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。     

Phase—Controlled and Gas—Washing Fractionations During the Formation of Petroleum Reservoirs

By PVT fractionation experiments to model phase-controlled and gaw-washing fractionations during the formation of petroleum reservoirs,the authors measured the physical and chemical properties of products formed in different fractionation staes and made a correlative analysis of the influence of depressurization and gas washing on oil/gas molecular composition and the rule of fractionation.The analytical results showed that gas washing is an important factor affecting the physical properties of crude oils.and also can be regarded as a good genetic interpretation of marine wax-high oils in the Tarim region,Xinjiang,China.Phase-controlled and gaw-washing fractionations can lead to the formation of condensates and their differences in chemical composition from crude oils are a direct reflection of evaporating fractionation.Phasecontrolled and gaw-washing fractionations have a great influence on the composition of molecular compounds and relevant parameters.So phase-controlled and gas-washing fractionations during the formation of petroleum reservoirs are not only favorable to identifying different processes of formation of petroleum reservoirs,but also to the scientific application of routine geochemical parameters.     

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.     

The effects of phase fractionation on the composition of oils, condensates and gases

The effects of phase fractionation on the composition of oils, condensates and gases have been studied in the laboratory by a differential evaporation experiment. The experiment was designed to simulate the behaviour of a single-charge petroleum during (upward) migration or in a reservoir undergoing uplift, scenarios thought to be typical for the Norwegian continental shelf. After each pressure reduction step, samples from all phases (oil, condensate, free and associated gas) were collected and analysed. The results show that, in general, small molecules are preferentially enriched in the gas phase, both at elevated pressure and temperature and under standard conditions (15°C, 1 bar). The solvent power of the gas phase for large molecules is drastically reduced at lower pressures (here below approx. 300 bar). Carbon isotope ratios of individual compound are not affected, while variations in isotope ratios of fractions can be explained by changes in the fraction compositions. Individual geochemical parameters behave as can be expected based on the molecular size and structure of the compounds involved. However, a generally valid, quantitative prediction of how geochemical parameters vary as a result of phase fractionation is very difficult, as this will depend not only on the pressure and temperature of fractionation, but also on the composition and amounts of the oil and gas that are involved. The observations from this study confirm the importance of phase fractionation as an important process to consider when interpreting geochemical data for both condensates and oils.     

Reservoir geochemistry of the Tazhong Oilfield in the Tarim Basin, China, Part II. Migration, accumulation and mixing of crude oils

Measurements of the absolute and relative concentrations of nitrogen-containing compounds in crude oils from different reservoir strata (Ordovician, Silurian and Carboniferous) in the Tazhong region of the Tarim Basin, Xinjiang, China, showed that even though there are quite a number of factors affecting the distributional and compositional characteristics of neutral nitrogen-containing compounds in crude oils, the distributional and compositional characteristics of crude oils whose source conditions are approximate to one another are influenced mainly by the migration and fractionation effects in the process of formation of oil reservoirs. In addition, crude oils in the Tazhong region show obvious migration-fractionation effects in the vertical direction. Carboniferous crude oils are characterized by high migration parameters and low compound concentrations, just in contrast to Ordovician crude oils. This indicates that crude oils from shallow-level oil reservoirs were derived from those of deep-level oil reservoirs via faults, unconformable contact or carrier beds. Crude oils from the Tazhong region show some migration-fractionation effects in the lateral direction, but mixing of crude oils derived from different hydrocarbon source rocks in the process of formation of oil reservoirs made it more complicated the migration and accumulation of crude oils, as well as the formation of oil reservoirs.     

