Evaluation of the petroleum composition and quality with increasing thermal maturity as simulated by hydrous pyrolysis: A case study using a Brazilian source rock with Type I kerogen

Hydrous pyrolysis (HP) experiments were used to investigate the petroleum composition and quality of petroleum generated from a Brazilian lacustrine source rock containing Type I kerogen with increasing thermal maturity. The tested sample was of Aptian age from the Araripe Basin (NE-Brazil). The temperatures (280–360 °C) and times (12–132 h) employed in the experiments simulated petroleum generation and expulsion (i.e., oil window) prior to secondary gas generation from the cracking of oil. Results show that similar to other oil prone source rocks, kerogen initially decomposes in part to a polar rich bitumen, which decomposes in part to hydrocarbon rich oil. These two overall reactions overlap with one another and have been recognized in oil shale retorting and natural petroleum generation. During bitumen decomposition to oil, some of the bitumen is converted to pyrobitumen, which results in an increase in the apparent kerogen (i.e., insoluble carbon) content with increasing maturation.The petroleum composition and its quality (i.e., API gravity, gas/oil ratio, C₁₅₊ fractions, alkane distribution, and sulfur content) are affected by thermal maturation within the oil window. API gravity, C₁₅₊ fractions and gas/oil ratios generated by HP are similar to those of natural petroleum considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. API gravity of the HP expelled oils shows a complex relationship with increasing thermal maturation that is most influenced by the expulsion of asphaltenes. C₁₅₊ fractions (i.e., saturates, aromatics, resins and asphaltenes) show that expelled oils and bitumen are compositionally separate organic phases with no overlap in composition. Gas/oil ratios (GOR) initially decrease from 508–131 m³/m³ during bitumen generation and remain essentially constant (81–84 m³/m³) to the end of oil generation. This constancy in GOR is different from the continuous increase through the oil window observed in anhydrous pyrolysis experiments. Alkane distributions of the HP expelled oils are similar to those of natural crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous age. Isoprenoid and n-alkane ratios (i.e., pristane/n-C₁₇ and phytane/n-C₁₈) decrease with increasing thermal maturity as observed in natural crude oils. Pristane/phytane ratios remain constant with increasing thermal maturity through the oil window, with ratios being slightly higher in the expelled oils relative to those in the bitumen. Generated hydrocarbon gases are similar to natural gases associated with crude oils considered to be sourced from similar Brazilian lacustrine source rocks with Type I kerogen of Lower Cretaceous, with the exception of elevated ethane contents. The general overall agreement in composition of natural and hydrous pyrolysis petroleum of lacustrine source rocks observed in this study supports the utility of HP to better characterize petroleum systems and the effects of maturation and expulsion on petroleum composition and quality.     

海相原油沥青质作为特殊气源的生气特征及其地质应用

应用高压封闭体系,对塔里木盆地海相原油中的沥青质组分进行了热裂解模拟实验,从气态烃产率及碳同位素演化、焦沥青的生成等方面,探讨了沥青质作为特殊气源的生气机理。运用Kinetics动力学软件,计算得到沥青质裂解的动力学参数（活化能和指前因子）,在此基础上,将模拟实验结果外推至地质条件下,探讨其动力学模型的实际应用。结果表明,沥青质裂解气在Easy%Ro值0.8左右开始生成,在Easy%Ro值2.65左右其转化率达到1。Easy%Ro为0.9时沥青质裂解进入主生气期（转化率0.1）,Easy%Ro为2.3时主生气期结束（转化率0.9）。研究成果可为中国海相层系裂解气的判识、资源评价及勘探决策等提供实验和理论依据。     

Comparison of petroleum generation kinetics by isothermal hydrous and nonisothermal open-system pyrolysis

