents and hydrocarbon-generative potential suggests that liptinites in mudstone in the Jurassic coal-measures of the Turpan Basin generate about 70% of oils, while the vitrinites produce about 30% of the oils. Therefore, the hydrogen-rich liptinites (alginite, sporinite and cutinite) are the main oil generating macerals, while vitrinites (collotelinite and collodetrinite) are secondary oil generating macerals in the Jurassic coal-bearing Turpan Basin and other Jurassic basins of northwest China. (C) 2017 Elsevier Ltd. All rights reserved.

A number of condensate reservoirs with high concentrations of H2S have been discovered in the deep dolomite reservoirs of the lower Ordovician Yingshan Formation (O1y) in the Tazhong Uplift, where the formation water has a high pH value. In the O1y reservoir, the concentrations of Mg2+ and SO4 2− in the formation water are higher than those in the upper Ordovician formation. The concentration of H2S in the condensate reservoirs and the concentration of Mg2+ in the formation water correlate well in the O1y reservoirs of the Tazhong Uplift, which indicates a presumed thermochemical sulfate reduction (TSR) origin of H2S according to the oxidation theory of contact ion-pairs (CIPs). Besides, the pH values of the formation water are positively correlated with the concentration of H2S in the condensate reservoirs, which may indicate that high pH might be another factor to promote and maintain TSR. Oil–source correlation of biomarkers in the sulfuretted condensates indicates the Cambrian source rocks could be the origin of condensates. The formation water in the condensate reservoirs of O1y is similar to that in the Cambrian; therefore, the TSR of sulfate-CIPs likely occurred in the Cambrian. High H2S-bearing condensates are mainly located near the No. 1 Fault and NE-SW strike-slip faults, which are the major migration pathway of deep fluids in the Tazhong Uplift. The redox between sulfate-CIPs and hydrocarbons is the generation mechanism of H2S in the deep dolomite condensate reservoirs of the Tazhong Uplift. This finding should be helpful to predict the fluid properties of deep dolomite reservoirs.

准噶尔盆地南缘地区发育40多个背斜构造、5套有效烃源岩,大多数背斜构造发现了油气或存在油气苗,原油性质呈现多样性变化,长期以来对该地区不同构造原油的来源存在多种观点.在南缘地区典型原油地球化学特征与油源对比的基础上,选择了南缘地区具有代表性的典型背斜构造油藏,对其原油油源进行了深入细致的解剖研究.结果表明,南缘地区绝大多数背斜构造油藏原油为单一烃源来源,不同时代烃源岩生成原油混合现象并不普遍,只有少数油藏存在混源油.不同区域背斜构造原油油源存在较大差异,原油来源严格受控于有效烃源岩的分布:二叠系来源的原油主要分布于南缘中部以东的山前构造,侏罗系来源的原油主要分布于南缘西部及南缘东部构造,白垩系来源的原油仅分布于南缘中部构造,古近系来源的原油仅分布于南缘西部构造;齐古油田侏罗系油藏原油为二叠系与侏罗系原油的混合,各自的贡献约占50％.

The first exploratory well, the ZS1C well, with 158,545 m(3) daily gas production was discovered in 6861-6944 m deep strata of the Cambrian gypsolyte layer of the Tarim Basin, China in 2014. The discovery opens a new target for the Cambrian-reservoired oil and gas exploration, and directly leads to large-scale oil and gas exploration of the deep-reservoired Cambrian oil and gas fields in the Basin. Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry and a comprehensive two-dimensional gas chromatography flame ionization detector revealed the presence of abundant adamantane compounds, 2-thiaadamantanes and 2-thiadiamantanes, and a large amount of sulfur-containing compounds in the condensate oil. The formation of organic sulfur-containing compounds, such as 2-thiaadamantanes, is an indication of sulfur incorporation from the gypsum in the stratum into oil and gas in the course of TSR. This reservoir has apparently suffered severe TSR alteration because (1) High content of H2S, (2) H2S sulfur isotopes, (3) CO2 carbon isotopes, and others abundant data to support this findings. Similar sulfur isotopic composition of H2S, oil condensate and the gypsum in the Cambrian strata indicate that the produced condensate is experienced TSR alteration. Therefore, the deep-accumulated Cambrian oil reservoir has experienced severe TSR alteration, and accumulated natural gas and condensate contains high sulfur content. (C) 2015 Elsevier Ltd. All rights reserved.

