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.     

Biodegradation and migrational fractionation of oils from the Eastern Carpathians, Poland

Nineteen oil samples from Silesian Unit of the eastern Carpathian Overthrust have been characterised geochemically in order to determine the causes of compositional differences among them and elucidating the processes responsible for their differences. Some of analysed crude oils have undergone post-emplacement alteration in the reservoir such as biodegradation and evaporative fractionation. This explains much of the chemical and physical properties variability across individual fields from one tectonic unit. Geochemical correlation based on biomarker distributions showed a close relationship between all oils (included biodegraded oils). However, data based on the whole oil GC analysis of selected oils suggest that the process of evaporative fractionation may change the composition of lower molecular weight hydrocarbons of the oils in this region. This paper outlines the probable mechanisms for oil mixing in the region and describes how this can lead to observable lateral differences in the composition of oils.     

凝析油形成新模式——原油蒸发分馏机制研究

经典的石油生产理论认为凝析油是源岩高成熟阶段、或是陆相有机质于低成熟阶段的直接产物。本文在资料、数据分析的基础上结合我国塔北海相油田的实际地质地化资料提出原油蒸发分馏机制是凝析油形成的一种重要途径，并确认了塔北油田的诸多原油曾曹受了程度不一的蒸发分馏作用具有成熟度正常的凝析油形成。     

青海柴达木盆地南八仙油气田油源与成藏机理

柴达木盆地南八仙油气田不同油气藏间原油性质特征变化明显,油层在纵向井段上分布范围逾3000余m。通过地球化学实验并结合构造等地质条件分析,认为该区各种深、浅层油气藏中的原油具有煤成烃的基本地球化学特征,并且为同一来源,油源为临近地区较深部位中侏罗统煤系地层;该区深、浅层原油物性间的差异也反映在地球化学指标上,其成因或成藏机理是：深部早期油藏在断裂等地质作用下发生了明显的蒸发分馏作用,其分馏出的产物在浅部再次聚集成藏。     

凝析油形成新模式─—原油蒸发分馏机制研究

经典的石油生成理论认为凝析油是源岩高成熟阶段、或是陆相有机质于低成熟阶段的直接产物。本文在资料、数据分析的基础上结合我国塔北海相油田的实际地质地化资料提出原油蒸发分馏机制是凝析油形成的一种重要途径，并确认了塔北油田的诸多原油曾遭受了程度不一的蒸发分馏作用且有成熟度正常的凝析油形成。     

Petroleum inclusions and residual oils: constraints for deciphering petroleum migration

The analysis of petroleum inclusions (PI) in parallel to residual oils (=bitumens) provided specific constraints for petroleum migration processes in the Prague Basin. Whereas organic geochemical information from bitumens is limited due to alteration, additional high quality information was obtained from C_1–14-range compounds inside crystals. This was obtained by the application of a closed system for the decrepitation and thermal extraction of PI with subsequent gas chromatography (GC) and gas chromatography–mass spectrometry (GC–MS). The resulting data not only show pristine live oils, gases, residual and precipitated bitumens and a broad variation of mixtures of the groups above, but also show that gas migration and fractionation processes played a major role in the Prague Basin.     

New aromatic biomarkers and possible maturity indicators found in New Albany Shale extracts

Aromatic hydrocarbons from benzene extracts of New Albany Shale were characterized. A biomarker that has a molecular weight of 546 and a structural configuration consistent with that of an alkyl-aromatic hydrocarbon (C₄₀H₆₆) was tentatively identified. It was found that the relative concentrations of the biomarker are indicative of differing levels of thermal maturity of the shale organic matter. A 40-carbon bicyclic carotenoid (C₄₀H₄₈) is proposed as the geochemical precursor of this biomarker. Thermal maturity of the shale organic matter can also be differentiated by observing differences in “fingerprints” as obtained by field-ionization mass spectrometry on the aromatic hydrocarbon fraction. Using this technique, we found that the more mature shale samples from southeastern Illinois contain more low molecular weight extractable aromatic hydrocarbons and the less mature shale samples from northwestern Illinois contain more high molecular weight extractable aromatic hydrocarbons. It was demonstrated that field-ionization and tandem mass spectrometric techniques through fingerprint and individual compound identification, are useful for shale aromatic hydrocarbon fraction characterization and for thermal maturation interpretation.     