REE distribution in the resin-asphaltene fractions as a geochemical criterion for identification of sources of trace elements in oil

This paper reports REE data on resin-asphaltene components of oil from six oil-gas-bearing provinces and on bitumoids from inferred oil-source rocks (domanikites and bazhenites). It was shown that, regardless of geological-tectonic structure of the regions, oil composition, depth of reservoirs, and host lithologies, oil exhibits significant REE fractionation, and, unlike bitumoids, positive Eu anomaly. The (Eu/Sm)_n ratio increases from asphaltenes to resins and further to oils. Based on REE distribution in oil, source rocks, and bitumoids, it was concluded that deep-seated fluids were one of the possible sources that defined the trace element composition of oil.     

Experimental investigation of the compositional variation of acyclic paraffins during expulsion from source rocks

Anhydrous non-isothermal heating experiments were conducted under controlled compressive stress on cylindrical plugs of six oil shales from Permian through Eocene age. The objective of this study was to compare the distribution of acyclic paraffins in initial, residual and expelled organic matter and to highlight causes of compositional differences resulting from expulsion. Pristane generation from kerogen is highest in the Eocene Messel shale and affects the pristane / phytane (pr / ph) ratio commonly used as a redox proxy. The isoprenoid to n-alkane ratios (pr / n-C₁₇, ph / n-C₁₈) decrease during generation and are lowest in the residual bitumen due to preferential generation and retention of n-alkanes. The n-alkane distribution shows that only lacustrine shales produce high wax oils. Evaporative fractionation leads to loss of n-alkanes up to n-C₂₀ with boiling points below 350 °C. This demonstrates that lacustrine and marine shales may lead to accumulation of low wax oils due to evaporative fractionation after expulsion.     

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。     

东营凹陷北部陡坡带稠油成藏机理与油气运聚特征

东营凹陷北部陡坡带的王庄—宁海地区发现了大量稠油 ,但原油的成因及其成藏机理仍不甚了解 .在以往原油成因与油源调查基础上 ,利用非烃含氮化合物及包裹体均一化温度对该区进行油气运移方向与相对距离、成藏时间与期次的分析 .结果表明 ,东营凹陷北部带原油具有由南而北的运移分馏效应 ,反映北部陡坡带原油来自南部的利津洼陷 ;王庄—宁海地区原油总体具有由东而西的运移分馏效应 ,其东、西两侧各有一主要油气注气点 .王庄—宁海原油烃类与非烃组成与分布特征揭示该区油气具有混合聚集特征 .包裹体测试反映王庄—宁海地区油藏主要为晚期成藏 ,距今小于 5Ma ;而靠近生油中心地带包裹体具有早期成藏的记录 (± 36Ma) .     

Correlation and migration studies of North Central Sumatra oils

The Central Sumatra Basin (CSB) is a prolific petroleum-producing basin and its petroleum systems have been extensively studied. The widely occurring Tertiary lacustrine shale, Brown Shale, has long been recognized as the main source rock for tens of billions of barrels of oil in place. The oils produced from different fields display significant variation in source characteristics that have been attributed to variation in source facies. Despite this generalized recognition of the oil source, the links between major oil accumulations and many possible source kitchens throughout the basin have not been established as detailed oil-source correlation, and oil migration routes are not well defined. For continued exploration in the region, detailed genetic grouping and migration routes of the oils were evaluated through geochemical characterization of numerous oils from various fields in the northern part of Central Sumatra. Biomarker and carbon isotopic data indicate that the oils are quite similar geochemically but form several genetic groups on the basis of subtle but persistent differences in source facies. The groupings appear geographically meaningful and show association of the different oil groups with various troughs. Carbazole distributions show consistency in suggesting differences in relative migration distance among the oils within each genetic group. By combining the oil grouping and carbazole distributions with geological data, models of migration directions and pathways for oils in North Central Sumatra have been constructed.     