This study compares kinetic parameters determined by open-system pyrolysis and hydrous pyrolysis using aliquots of source rocks containing different kerogen types. Kinetic parameters derived from these two pyrolysis methods not only differ in the conditions employed and products generated, but also in the derivation of the kinetic parameters (i.e., isothermal linear regression and non-isothermal nonlinear regression). Results of this comparative study show that there is no correlation between kinetic parameters derived from hydrous pyrolysis and open-system pyrolysis. Hydrous-pyrolysis kinetic parameters determine narrow oil windows that occur over a wide range of temperatures and depths depending in part on the organic-sulfur content of the original kerogen. Conversely, open-system kinetic parameters determine broad oil windows that show no significant differences with kerogen types or their organic-sulfur contents. Comparisons of the kinetic parameters in a hypothetical thermal-burial history (2.5 °C/my) show open-system kinetic parameters significantly underestimate the extent and timing of oil generation for Type-IIS kerogen and significantly overestimate the extent and timing of petroleum formation for Type-I kerogen compared to hydrous pyrolysis kinetic parameters. These hypothetical differences determined by the kinetic parameters are supported by natural thermal-burial histories for the Naokelekan source rock (Type-IIS kerogen) in the Zagros basin of Iraq and for the Green River Formation (Type-I kerogen) in the Uinta basin of Utah. Differences in extent and timing of oil generation determined by open-system pyrolysis and hydrous pyrolysis can be attributed to the former not adequately simulating natural oil generation conditions, products, and mechanisms.     

Stable sulfur isotope partitioning during simulated petroleum formation as determined by hydrous pyrolysis of Ghareb Limestone, Israel

Hydrous pyrolysis experiments at 200 to 365°C were carried out on a thermally immature organic-rich limestone containing Type-IIS kerogen from the Ghareb Limestone in North Negev, Israel. This work focuses on the thermal behavior of both organic and inorganic sulfur species and the partitioning of their stable sulfur isotopes among organic and inorganic phases generated during hydrous pyrolyses. Most of the sulfur in the rock (85%) is organic sulfur. The most dominant sulfur transformation is cleavage of organic-bound sulfur to form H₂S_(gas). Up to 70% of this organic sulfur is released as H₂S_(gas) that is isotopically lighter than the sulfur in the kerogen. Organic sulfur is enriched by up to 2‰ in ³⁴S during thermal maturation compared with the initial δ³⁴S values. The δ³⁴S values of the three main organic fractions (kerogen, bitumen and expelled oil) are within 1‰ of one another. No thermochemical sulfate reduction or sulfate formation was observed during the experiments. The early released sulfur reacted with available iron to form secondary pyrite and is the most ³⁴S depleted phase, which is 21‰ lighter than the bulk organic sulfur. The large isotopic fractionation for the early formed H₂S is a result of the system not being in equilibrium. As partial pressure of H₂S_(gas) increases, retro reactions with the organic sulfur in the closed system may cause isotope exchange and isotopic homogenization. Part of the δ³⁴S-enriched secondary pyrite decomposes above 300°C resulting in a corresponding decrease in the δ³⁴S of the remaining pyrite. These results are relevant to interpreting thermal maturation processes and their effect on kerogen-oil-H₂S-pyrite correlations. In particular, the use of pyrite-kerogen δ³⁴S relations in reconstructing diagenetic conditions of thermally mature rocks is questionable because formation of secondary pyrite during thermal maturation can mask the isotopic signature and quantity of the original diagenetic pyrite. The main transformations of kerogen to bitumen and bitumen to oil can be recorded by using both sulfur content and δ³⁴S of each phase including the H₂S_(gas). H₂S generated in association with oil should be isotopically lighter or similar to oil. It is concluded that small isotopic differentiation obtained between organic and inorganic sulfur species suggests closed-system conditions. Conversely, open-system conditions may cause significant isotopic discrimination between the oil and its source kerogen. The magnitude of this discrimination is suggested to be highly dependent on the availability of iron in a source rock resulting in secondary formation of pyrite.     