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.     

渤海湾盆地渤中19-6气田凝析油特征及油气关系

2018年中国东部最大整装凝析气田——渤中19-6气田的发现,一举打开了渤海湾盆地深层天然气勘探的新领域,展现了油型盆地天然气勘探的巨大潜力。渤中凹陷发育多套不同成熟度的烃源岩,位于渤中凹陷西南部的渤中19-6气田凝析油与天然气并存,近年来于该地区的天然气成因及形成条件多有研究,但是对于凝析油的形成则少有研究。本文以渤中19-6气田为例,探讨多套烃源岩发育的复杂油气区凝析油的成因及与天然气的关系。渤中19-6气田6个凝析油样品化学组成及其变化特征精细分析对比表明:该区凝析油以低碳数正构烷烃为主,含有较丰富的环己烷、甲基环己烷等环烷烃和苯、甲苯等低碳数芳烃,金刚烷类化合物较发育,高碳数生物标志化合物含量较低,综合判断凝析油属于混合偏腐殖型高熟原油。结合该区凝析油与天然气对比结果认为,该区凝析油和天然气属于同一套烃源岩在同一阶段所生成的烃类化合物,两者相伴而生,均属于烃源岩原生产物。     

Geological and laboratory evidence for early generation of large amounts of liquid hydrocarbons from suberinite and subereous components

Suberinite, and subereous components of amorphous nature, comprise largely unrecognized, proficient sources of liquid hydrocarbons. Due to difficulties in recognizing the presence of subereous components and suberinite in organic sediments, the contributions of these liptinitic components to the organic input of source rocks are easily underestimated. Severe chemical alterations of suberinite in the vitrinite reflectance range of R_o = 0.35–0.60% are demonstrated. Organic geochemical data, obtained from samples subjected to natural maturation, reveal that subereous components/suberinite undergoes early thermal degradation to generate large amounts of hydrocarbons below R_o = 0.60%. Data obtained from laboratory maturation of immature, suberinite-rich coals indicate that about 50% of the potential of suberinite for generating C₁₂₊ hydrocarbons has already been exhausted during natural maturation of the samples, prior to the onset of the traditionally defined “oil window”. The present data (a) contradict the assumption that suberinite is mainly sourced by selective preservation/enrichment of a stable, highly aliphatic biopolymer, i.e. “suberan” and (b) suggest that suberinite contains appreciable amounts of aliphatic and aromatic moieties which are released at low thermal stress.     

Hydrogen isotopic compositions of individual alkanes as a new approach to petroleum correlation: case studies from the Western Canada Sedimentary Basin

Isotopic compositions of carbon-bound hydrogen in individual n-alkanes and acyclic isoprenoid alkanes, from a number of crude oil samples, were measured using gas chromatography-thermal conversion-isotope ratio mass spectrometry. The precision of this technique is better than 3‰ for most alkanes, compared to the large range of δD variation among the samples (up to 160‰). The oils were selected from major genetic oil families in the Western Canada Sedimentary Basin, with source rocks ranging in age from Ordovician (and possibly Cambrian) to Cretaceous. The hydrogen isotopic composition of alkanes in crude oils is controlled by three factors: isotopic compositions of biosynthetic precursors, source water δD values, and postdepositional processes. The inherited difference in the lipid's biosynthetic origins and/or pathways is reflected by a small hydrogen isotopic variability within n-alkanes, but much larger differences in the δD values between n-alkanes and pristane/phytane. The shift toward lighter hydrogen isotopic compositions from Paleozoic to Upper Cretaceous oils in the WCSB reflects a special depositional setting and/or a minor contribution of terrestrial organic matter. The strong influence of source water δD values is demonstrated by the distinctively lower δD values of lacustrine oils than marine oils, and also by the high values for oils with source rocks deposited in evaporative environments. Thermal maturation may alter the δD values of the alkanes in the oil to some extent, but secondary oil migration does not appear to have had any significant impact. The fact that oils derived from source rocks that could be of Cambrian age still retain a strong signature of the hydrogen isotopic compositions of source organic matter, and source water, indicates that δD values are very useful for oil-source correlation and for paleoenvironmental reconstructions.     