Hydrocarbon Vertical Continuity Evaluation in the Cretaceous Reservoirs of Azadegan Oilfield, Southwest of Iran: Implications for Reservoir Geochemistry

A collection of data obtained from analytical methods in geochemistry along with the reservoir engineering and geologic data were used to investigate the reservoir continuity in the Cretaceous Fahliyan, Gadavan, Kazhdumi and Sarvak reservoirs of the super-giant Azadegan oilfield, SW Iran. The geochemical data indicate that the oil samples, with medium to high level of thermal maturity, have been generated from the anoxic marine marl/carbonate source rock(s). The Sargelu (Jurassic) and Garau (Cretaceous) formations are introduced as the main source rocks for the studied oils. The dendrogram obtained from the cluster analysis of high-resolution gas chromatography data introduces two main oil groups including Fahliyan reservoir, and Kazhdumi along with Sarvak/Gadvan reservoirs. This is confirmed by C7 Halpern star diagram, indicating that, the light oil fraction from Fahliyan reservoir is distinct from the others. Also, different pressure gradient of the Fahliyan Formation (over-pressured) relative to other reservoirs (normally-pressured) show the presence of compartments. The relation between toluene/n-heptane and n-heptane/methylcyclohexane represents the compartmentalization due to maturation/evaporative fractionation for Fahliyan and water washing for other studied reservoirs. Also, the impermeable upper part of the Fahliyan Formation and thin interbedded shaly layers in the Kazhdumi, Sarvak and Gadvan formations have controlled reservoir compartmentalization.     

Correlation of crude oils with their oil source formation,using high resolution GLC C6-C7 component analyses

A new method has been devised, based on high resolution GLC component analyses of the C₆-C₇ hydrocarbons from shales and from crude oils, whereby composition parameters in an oil are compared with the corresponding parameters in a shale. Ideally, a given composition parameter should have the same value for a crude oil and the source rock which generated and expelled that crude oil. A Similarity Coefficient has been devised, to measure the degree of correlation between crude oil and source rock hydrocarbons or between the hydrocarbons from different groups of crude oils. The maximum value of the Similarity Coefficient is 1.00, and the theoretical minimum is a positive fraction close to zero. Based on the natural variation in composition of primary (not biodegraded) crude oils of the same basin and origin, it was found that if the Similarity Coefficient is about 0.80 or higher, correlation between the natural hydrocarbons considered is good. If the Similarity Coefficient is less than 0.73, correlation is poor.Based on strict rules for sample selection (e.g. maturity of shales and lack of biodegradation in the oils), ten presumed crude oil-source formation pairs were selected. Most of these pairs have high Similarity Coefficients of 0.80 or more. Erroneous crude oil-source rock combinations from areas with more than one source formation, as in West Texas, have low Similarity Coefficients. This indicates that the crude oil-source formation correlation method based on the Similarity Coefficient generally is functioning properly.     

Comparison of bulk kinetic parameters for asphaltenes from long-time stored and related fresher-produced crude oils

Two investigated long-time stored oils, which were produced in the 1960s, show strong compositional changes compared to fresher oil samples from the same well and production zones. Asphaltenes isolated from stored and fresher-produced oil pairs show highly similar results from open-system pyrolysis. However, asphaltenes from long-time stored oils show higher reactivity compared to those from fresher oils. The study shows that differences exist in kinetic models based on asphaltenes from fresh-produced oil and those based on oil that has undergone long-term storage, and that these differences may impact geological predictions using such models. Factors controlling the chemical differences between stored and fresher asphaltenes are unclear and hard to determine, because of a broad range of factors controlling compositional differences between these oil pairs. The difference in chemical kinetics might be related to chemical storage effects, but different well-site sampling techniques between decades, or even incomplete homogenization of the long-time stored oils in barrels before sampling may also have an impact.     