Oil-generation kinetics for organic facies with Type-II and -IIS kerogen in the Menilite Shales of the Polish Carpathians

The Menilite Shales (Oligocene) of the Polish Carpathians are the source of low-sulfur oils in the thrust belt and some high-sulfur oils in the Carpathian Foredeep. These oil occurrences indicate that the high-sulfur oils in the Foredeep were generated and expelled before major thrusting and the low-sulfur oils in the thrust belt were generated and expelled during or after major thrusting. Two distinct organic facies have been observed in the Menilite Shales. One organic facies has a high clastic sediment input and contains Type-II kerogen. The other organic facies has a lower clastic sediment input and contains Type-IIS kerogen. Representative samples of both organic facies were used to determine kinetic parameters for immiscible oil generation by isothermal hydrous pyrolysis and S₂ generation by non-isothermal open-system pyrolysis. The derived kinetic parameters showed that timing of S₂ generation was not as different between the Type-IIS and -II kerogen based on open-system pyrolysis as compared with immiscible oil generation based on hydrous pyrolysis. Applying these kinetic parameters to a burial history in the Skole unit showed that some expelled oil would have been generated from the organic facies with Type-IIS kerogen before major thrusting with the hydrous-pyrolysis kinetic parameters but not with the open-system pyrolysis kinetic parameters. The inability of open-system pyrolysis to determine earlier petroleum generation from Type-IIS kerogen is attributed to the large polar-rich bitumen component in S₂ generation, rapid loss of sulfur free-radical initiators in the open system, and diminished radical selectivity and rate constant differences at higher temperatures. Hydrous-pyrolysis kinetic parameters are determined in the presence of water at lower temperatures in a closed system, which allows differentiation of bitumen and oil generation, interaction of free-radical initiators, greater radical selectivity, and more distinguishable rate constants as would occur during natural maturation. Kinetic parameters derived from hydrous pyrolysis show good correlations with one another (compensation effect) and kerogen organic-sulfur contents. These correlations allow for indirect determination of hydrous-pyrolysis kinetic parameters on the basis of the organic-sulfur mole fraction of an immature Type-II or -IIS kerogen.     

Secondary hydrocarbon generation potential from heavy oil, oil sand and solid bitumen during the artificial maturation

Secondary hydrocarbon generation potentials from natural bitumen, oil sand and heavy oil, representing different residual oil accumulations, were determined by artificial maturation in a closed pyrolysis system. Simulated results indicate that their thermal behavior and reactivity are similar to those of kerogen, and that they can generate hydrocarbons once subjected to suitable geological processes. Overall differences in oil and gas generation potentials among the samples result from differences in the chemical structure of precursor components, physical compaction status, and mineral matrices. Hydrogen rich precursors, such as oil sand and heavy oil, have greater potential to generate hydrocarbons than hydrogen poor ones. Naturally compacted oil sand has slightly higher conversion efficiency than artificially compacted heavy oil as indicated by lower residual bitumen content. However, total gas and liquid oil recovery from oil sand is lower than from heavy oil due to the poor release of pyrolytic products from well compacted and cemented networks in the experiments. Mineral matrices of previous oil deposits also affect further hydrocarbon generation potential. Carbonate matrices inhibit total oil and gas generation, which consequently retains high gas potential at the postmature stage. Traditional oil generation models mainly consider the thermal alteration of kerogen; this study provides supplemental information for superimposed basins where previous oil accumulations may have been destroyed and reburied to serve as secondary sources of oil and gas. Consideration of previous oil residues as potential source rock allows better estimates of available oil resources and the risks associated with their exploration.     

Evaluating transition-metal catalysis in gas generation from the Permian Kupferschiefer by hydrous pyrolysis

Transition metals in source rocks have been advocated as catalysts in determining extent, composition, and timing of natural gas generation (Mango, F. D. (1996) Transition metal catalysis in the generation of natural gas. Org. Geochem.24, 977-984). This controversial hypothesis may have important implications concerning gas generation in unconventional shale-gas accumulations. Although experiments have been conducted to test the metal-catalysis hypothesis, their approach and results remain equivocal in evaluating natural assemblages of transition metals and organic matter in shale. The Permian Kupferschiefer of Poland offers an excellent opportunity to test the hypothesis with immature to marginally mature shale rich in both transition metals and organic matter. Twelve subsurface samples containing similar Type-II kerogen with different amounts and types of transition metals were subjected to hydrous pyrolysis at 330° and 355 °C for 72 h. The gases generated in these experiments were quantitatively collected and analyzed for molecular composition and stable isotopes. Expelled immiscible oils, reacted waters, and spent rock were also quantitatively collected. The results show that transition metals have no effect on methane yields or enrichment. δ¹³C values of generated methane, ethane, propane and butanes show no systematic changes with increasing transition metals. The potential for transition metals to enhance gas generation and oil cracking was examined by looking at the ratio of the generated hydrocarbon gases to generated expelled immiscible oil (i.e., GOR), which showed no systematic change with increasing transition metals. Assuming maximum yields at 355 °C for 72 h and first-order reaction rates, pseudo-rate constants for methane generation at 330 °C were calculated. These rate constants showed no increase with increasing transition metals. The lack of a significant catalytic effect of transition metals on the extent, composition, and timing of natural gas generation in these experiments is attributed to the metals not occurring in the proper form or the poisoning of potential catalytic microcosms by polar-rich bitumen, which impregnates the rock matrix during the early stages of petroleum formation.     