强烈气洗作用导致原油成分变化的定量计算:以库车坳陷天然气藏为例

库车坳陷以产天然气为主,同时产出少量原油和凝析油,油气充注不同步,普遍具有"油早气晚"的特点,晚期大量天然气的侵入,必然对早期聚集的油藏发生改造作用。对气藏中原油成分变化的定量计算和讨论,可为天然气的注入强度定量评价提供直接证据。未遭受气洗的原油,正构烷烃摩尔浓度的对数与相应的碳数呈线性关系,而气洗作用可使轻组分的正构烷烃最先溶解于干气中,并随着天然气继续向前运移,原始油藏中的轻组分正构烷烃大大减少。以此为理论基础,建立了正构烷烃损失的定量计算模型。结果表明克拉2构造原油正构烷烃损失程度最高,平均可达70%左右,大北构造带原油正构烷烃损失程度差异较大,与该地区断块发育有关。气洗作用导致原油正构烷烃减少,而金刚烷、多环芳烃等在天然气中溶解度较低的化合物得以浓缩富集,相对含量大大增加。轻芳烃含量也会随之而增加,原油芳香度增加,石蜡度降低。在模拟实验基础上对气洗程度进行了定量评价,初步估算表明,克拉2构造原油遭受的气洗作用最强,是其他构造带的2～5倍。     

HPLC fractionation and GC-MS determination of aromatic hydrocarbons from oils and sediments

Normal-phase HPLC using a cyano/amino bonded silica stationary phase can provide reliable and rapid fractionation of total neutral fractions from oils and sediments. Separation of aliphatic from aromatic hydrocarbons is possible, but in addition HPLC is able to fractionate aromatic hydrocarbons according to their number of double bonds.Applications of the HPLC fractionation in organic geochemistry are described and illustrated by GC-FID and GC-MS data from an oil and a recent sediment exhibiting a high degree of petrogenic contamination. Alkylphenanthrenes can be obtained free of alkyldibenzothiophenes, allowing evaluation of the methylphenanthrene index by GC-FID without the need for quantification of co-eluting sulphur compounds by GC-FPD.The choice of capillary GC stationary is also considered, particularly with regard to resolution of aromatic steroidal hydrocarbons and alkylphenanthrenes.     

Classification and thermal history of petroleum based on light hydrocarbons

Classifications of oils and kerogens are described. Two indices are employed, termed the Heptane and IsoheptaneValues, based on analyses of gasoline-range hydrocarbons. The indices assess degree of paraffinicity. and allow the definition of four types of oil: normal, mature, supermature, and biodegraded. The values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type. Aliphatic and aromatic kerogens are definable. Only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group (41%) in the sample set. This group was evidently generated at subsurface temperatures of the order of 138°–149°C, (280°–300°F) defined under specific conditions of burial history. It is suggested that all other petroleums are transformation products of normal oils.     

石油碳、氢同位素组成的研究

通过对我国18个含油气区、385个石油样品进行碳、氢同位素和部分馏份碳同位素分析,将所获数据对两种不同性质的石油如正常原油和轻质（凝析）油分别研究其碳、氢同位素地球化学特征,提出轻质（凝析）油的碳同位素值（δ13C为-32.5‰～ -24.3‰）比正常原油δ13C为-34.4‰～ -24.6 ‰．6‰）相对偏高;石油馏份中芳烃碳同位素组成的变化受母质继承效应更为明显。因此,用芳烃碳同位值可以判识不同母质来源的石油。与海相有关的轻质（凝析）油的氢同位素值大于-15.0‰,而非海相轻质（凝析）油的δD值( δD为-21.0‰～-1.05‰）基本覆盖了海相轻质油的分布范围,从淡水－微咸水－半咸水和海水环境其氢同位素有明显变重趋势,表明氢同位素主要与沉积环境密切相关。     

