Changes in gas composition during simulated maturation of sulfur rich type II-S source rock and related petroleum asphaltenes

Asphaltenes extracted from crude oils are proposed to possess structural features of the related source rock kerogen. For the present study micro-scale sealed vessel pyrolysis (MSSV) and combustion isotope ratio mass spectrometry (GC–C–IRMS) were used to compare gas generation from a whole rock (type II-S kerogen) from southern Italy with that from related sulfur rich asphaltenes isolated from a low maturity heavy crude oil. The purpose of was to determine whether experimental pyrolysis of oil asphaltenes can be used to predict the timing and the chemical and isotopic composition of hydrocarbon gases generated from genetically related kerogen in the source rock during burial maturation. The results show that parameters such as (gas to oil ratio) GOR and oil and gas formation timing are very similar for these two sample types, whereas gas composition, product aromaticity and sulfur content are remarkably different. Slight differences in GOR are mainly due to differences in gas formation characteristics at very high levels of thermal alteration. Secondary gas formation from the whole rock covers a much broader temperature range under geological conditions than that from the asphaltene products. However, it is remarkable that both the onset and the maximum temperature are nearly identical under geological conditions. The observed differences in gas generation characteristics are supported by discrepancies in the carbon isotopic characteristics of the gas range compounds and indicate different precursors and/or mechanisms for gas generated from whole rock and asphaltenes.     

Geochemical significance of chemical composition and ESR properties of asphaltenes in crude oils from Alberta,Canada

More than 100 crude oils and 12 oil sands from Alberta, Canada, from stratigraphie units ranging in age from Upper Cretaceous to Middle Devonian, were processed to separate the asphaltenes which were then analysed for C, H, N, O and S and the ESR spectra run to determine g-value, spin number and line width. A factor analysis of these data, together with selected crude oil and reservoir properties, was interpreted in terms of a number of non-thermal processes which can affect the content and composition of asphaltenes during and after thermal maturation. Experiments on the effect of temperature on the signal intensity of free radicals indicate that more than half, and possibly all, the free radicals observed in asphaltenes must arise from charge transfer or closely related equilibria. An important solvent effect is demonstrated in further experiments, in which the more polar the solvent, the more the solvent dispersive forces overcome the associative forces of the asphaltene layers thus leading to lower molecular weights and a change in the charge transfer forces responsible for the ESR signal. The average molecular weight of asphaltenes from heavy, asphaltic, biodegraded crude oils and oil sands is considerably greater than the average molecular weight of asphaltenes from the more paraffinic, mature, non-biodegraded crude oils, and it seems likely that the differences in molecular weight of these asphaltenes are due to the solvent effect of the crude oils, and thus the laboratory observations have been confirmed in the Alberta basin.     

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。     

Hydrous pyrolysis of Monterey asphaltenes

Hydrous pyrolysis of asphaltenes has been tested as a method to reconstruct the chemical composition of biodegraded oils and oil seeps. The asphaltenes of three oils (a nondegraded oil, a biodegraded oil, and a biodegraded oil seep) from the Monterey Formation were studied. Results show that the aliphatic fraction generated by hydrous pyrolysis is very similar in chemical composition to the non-degraded oil. This makes the method very useful in correlation studies of biodegraded and nondegraded oils. It also allows to roughly estimate the maturity of the source of the biodegraded oil or oil seep.     

Carbon isotopic composition of individual n-alkanes in asphaltene pyrolysates of biodegraded crude oils from the Liaohe Basin, China

Biodegraded oils are widely distributed in the Liaohe basin, China. In order to develop effective oil-source correlation tools specifically for the biodegraded oils, carbon isotopic compositions of individual n-alkanes from crude oils and their asphaltene pyrolysates have been determined using the gas chromatography–isotope ratio mass spectrometry technique. No significant fractionation in the stable carbon isotopic ratios of n-alkanes in the pyrolysates of oil asphaltenes was found for anhydrous pyrolysis carried out at temperatures below 340°C. This suggests that the stable carbon isotopic distribution of n-alkanes (particularly in the C₁₆–C₂₉ range) in the asphaltene pyrolysates can be used as a correlation tool for severely biodegraded oils from the Liaohe Basin. Comparison of the n-alkane isotopic compositions of the oils with those of asphaltene pyrolysates shows that this is a viable method for the differentiation of organic facies variation and post-generation alterations.     

Thermal treatment of the Athabasca oil sand bitumen and its component parts

The Athabasca oil sand bitumen, and its major component fractions, have been heated with and without added montmorillonite to simulate natural geological diagenesis. The products of the treatment of the whole bitumen were found to be essentially the sum of the products of the treatment of the component parts. The saturated hydrocarbon fraction underwent little change, with only minor cracking apparent. Some cracking and polymerization of the aromatic hydrocarbon fraction was noted. Geochemical analysis of these products indicated that the simulation had caused a continuation of the normal petroleum maturation sequence towards coke and gas. Treatment of the polar fraction and asphaltenes caused a continuation of the maturation sequence towards a ‘crude oil’, further treatment of which continued towards coke and gas. Geochemical examination of this ‘crude oil’ yielded information on the past history of the bitumen from the time of asphaltene formation. The geochemical evidence indicates that the Athabasca bitumen may have originated in an unusual marine environment, and that the maturation and exodus from its source rock differed from that of conventional crude oils. The overall results indicate that, except for the maturation of the polar and asphaltene fractions, the bio-degradation of a crude oil is not reversible by diagenetic means.     