Huge, high gas-oil ratio, hydrogen sulfide (H2S)-bearing gas condensate accumulations were recently discovered in the Ordovician carbonate reservoirs of the Tazhong uplift in the Tarim Basin, northwest China. Distinct differences exist between the eastern and western condensates in terms of chemical and isotopic compositions. Condensates from the western part of the uplift were characterized by high dibenzothiophenes (generally >500 mu g/g), a high H2S concentration (similar to 7%, vol./vol.), and relatively depleted C-13 methane (delta C-13(1) = -55.5 parts per thousand to -36 parts per thousand). The H2S concentration in the Tazhong gas condensates shows a positive correlation to Mg2+ concentration in the formation water. Formation water in Lower Ordovician-Cambrian strata in the Tazhong uplift is rich in Mg2+, which facilitates the thermochemical sulfate reduction (TSR) of sulfate contact ion pairs (CIPs) to produce H2S and dibenzothiophenes. A detailed comparison of the chemical compositions of the formation waters in different strata indicates that a high H2S concentration in the Tazhong gas condensates originates from the TSR of sulfate CIPs in the Lower Ordovician Cambrian strata, where a primary oil accumulation may have existed. The concentrations of 3- and 4-methyldiamantanes in the western condensates (80 to 150 mu g/g) are relatively lower than those from the eastern part of the uplift. Also, the delta C-13(1) in the western H2S-bearing gas condensates was more negative, and the delta C-13(2)-delta C-13(1) value was larger than that from typical TSR-altered gases. These features indicate that the western Tazhong samples had just entered the initial stage of TSR. According to the pressure, volume, temperature (PVT) phase diagram, the lower Paleozoic section was quickly buried after the Tortonian. High-H2S hydrocarbon inclusions formed during the last 10 m.y. when paleotemperatures reached 140 degrees C (284 degrees F). Because the reaction rate of the sulfate CIPs oxidation was relatively slower than that of H2S autocatalysis during the entire TSR process, advanced TSR has not been accomplished yet. It is also inferred that the Tortonian was the key period for accumulation of secondary H2S-bearing gas condensates, resulting from abundant gas washing along deep fractures and charging in the early reservoirs. An increased aromaticity parameter (toluene/n-heptane) and an increased fractionation index from east to west indicate an intensified degree of gas washing. Different gas-washing intensities in the eastern and western gas condensates led to diverse PVT states as well. Deep strata in the Tazhong uplift were characterized by multiple charges and mixing, coupled with periodic TSR, leading to the occurrence of variable H2S-bearing gas condensates.

准噶尔盆地南缘地区背斜众多,油气分布广泛,油气性质多样.该地区存在5套有效烃源岩,油-源关系十分复杂,长期以来对原油来源存在很大争议.系统采集了南缘地区5套烃源岩及不同构造原油样品,对烃源岩抽提物与原油碳同位素组成、正构烷烃及类异戊二烯烷烃分子碳同位素组成、生物标志物组成特征等进行了全面的对比分析.结果表明,南缘地区4种典型类型原油分别来源于不同时代的烃源岩:①第1类原油碳同位素组成轻,正构烷烃分子碳同位素分布比较平缓,含有较丰富的β-胡萝卜烷,甾烷以C28、C29规则甾烷为主,来源于二叠系烃源岩;②第2类原油碳同位素组成重,正构烷烃分子碳同位素分布比较平缓,Pr/Ph比值高,甾烷以C29规则甾烷和重排甾烷为主,三环萜烷以C19为主,伽马蜡烷含量很低,来源于侏罗纪煤系烃源岩;③第3类原油碳同位素组成轻,正构烷烃分子碳同位素分布随碳数增高大幅下降,Pr/Ph比值小于1.0,C27、C28、C29甾烷呈“V”型分布,异胆甾烷、重排甾烷、Ts、C29Ts、C30重排藿烷、伽马蜡烷含量高,来源于白垩系清水河组烃源岩;④第4类原油碳同位素组成重,正构烷烃分子碳同位素分布随碳数增高下降幅度大,C27、C28、C29甾烷呈“V”型分布且以ααα-20R为主,甲藻甾烷尤为丰富,来源于古近系安集海河组烃源岩.目前在南缘地区尚未发现三叠系烃源岩生成的原油.