Controls of oil family distribution and composition in nonmarine petroleum systems: A case study from Inner Mongolia Erlian basin,Northern China

The Erlian basin is a continental rift basin located in Inner Mongolia, Northern China. It is a typical representative of Cretaceous Northeast Asian Rift System, which includes many small petroliferous basins in Mongolia Republic and Northern China. Although Lower Cretaceous source rocks are understood to be most important in the Erlian petroleum systems, the precise identification of these source rock intervals and their determination on oil families distribution and composition are poorly understood in this tectonically complicated, nonmarine basin. New bulk data have been gathered from source rock intervals, oil sands and crude oil samples in eight main oil-producing subbasins. Geochemical analyses indicate that Lower Cretaceous Aershan formation (K₁ba) and Tengger 1 formation (K₁bt₁) are two main source intervals in the Erlian basin and their source rock facies vary from profundal lacustrine to marginal lacustrine according to biomarker and trace elements calibration, the profundal lacustrine facies is characterised by brackish water and anoxic environment, which is similar to their correlative oils (Family 1 oils). The marginal lacustrine facies is characterised by freshwater and suboxic environment, which sourced the most common Family 2 oils. Meanwhile, different maturation processes exercise the second control on oil groups and their compositions, the profundal lacustrine source rocks characterised by their sulphur-rich kerogens lead to two oil groups (group 1 and group 2 oils), whose maturity range from low to normal; while, the marginal lacustrine source rock only lead to normal-maturity oils. At last, biodegradation affected the composition of a certain oils and formed group 4 heavy oils. In addition, short migration distance in small subbasins made the contamination or fractionation less notable in the Erlian basin.     

原油中吡咯类化合物的地球化学特征及其意义

本文分析了塔里木盆地轮南海相、松辽盆地新站淡水湖相、朝阳沟-长春岭淡水-半咸水相、柴达木盆地尕斯库勒盐湖相原油中吡咯类化合物的分布特征,发现不同成因类型原油中吡咯类含氮化合物的丰度与相对分布有着显著的差异。文中进一步探讨了沉积环境、成熟度、母源、运移作用对该类化合物的可能控制作用。结果表明原油中吡咯类化合物的丰度与相对分布可能受控于多种地球化学因素,运移分馏作用与沉积环境可能是控制该类化合物的主要因素。成熟度、原始生源对该类化合物也有一定的影响。非烃中吡咯类含氮水合物在指示油气运移方向、指相以及成熟度方面有着潜在地球化学意义。     

徐家围子断陷深层天然天的形成

通过对松辽盆地徐家围子断陷深层天然气的地质和地球化学分析,揭示了天然气的成因特征和形成过程。该区天然气的分子组成和同位素组成均显示出较大的变化,甲烷是天敢中的主要组分,质量分数在５７．４％－９８．２％之间,平均为９０．１％,主要非烃气体是ＣＯ２和Ｎ２,平均质量分数分别为４．９％和３．２％,且ＣＯ２质量分数变化范围较大,气体同位素分析结果表明,甲烷同位素显示煤型气特征,而乙烷和丙烷的同位素显示油型 的特征,表明徐家子深部除煤和ＩＩＩ型干酪根作为主要气源外,仍有一定含量趋于生油的ＩＩ型干酪根作为次要气源,实测和计算结果证实天然气主要是有机质在高成熟阶段形成的产物,运移过程中和成藏后的次生变化使天然气的组成和同位素面目变得非常复杂,尤其是由盖层微渗漏造成的蒸发分馏的作用使同位素出现例转,徐家围子断陷深层煤型气的发现为     

徐家围子断陷深层天然气的形成

通过对松辽盆地徐家围子断陷深层天然气的地质和地球化学分析 ,揭示了天然气的成因特征和形成过程。该区天然气的分子组成和同位素组成均显示出较大的变化 ,甲烷是天然气中的主要组分 ,质量分数在 57.4 % - 98.2 %之间 ,平均为 90 .1%。主要非烃气体是CO2 和N2 ,平均质量分数分别为 4 .9%和 3.2 % ,且CO2 质量分数变化范围较大。气体同位素分析结果表明 ,甲烷同位素显示煤型气特征 ,而乙烷和丙烷的同位素显示油型气的特征 ,表明徐家围子深部除煤和Ⅲ型干酪根作为主要气源外 ,仍有一定含量趋于生油的Ⅱ型干酪根作为次要气源 ,实测和计算结果证实天然气主要是有机质在高成熟阶段形成的产物。运移过程中和成藏后的次生变化使天然气的组成和同位素面目变得非常复杂 ,尤其是由盖层微渗漏造成的蒸发分馏作用使同位素出现倒转。徐家围子断陷深层煤型气的发现为该区油气勘探展示出了更广阔的前景。     