开放体系下平凉组页岩干酪根的生烃动力学研究

对鄂尔多斯盆地奥陶系平凉组海相页岩进行了3种升温速率下的 Rock-Eval 热解模拟实验,研究了其干酪根在开放体系下的热解生烃演化特征.通过分析热解烃 S2的产率随温度的变化,结合 Kinetics 生烃动力学专用软件计算,获得了其开放体系下的生油动力学参数,活化能分布范围为(57~81)×4.185 kJ/mol.在此基础上进行的动力学模拟结果与实验数据非常吻合,可较好地将实验数据外推到地质实际过程.此外,发现热解残渣中的 H/C(原子比)值与热解温度、干酪根转化率有较好的相关关系,可建立 H/C 值与转化率或者等效镜质组反射率的可靠模板,表明 Rock-Eval 热解实验与 H/C 值结合,可快速评价下古生界源岩的成熟度、转化率和生烃量等指标,将其应用于资源量计算等方面     

Biomarker distributions in asphaltenes and kerogens analysed by flash pyrolysis-gas chromatography-mass spectrometry

Biomarker distributions in a suite of asphaltenes and kerogens have been analysed by flash pyrolysis directly coupled to a GCMS system. Attention has been focussed on biomarkers of the sterane and triterpane types. The sample suite under investigation consists of sediments with different kerogen types and some crude oils. Biomarker distributions in the pyrolysates have been compared with the “free” biomarkers in the corresponding saturated hydrocarbon fractions.The analyses show significant differences between the distributions of the free biomarkers and those in the pyrolysates. The latter have lower amounts of steranes while diasteranes are absent or present at low concentrations only. In the triterpane traces a shift of maximum intensity from C₃₀ (free compounds) to C₂₇/C₂₉ is observed. Furthermore, the pyrolysates contain a set of triterpenes (not present among the free compounds), and there is a selective loss of “non-regular” triterpanes that are present in the saturated hydrocarbon fractions. The observed differences between pyrolysates and free hydrocarbons can be explained partly by the processes occurring during pyrolysis such as bond rupture and subsequent stabilisation of primary pyrolysis products. To a certain extent these differences also show that maturation processes occurring in sediments have effects on free biomarker molecules different from those on molecules that are enclosed in a macromolecular matrix (kerogen or asphaltenes).Differences between biomarker distributions of asphaltene and kerogen pyrolysates are relatively small. A comparison with the pyrolysates from extracted whole sediments suggests that these differences are mainly caused by interactions between the organic material and the mineral matrix during pyrolysis.Oil asphaltenes behave differently from sediment asphaltenes as their pyrolysates are more similar to the corresponding saturated hydrocarbon fractions, i.e. the differences described above are observed to a much smaller extent. This different behaviour appears to be the result of coprecipitation of a part of the maltene fraction with the oil asphaltenes.     