准噶尔盆地南缘地区原油分布广泛,不同地区原油具有不同的地球化学特征.按照碳同位素与生物标志物组成特征可以分为4种典型类型:①第1类原油典型特征是碳同位素组成轻,全油δ13C值通常小于-29‰,正构烷烃分子碳同位素分布比较平缓,δ"C值通常小于-28‰,含有较丰富的β-胡萝卜烷,甾烷以C28、C"规则甾烷为主,重排甾烷低,伽马蜡烷含量变化较大;②第2类原油碳同位素组成重,全油δ13C值通常在-28‰～-26‰,正构烷烃分子碳同位素分布比较平缓,δ"C值通常大于-28‰,Pr/Ph比值高,甾烷以C29规则甾烷和重排甾烷为主,三环萜烷以C19为主,伽马蜡烷含量很低;③第3类原油碳同位素组成轻,全油δ"C值通常小于-29‰,C9以上正构烷烃分子碳同位素分布随碳数增高大幅下降,δ"C值在-27‰～-34‰,Pr/Ph比值小于1.0,C27、C28、C29甾烷呈"V"型分布,异胆甾烷、重排甾烷含量高,Ts、C29 Ts、C30重排藿烷高,Ts/Tm＞1,伽马蜡烷含量高且具有两个异构体,甾/萜烷比值高;④第4类原油碳同位素组成重,全油δ"C值通常在-28‰～-26‰,C9以上正构烷烃分子碳同位素分布随碳数增高下降幅度大,δ13C值在-23‰～-29‰,C27、C28、C29甾烷呈"V"型分布且以ααα-20R为主,异胆甾烷丰度低,甲藻甾烷尤为丰富,三环萜烷以C19为主,伽马蜡烷含量较低.

The aromatic hydrocarbon fractions of five crude oils representing a natural sequence of increasing degree of biodegradation from the Liaohe Basin, NE, China, were analyzed using conventional gas chromatography-mass spectrometry (GC-MS) and comprehensive two-dimensional gas chromatography (GC×GC). Because of the limited peak capability and low resolution, compounds in the aromatic fraction of a heavily biodegraded crude oil that were analyzed by GC-MS appeared as unresolved complex mixtures (UCMs) or GC "humps". They could be separated based on their polarity by GC×GC. UCMs are composed mainly of aromatic biomarkers and aromatic hydrocarbons with branched alkanes or cycloalkanes substituents. The quantitative results achieved by GC×GC-FID were shown that monoaromatic hydrocarbons account for the largest number and mass of UCMs in the aromatic hydrocarbon fraction of heavily biodegraded crude oil, at 45% by mass. The number and mass of diaromatic hydrocarbons ranks second at 33% by mass, followed by the aromatic biomarker compounds, triaromatic, tetraaromatic, and pentaaromatic hydrocarbons, that account for 10%, 6%, 1.5%, and 0.01% of all aromatic compounds by mass, respectively. In the heavily biodegraded oil, compounds with monocyclic cycloalkane substituents account for the largest proportion of mono- and diaromatic hydrocarbons, respectively. The C4-substituted compounds account for the largest proportion of naphthalenes and the C3-substituted compounds account for the largest proportion of phenanthrenes, which is very different from non-biodegraded, slightly biodegraded, and moderately biodegraded crude oil. It is inferred that compounds of monoaromatic, diaromatic and triaromatic hydrocarbons are affected by biodegradation, that compounds with C1-, C2-substituents are affected by the increase in degree of biodegradation, and that their relative content decreased, whereas compounds with C3-substituents or more were affected slightly or unaffected, and their relative content also increased. The varying regularity of relative content of substituted compounds may be used to reflect the degree of degradation of heavy oil. Moreover, biomarkers for the aromatic hydrocarbons of heavily biodegraded crude oil are mainly aromatic steranes, aromatic secohopanes, aromatic pentacyclotriterpanes, and benzohopanes. According to resultant data, aromatic secohopanes could be used as a specific marker because of their relatively high concentration. This aromatic compound analysis of a series of biodegraded crude oil is useful for future research on the quantitative characterization of the degree of biodegradation of heavy oil, unconventional oil maturity evaluation, oil source correlation, depositional environment, and any other geochemical problems.