东海盆地西湖凹陷轻质原油芳烃地球化学特征

采用GC-MS实验技术,对西湖凹陷8口井12个轻质原油样品的芳烃地球化学特征进行了系统研究。平湖斜坡带北部地区NB14-1井、NB14-2井和NB14-3井这3口井的轻质原油与平湖斜坡带中部地区的NB25-1井和NB25-2井、西次凹的HY1-1井以及黄岩构造带的HY2-1井和HY2-2井这5口井的轻质原油芳烃化合物的分布特征和相对含量存在明显差别。轻质原油芳烃馏分中以二环和三环的化合物为主,前者富含菲系列化合物,萘系列和联苯系列化合物含量相对较低,后者则相反,其富含萘系列和联苯系列化合物,菲系列化合物含量相对较低。轻质原油生源和沉积环境的研究表明,原油的母源都主要为陆源高等植物来源,成因环境都为氧化环境,但后者的1,2,5-TMN和1,2,5,6-TeMN相对含量更高,氧芴相对含量和Pr/Ph值更大,反映其母源的陆源高等植物输入程度更大,沉积环境的氧化性更强。烷基萘、烷基菲和烷基二苯并噻吩成熟度指标表明,轻质原油以达到成熟到高熟演化阶段,平湖斜坡带北部地区的NB14-1井、NB14-2井和NB14-3井这3口井的轻质原油热演化程度相对更高。     

Hydrous pyrolysis of sediments: Composition and proportions of aromatic hydrocarbons in pyrolysates

Hydrous pyrolysis (closed vessel autoclaving in the presence of excess water) of organic-rich rocks is said to generate oils which closely resemble natural crude oils in their broad characteristics and composition. However there are only a few accounts of the proportions and compositions of hydrocarbons in hydrous pyrolysates and none of these discuss the aromatic hydrocarbon composition in detail. The present paper presents some data on the latter.Hydrous pyrolysis (3 days) of a dolomitic siltstone (Permian, Marl Slate) at 280, 300,320, 340 and 360°C produced significant amounts of oils in which the aromatic hydrocarbons were one and a half to two times as abundant as the saturated hydrocarbons.The overall composition of the aromatic hydrocarbons was similar to most crude oils; the major components isolated by our methods from natural oils and from pyrolysates were C_1–4 alkylnaphthalenes. At the lowest pyrolysis temperature (280°C) the distributions of the more minor components of the pyrolysates (e.g. alkylphenanthrenes, aromatic steroids) were also generally similar to those found in natural crudes. However, a number of components (e.g. methylanthracenes, Diels' hydrocarbon) which are not usually reported in crudes, were also detected and the relative proportions of these increased at the higher temperatures. Hydrous pyrolysis (340°C) of an organic-rich oil shale (Jurassic, Kimmeridge) and an asphaltic-material containing no minerals produced pyrolysates in which many of these unusual compounds were also present. In addition the pyrolysate of the oil-shale contained higher proportions of organic sulphur compounds. It appears that the formation of the unusual compounds is not simply a function of the type of organic matter or mineralogy but rather of the high temperatures or fast heating rates employed.     