Petroleum Origins and Accumulation Patterns in the Weixinan Sag in the Beibu Gulf Basin, Using Subsag B as an Example

Crude oil has been discovered in the Paleogene and Neogene units of the Weixinan Sag in the Beibu Gulf Basin. To determine the source and accumulation mode of this crude oil, 12 crude oil samples and 27 source rock samples were collected and an extensive organic geochemical analysis was conducted on them. Based on the geological conditions and the analytical results, the types, origins and accumulation patterns of crude oil in the study area were elucidated. Except for a shallowly-buried and biodegraded crude oil deposit in Neogene rocks, the crude oil samples in the study area were normal. All of the crude oils were derived from lacustrine source rocks. According to biomarker compositions, the crude oils could be divided into two families, A and B, distinctions that were reinforced by differences in carbon isotope composition and spatial distribution. Oil-source correlation analysis based on biomarkers revealed that Family A oils were derived from the mature oil shale at the bottom of the second member of the Liushagang Formation, while the Family B oils formed in the mature shale of the Liushagang Formation. The Family A oils, generated by oil shale, mainly migrated laterally along sand bodies and were then redistributed in adjacent reservoirs above and below the oil shale layer, as well as in shallow layers at high structural positions, occupying a wide distribution range. The Family B oils were generated by other shale units before migrating vertically along faults to form reservoirs nearby, resulting in a narrow distribution range.     

Quantitative assessment and significance of gas washing of oil in Block 9 of the Tahe Oilfield, Tarim Basin, NW China

Some Ordovician and Triassic oils in Block 9 are characterized by light oils,which have distinctly differentiated from heavy oils in other blocks in the Tahe Oilfield,Tarim Basin.Based on the whole oil gas chroma- tograms,this paper estimates the effect of oil migration and fractionation and the amount of depletion(Q)in terms of the n-alkanes depletion model.The results showed that the amount of depletion in the Ordovician reservoir is highest in the east of this block,e.g.the depletion is 97% in Well T904.The amount of Q gets lower to the west,e.g.the depletion is 53.4%in Well T115 and there is no sign of depletion in Well S69.It is suggested that the direction of gas washing is from the east to the west.The compositions and isotopic characteristics of associated gas in Ordovician oils indicated that the gas might be derived from Cambrian source rocks of the Caohu Depression which lies to the east of Block 9.In contrast,no obvious depletion of n-alkanes in Triassic oils was found,suggesting that the migration pathway of natural gas has been limited to the Ordovician karst fracture system formed in the Early Hercynian Orogeny.Different depletions of the Ordovician and Triassic oils can reveal fault activities in this region.     

生排烃过程中正构烷烃单体碳同位素组成的变化特征及其研究意义

通过对生排烃模拟实验产物 (残留油和排出油 )中正构烷烃单体碳同位素组成的测定,揭示出生排烃过程中正构烷烃碳同位素组成的变化特征。研究表明,生烃初期,液态正构烷烃主要来自干酪根的初次裂解,它们的碳同位素组成不论是在排出油中还是在残留油中,随温度的变化都不明显,呈现较相似的分布特征;在生烃高峰期,早期形成的沥青质和非烃等组分的二次裂解以及高碳数正构烷烃可能存在的裂解,使得正构烷烃单体碳同位素组成明显富集13 C,尤其在高碳数部分呈现出较大的差异。另外,实验结果显示排烃作用对液态正烷烃单体碳同位素组成的影响不太显著。