中国低熟油的几种成因机制

在对国内外文献详尽检索研究基础上，根据对国内９个二级沉积凹陷和５个中、小型盆地的有机地球化学、有机岩石学和同位素地球化学的系统研究，作者建立了木栓质体、树脂体、细菌改造的陆源有机质、藻类与高等植物生物类脂物以及富硫大分子（非烃、沥青质和干酪根）等五种早期生烃形成低熟油的成因机制。     

Pyrolysis of asphaltenes: a source of geochemical information

Pyrolysis of asphaltenes from crude oils yields significant amounts of crude oil-like material. Studies of asphaltenes and their pyrolysis products from biodegraded and non-biodegraded oils show that biodegradation does not affect the composition of asphaltene. The overall composition of the oil produced from them on pyrolysis is similar to, yet significantly different from, that of the parent oil. From these compositional differences, it is concluded that asphaltene and its pyrolysis products contain geochemical information which is characteristic, and therefore may shed light on the history of the oil prior to asphaltene formation.     

三塘湖盆地马东地区卡拉岗组烃源岩特征与致密油意义

      凝灰岩致密油藏作为一种特殊的油藏类型,已受到高度关注,而烃源岩的质量对致密油藏形成起着至关重要的作用.为了研究新疆三塘湖盆地马朗凹陷石炭系火山岩油气藏成藏规律,本文在基本地质特征研究的基础上,通过岩石样品的岩石热解、干酪根碳同位素和饱和烃色质谱等分析测试,对马东地区卡拉岗组不同岩性烃源岩特征进行描述与评价.研究结果表明:凝灰质泥岩和白云质泥岩均具有一定生烃潜力,其中凝灰质泥岩生烃潜力大,有机质丰度高,总有机碳质量分数平均值大于２％,以Ⅱ型有机质为主;卡拉岗组烃源岩埋藏深度较小,镜质体反射率主要分布在０．５％~０．８％,总体上处于低熟—成熟阶段.纵向上各种岩性烃源岩与凝灰岩交错叠置,有利于形成源内自生自储型油藏,对卡拉岗组致密凝灰岩油藏的形成具有重要意义.     

Source rock and asphaltene biomarker characterization by pyrolysisgas chromatography-mass spectrometry-multiple ion detection

Biomarkers produced by microscale pyrolysis of extracted source rocks, kerogens and asphaltenes have been analysed directly by gas chromatography-mass spectrometry in the MID mode. A series of experiments have been undertaken. These include an investigation into the production of the hopanes at different pyrolysis temperatures and a comparison of results obtained from Curie point pyrolysis and the Chemical Data Systems pyroprobe. Steranes and triterpanes produced by pyrolysis of a series of vitrinites were investigated and correlated with maturity variations. Finally, the biomarkers produced from asphaltenes were compared with those produced from the extract and extracted rock in order to further investigate the theory that asphaltenes are a liquid kerogen. The results of these experiments showed that the biomarkers released by pyrolysis of source rocks have a potential use for source and maturity determinations. The method is advantageous in that it can be said with some certainty that the biomarkers are indigenous to the rock and not present in the rock as a possible result of migration.     

海相烃源岩二次生烃潜力定量评价新方法

烃源岩二次生烃的演化过程是残余干酪根热解演化与残留油热裂解转化两个既相互联系又完全不同的物理化学反应过程的叠加。本文利用自制高压釜热压生烃模拟实验装置,采取分阶段连续递进模拟实验方式,以海相烃源岩样品为例分别评价了残余干酪根的生烃潜力与残留可溶有机质转化油气潜力,建立了一套不同起始与终止成熟度海相烃源岩二次生烃潜力的定量评价方法,并首次明确提出了干酪根生油指数KIo、干酪根生气指数KIg、干酪根生烃指数KIh等评价烃源岩生烃潜力的参数,弥补了ROCK EVAL热解评价方法无法分别评价烃源岩在不同生烃演化阶段所生成的“油”或“烃气”潜力的不足。     

分散可溶有机质的气源意义

广泛分布在烃源岩及输导通道中的分散可溶有机质作为一种新的气源逐渐受到重视。分散可溶有机质成气不仅关系到原油的消耗, 还关系到天然气的资源评价结果, 因而定量评价这一过程对油气勘探有着重要意义。笔者总结对比了传统成烃模式与考虑分散可溶有机质成气模式的异同, 建立了分散可溶有机质成气的地质模型, 初步计算了塔里木盆地分散可溶有机质的成气量。指出分散可溶有机质这一气源使得地层成气区域突破了烃源岩的分布范围, 成气中心向构造高部位偏移, 成气期推迟。计算得到塔里木盆地分散可溶有机质成气模式中源内分散可溶有机质裂解成气与源外的比例为1:2.88, 在成气晚期白垩纪末至现今这一阶段油裂解成气量为799 千亿m³, 是传统油成气模式的4.23倍。     