准噶尔盆地南缘发育二叠系、三叠系、侏罗系、白垩系和古近系5套可能的烃源岩,并在很多构造圈闭发现了不同物理化学性质的油气.长期以来对该地区有效烃源岩及所发现油气的来源存在很大争议.通过对南缘地区24条地面剖面及28口探井烃源岩岩心系统采样分析研究认为,不仅二叠系与侏罗系是南缘地区重要的烃源岩,三叠系是可能的烃源岩,白垩系与古近系也是非常重要的烃源岩.二叠系烃源岩有机质丰度很高、类型好,以Ⅰ、Ⅱ型有机质为主;三叠系与侏罗系烃源岩有机质丰度变化大,且类型较差,以Ⅱ、Ⅲ型有机质为主;白垩系和古近系烃源岩有机质丰度中等,但有机质类型好,以Ⅰ、Ⅱ型有机质为主.5套烃源岩目前成熟度差异较大,二叠系、三叠系、侏罗系烃源岩处于低成熟—高、过成熟阶段,白垩系烃源岩处于未成熟—高成熟阶段,古近系烃源岩处于未成熟—成熟演化阶段.5套烃源岩大量生烃时期明显不同:中二叠统烃源岩主要在晚侏罗世—古近纪,侏罗系在晚白垩世—新近纪;白垩系从始新世延续现今,在上新世初达到生油高峰;古近系中新世末期进入生油门限开始生油,目前仍未达生油高峰.白垩系在南缘中部地区为有效生烃源岩,古近系在南缘西部地区是有效的生油源岩.

A continental condensate field, with the deepest burial depth (7084 m) so far in China, was recently discovered from the Cretaceous sandstone reservoir in the Bozi area of the Kuqa Depression, Tarim Basin. In this paper, we report general features of this field, including geochemistry of the condensates, their origin, and migration and accumulation. General geochemistry of the condensate reflects that it is an over pressured condensate reservoir. Analytical results by newly-developed high-resolution GC x GC-TOFMS method indicate that the condensate is rich in n-alkanes from nC(3) to nC(34) and in diamondoid hydrocarbons with a high content of adamantanes. The condensate has a high abundance of aromatic hydrocarbons. The gas delta C-13(2) values in these are -23.3 parts per thousand, which is characteristic of coal-derived gases. Compared with the contents of conventional condensate oils, the BZ1 well has a high content of aromatic hydrocarbons, suggesting that the condensate is in a stage of highly mature cracking. Analyses of oil biomarkers, carbon isotopes, gas compositions, etc., revealed that the condensate is dominantly sourced from the highly mature coals of the Jurassic Qiakemake Formation in the western Kuqa Depression. This set of coal source sequences has generated large quantities of condensate since 3 Ma, when the study area was subjected to a rapid subsidence with sediments over 2000 m and consequently attained at the condensate-generating stage. Such condensates then migrated vertically into the Cretaceous traps along faults. Therefore, the Bozi condensate field is a typical case of large-scale oil/gas field characterized by late-stage accumulation. Multiple elements favor the formation of this large-scale condensate field. The timing of trap formation accords with that of the condensate generation (since 3 Ma). The traps and source rocks are stacked vertically. Faults connect the source rock sequences and the reservoirs. The hydrocarbon charge is strong and efficient. Thick Paleogene gypsum beds act as an excellent cap rock, favoring the accumulation and preservation of the condensate. (C) 2015 Elsevier Ltd. All rights reserved.

In this paper, we analyse the Ordovician condensate in the Tazhong area of the Tarim Basin, NW China, using a new high-resolution comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (GC x GC-TOFMS). Results show that 4778 compounds at a signal-to-noise ratio of more than 100 were detected, reflecting the advantage of this method in the analysis of condensate. Abundant adamantane series compounds and some diamantane series compounds, as well as a series of sulphur-containing compounds (including the benzothiophene series, and the dibenzothiophene series) were detected for the first time in the study area. Combined with the results of a study on oil and gas sources and migration/accumulation, we address the origin of these compounds. It is implied that the diamondoids were not formed in-situ due to oil cracking (in reservoir approximately 5500 m deep at temperature around 139 degrees C), whereas the cracking of oil to gas may have occurred in the deeper-buried Cambrian dolomite reservoir at a reservoir temperature of 180-220 degrees C (8500-11,000 m depth). The oil-cracked gas (secondary cracking product) might have migrated to and accumulated in the shallower-buried Ordovician reservoir, accompanied by abundant diamondoids. The formation of sulphur-containing compounds may be related to thermochemical sulphate reduction (TSR), which, similar to the oil cracking, took place in the deeper-buried Cambrian reservoir. Under high-temperature TSR, SO42- in the formation water is reduced to H2S and organic sulphur compounds, leading to the enrichment of sulphur in crude oils (e.g., benzothiophene, and dibenzothiophene series). As such, it is implied that the Tazhong area could potentially be explored for deep-buried oil-cracked gas. (C) 2015 Elsevier Ltd. All rights reserved.

A series of pairwise alkyl-decahydronaphthalene isomers were identified in petroleum geological samples by GC × GC/TOFMS.