山东东营凹陷八面河油田稠油成因分析

东营凹陷八面河地区原油物性呈规律性的变化,偏离生油中心的构造高部位主要分布稠油,靠近生油中心的构造低部位主要分布正常油。对原油族组成与化学成分的分析表明,八面河油田稠油具有低饱芳比、饱和烃含量低、链烷烃与低分子量萘、菲等轻质馏分严重缺失等轻度-中等降解油特征,邻区草桥油田稠油含较为完整的生物降解标志物--25-降藿烷系列,系严重降解油,反映该区稠油的形成与次生变化有关。同区具有相同或相似油气成因的沙子岭原油的成熟度（C₂₉甾烷ααα20S/(S+R)值为0.24~0.25）低于八面河的（C₂₉甾烷ααα20S/(S+R)值为0.31~0.44）,为典型未熟-低熟油。沙子岭的轻度或未降解油同样表现为正常油,反映八面河地区低温成烃与稠油无必然的联系,进一步验证八面河稠油主系次生成因。处于构造高部位的油藏由于埋深浅、保存条件差,导致水洗、生物降解等次生变化相对较强,最终形成稠油。     

Application of light hydrocarbons (C4–C13) to oil/source rock correlations:: a study of the light hydrocarbon compositions of source rocks and test fluids from offshore Mid-Norway

Relatively little work has been published on the correlation between the light hydrocarbon distributions in reservoir fluids and their proposed source rocks [Philippi, G. T. (1981)]. The aim of our work was to study this relationship in detail for samples from Mid-Norway. The main source rocks offshore Mid-Norway are the marine shales of the Late Jurassic Spekk Formation and the coals and paralic shales of the Early Jurassic Åre Formation. Reliable light hydrocarbon (C₄–C₁₃) data for source rock samples were acquired by thermal extraction-GC of the source rocks. Of these, notably the hydrocarbons in the C₆–C₈ range (routinely measured in test fluids) were used to discriminate between the Spekk and Åre Formation samples. A total of twenty-six samples from the Spekk Formation and twenty-four samples from the Åre Formation at different maturity levels and facies were analyzed. In general, the two source rock types differ in their light hydrocarbon composition by the presence of relatively more aromatics and cyclohexanes in the Åre samples, while the Spekk samples are richer in cyclopentanes and acyclic hydrocarbons. We show that source rock facies is a more important indicator of light hydrocarbon composition than maturity variation. Differences in the chemical composition, which can be used to discriminate between the two source rocks, were supported by differences in the carbon isotope composition of individual components of the same fraction, as determined by GHM-IR-MS analysis of eleven samples. Further, the light hydrocarbon compositions of reservoir fluids (oils and condensates) were compared with those for the source rock(s). Sixty-six gas chromatograms of oils and condensates, representing most of the known petroleum accumulations in Mid-Norway, were collected. Of these, five oil samples were selected for detailed isotopic analysis of individual components (GC-IR-MS). When using a classification scheme based on data from sediment samples, data for the light hydrocarbon fraction of oils and condensates indicate that the Spekk Formation is the dominant source for most of the fields from Mid-Norway, with a significant contribution from the Åre Formation being detected principally in one field. Differences in the chemical composition of the C₆–C₈ fractions were supported by differences in the carbon isotope composition of individual components, which also discriminate between the oils. Although the classification diagrams used in this study are based on source rock data from Mid-Norway, the method can be applied to other areas, providing that the diagrams are calibrated with source rock data from the area of interest.     

Petroleum potential of Baikal deposits

We analyzed oils, gases, and bitumens of bottom sediments from natural shows on the southeastern shore of Lake Baikal, in the mouth of the Stvolovaya River near Capes Tolstyi and Gorevoi Utes. Based on a set of geological data, we have established that: (1) the lake oils underwent biodegradation to a variable degree: “Fresh” nondegraded paraffin oil floats up near Cape Gorevoi Utes; in the mouth of the Stvolovaya River and near Cape Tolstyi, aromatic-naphthene oil lacks n-alkanes, monomethyl alkanes, and acyclic isoprenoids; (2) Cenozoic oil originated from the organic matter of fresh-water basins with significant amounts of higher land plant remains, including angiosperm plants (oleanane), which suggests the lake or delta genesis of oil source formations of Cretaceous and younger ages. Judging from the carbon isotope composition (average δ¹³C = −43.84‰), methane from the bottom sediments near Cape Gorevoi Utes is catagenetic. The initial in-place resources in the Baikal sedimentary basins are estimated by the volumetric-statistical method at 500 mln tons of equivalent hydrocarbons.