海相油气成藏定年技术及其对元坝气田长兴组天然气成藏年代的反演

我国海相叠合盆地的油气特征及成藏过程表现出很强的复杂性,确定油气成藏年代极其困难,建立有效的成藏定年技术显得尤为迫切.为此基于稀有气体He年代积累效应和油气藏保存机制,建立了油气藏4He成藏定年地质模型及年龄估算公式,明确其为油气成藏定型时间;基于天然气40Ar/36Ar比值与源岩钾丰度及地质时代的关系,建立了追溯油气源岩时代的Ar同位素估算模型.磷灰石、锆石（U-Th）/He定年体系的封闭温度与含油气盆地生油气窗的温度范围较为一致,磷灰石、锆石（U-Th）/He年龄可以揭示含油气盆地抬升剥蚀时间、由构造抬升导致的油气藏调整时间,建立了固体沥青、原油中沥青质提取、溶样、Re-Os纯化富集及分离等Re-Os同位素测年前处理技术,可以直接确定固体沥青、原油等的形成时间.按照含油气系统成藏地质要素形成时间或发生时间先后顺序,提出了从确定源岩形成-油气生成-运移充注-调整改造-成藏定型等成藏过程的定年技术序列.开展四川盆地元坝气田源岩时代、生排烃、运移充注、调整改造及成藏定型等关键过程的时间节点综合研究,明确了元坝气田的主力气源为上二叠统龙潭组烃源岩,两期原油充注时间分别为220~175 Ma、168~140 Ma;油裂解气发生在140~118 Ma,元坝地区约97 Ma以来发生构造抬升,尤其15 Ma以来气-水界面发生调整,约在12~8 Ma气藏最终定型并形成现今的气藏格局.      

Kinetic studies of asphaltene pyrolyses and their geochemical applications

Pyrolysis kinetics are determined for a series of asphaltene samples associated with different types of kerogens. The activation energy distributions obtained for asphaltenes related to type I kerogens cover a wide range, with significant hydrocarbon generation with activation energies above 350 kJ/mol. The ranges of activation energy distributions are relatively narrow for asphaltenes associated with type II and III kerogens, with little hydrocarbons generated with activation energies above 350 kJ/mol. If the type of associated kerogen can be reasonably constrained, the pyrolysis kinetics of the asphaltenes is potentially a very useful tool for assessing the maturity levels of reservoired oils. The results of pyrolysis kinetics indicate that there is still significant potential for asphaltenes to regenerate hydrocarbons.     

Water Role and Its Influence on Hydrogen Isotopic Composition of Natural Gas during Gas Generation

In order to discuss the role and influence of water during the generation of natural gas, the participation mechanism of water during the evolution of organic matter and its influences were summarized. In addition, we carried out an anhydrous cracking experiment of oil extracted from the Feixianguan Formation source rock in a closed system, which led to the establishment of the kinetic models for describing carbon and hydrogen isotopic fractionation during gas generation from organic matter. The models were calibrated and then applied to the northeastern Sichuan Basin. By combining a series of gas generation experiments from octadecane pyrolysis without water or with distilled water in varying mass proportions, several results were proved: (1) the hydrogen isotopic composition of natural gas becomes lighter with the participation of formation water; (2) we can quantitatively study the hydrogen isotopic fractionation with the kinetic model for describing carbon isotopic fractionation; (3) more abundant and reliable geological information can be obtained through the combined application of carbon and hydrogen isotopic indices.     

准噶尔盆地呼图壁气田油气成因及成藏分析

应用原油、天然气及烃源岩样品有机地球化学资料,对准噶尔盆地呼图壁气田油气地球化学特征及成藏时间进行分析.研究表明,呼图壁气田原油来源于白垩系烃源岩,天然气来源于侏罗系煤系烃源岩干酪根裂解气,源岩大量生气始于12 Ma左右,天然气成藏时间约在5 Ma.