Gas condensate and natural gas samples from wells ZS1 and ZS1C in the Cambrian strata of the Tarim Basin, NW China are analyzed geochemically to assess thermal maturity, extent of oil cracking and secondary alteration. Hydrocarbons are derived from the Cambrian source rocks with gas condensate and solution gas reservoired in the Middle Cambrian Awatage Formation and dry gas in the Lower Cambrian Wusonggeer and Xiaoerbulake Formations, where caverns from dolomitization and local fractures provide spaces for petroleum accumulation. Gas and gas condensate in the Middle Cambrian are mainly generated from kerogen thermal degradation at high maturity stage (condensate-wet gas window) rather than from in-reservoir thermal cracking of preexisted oils. Evaporative fractionation exerts certain impacts on gas condensates as evidenced by enrichment of light molecular weight alkanes, depletion of diamantanes in one condensate, and low aromaticity. Condensate in well ZS1C is a thermochemical sulfate reduction (TSR) alteration residual. The dry gas in the Lower Cambrian reservoirs is mainly derived from thermal degradation of kerogen with minor contribution from secondary cracking of preexisted oil. Variable H2S content in the Lower Cambrian reservoirs is primarily migrated from deep buried strata as in situ TSR alteration is on its initial stage. The Cambrian in the Tarim Basin is an unexplored zone. Condensate without extensive oil cracking in the Middle Cambrian and TSR altered condensate in the Lower Cambrian discovered at ZS1 structure suggest a likelihood of down-dip oil. Geologic framework of the Tarim Basin also favors the preservation of liquid hydrocarbons in ultradeep strata as it reaches its maximum burial depth in last 5 millions of anna (Ma). A short heating time requires much a higher temperature than the current reservoir temperature for completion of oil thermal cracking.

Twenty gas condensates, six oils and their associated formation water from the Tazhong Uplift in the Tarim Basin, NW China were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates. Thermochemical sulfate reduction (TSR) is the main secondary alteration process relevant to the generation of sour gas condensates. Those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations. The hydrogen sulfide (H2S) concentrations in gas phase show a strong positive correlation to gas/oil ratio, organosulfur compound (OSC) concentrations in the liquid phase. While methane isotopic values are mainly controlled by thermal maturity, wet gas components show unusual C-13 enrichment at a low degree of thermal cracking. Geochemical anomalies occur mainly in the Lower Ordovician Yingshan Fm. where more H2S has been discovered, while the Upper Ordovician Lianglitage Fm. shows less irregularity and H2S concentration is much lower. Generation of sour gas condensate is most likely caused by TSR alteration, which could oxidize liquid hydrocarbons and form a large quantity of OSCs. Isotopic enrichment of S-34 in formation water sulfate and elevated OSC concentrations provide supplemental evidence of TSR alteration. Both burial history reconstructions based on fluid inclusion analysis and PVT simulation indicate that in situ TSR reactions mainly occurred around last 10 Ma during the late Himalayan orogeny and overall TSR extent is relatively low in the Tazhong Uplift. (C) 2015 Elsevier Ltd. All rights reserved.

针对烃源岩生排烃效率评价现有方法的不足和第四次油气资源评价研究的需要,基于模拟地质条件的最新生排烃实验平台,以白垩系烃源岩为例,开展了松辽盆地烃源岩生排烃条件和过程的模拟实验.结合包括轻烃在内的产物计量方法探索,对实验产烃量进行计量;通过实验产物的H/C原子比数据,对干酪根热成熟度进行标定,获得实验点所对应的地质RO值;从而实现了对松辽盆地白垩系烃源岩在各热演化阶段的生排烃效率厘定.结果表明,轻烃在总烃中的含量为5％～29％,并有随实验热演化程度升高的特征,在生油高峰期的轻烃含量约为25％,从而弥补了传统实验和利用岩心残留烃分析排烃效率方法中缺少轻烃组分的缺憾;通过生排烃效率曲线显示,松辽盆地白垩系烃源岩的最大生油效率为680mg/gTOC,生油高峰期对应的绝对排烃效率为57％.对压力的影响分析认为,压力确实影响热成熟演化,在生油阶段对RO的抑制约为0.2％;压力的释放促进排烃,是导致岩心滞留烃量降低和轻烃散失的主要原因,仅轻烃散失造成的生油高峰期绝对排烃效率偏差值就达18％.常规方法低估了地质实际的滞留烃量,深层找气的前景乐观.