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.     

A comparison of organosulphur compounds produced by pyrolysis of asphaltenes and those present in related crude oils and tar sands

Asphaltenes and NSO fractions from a variety of oils and tar sands have been characterized by pyrolysis-gas chromatography (Py---GC) using both flame ionization and flame photometric detectors. Organosulphur compounds were not always produced by pyrolysis of the asphaltenes isolated from the biodegraded tar sand extracts although pyrolysis of the NSO fractions produced a series of thiophenes from all of the samples examined. These observations suggest that under certain conditions, asphaltene fractions may be affected by biodegradation, leading to preferential removal of the sulphur containing compounds. In addition, it was observed that Py---GC of asphaltenes permitted oils to be differentiated on the basis of their source rock lithology. Oils derived from carbonate source rocks produced thiophene/benzothiopene ratios <1 whilst the same ratio was >1 for oils derived from shales. Thiophene/dibenzothiophene ratios could be used in a similar manner.     

Field-ionization mass spectrometry: application to geochemical analysis

The thiourea adduct, thiourea non-adduct and aromatic fractions from a series of Alberta oil sand bitumens have been subjected to field ionization mass spectrometric analyses. Field ionization mass spectra feature little fragmentation and the molecular weight distributions of compounds in the fractions are readily obtained. Using this method, the various acyclic and cyclic saturated hydrocarbons and the hydrocarbon and thiophenic aromatic materials may be distinguished by compound type and number of carbon atoms. The effect of biodegradation on crude oils is illustrated in the results from the series of oil sand bitumens and their asphaltene pyrolysis products. Field ionization mass spectrometry is potentially an important new analytical tool in organic geochemistry.     

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.     

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.     

Tmax of asphaltenes: a parameter for oil maturity assessment

A new maturity parameter determined on both oil and bitumen samples, the asphaltene T_max, is proposed and discussed. This parameter could be very useful to address the maturity of the source rock. The asphaltene T_max is measured by programmed Rock-Eval pyrolysis, using a modified temperature program. Some phases of the experimental procedure, such as the asphaltene preparation and the Rock-Eval measurement substratum choice, are crucial in order to achieve reliable data. Laboratory simulations were carried out in order to assess the possible effects of both primary and secondary migration on asphaltene T_maxin the expelled oil: the original value of the asphaltene T_max in the bitumen is not substantially modified and it is very close to that measured on kerogen. Examples of the determination of asphaltene T_max on many samples, collected from different areas and with different organic matter composition, are given. Results show that T_max values from oil asphaltenes are reasonable indicators of source rock maturity.     

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.     

Geochemical study of a series of biodegraded oils: Microtexture and structure of their coked asphaltenes

The asphaltenes extracted from eight oils and a bituminous sand were coked and studied by transmission electron microscopy and X-ray diffraction to observe the microtextural and structural changes caused by increasing biodegradation. The pyrolysis effluents of asphaltenes were analyzed by chromatography. Biodegradation causes an increase of aromatic hydrocarbons and sulfur during heat-treatment of isolated asphaltenes. During thermal treatment, the more biodegraded the sample, the earlier and the greater the release of aromatics. The microtexture of the asphaltene coke is very sensitive to this early release of aromatics and is used to classify samples as to their degree of biodegradation. In the series, two groups were distinguished.     

Carbon isotope geochemistry of Paleozoic oils from Big Horn Basin

Twenty crude oils from Paleozoic reservoirs in the Big Horn Basin. Wyoming were fractionated into light hydrocarbons, saturates, monoaromatics. diaromatics, polyaromatics-and-polars. and asphaltenes. Amounts and isotopic composition of each fraction were found to be internally consistent with the degree of maturation of the oil. confirming the established single source origin for these oils. A variation of approximately three per mil in the carbon isotopic composition of the whole oil and individual fraction was explained as being caused by maturation. The isotope-type-curves for these oils, the variations in isotopic composition as a function of compound group-type, were not found to be as simple as commonly believed. Other alteration processes, such as migration and deasphalting, might have influenced the pattern of the isotope distributions among fractions. The conservative nature of mass balance and subsequent isotope flows among various fractions of crude oil showed that the maturation of crude oil consists of consecutive processes leading from larger and more polar components into smaller and less polar components culminating in natural gas. Calculations were made to project the amount of condensates associated with the crude oils, and the amount of natural gas which had been generated from crude oils of a given maturity.     

柯坪隆起阿克苏油苗的地球化学特征及其成因研究

柯坪隆起区是塔里木盆地海相烃源岩露头发育较好的地区之一,但相关的油气研究报道不多。本文详细研究了塔里木盆地柯坪隆起区油苗的有机地球化学特征并进行了相关的成因分析。阿克苏油苗经历了严重的生物降解作用,饱和烃生物标志物所能提供的成因信息十分有限。原油沥青质组分及储层吸附/包裹烃受后生作用影响较小,综合分析油苗沥青质热解产物与吸附/包裹烃的生物标志物及其碳同位素组成可以获得更多有效的成因信息。研究结果表明,阿克苏油苗与其储层吸附/包裹烃具有不同来源。油苗、沥青质以及沥青质热解产物的碳同位素特征与已经报道的多数海相原油及其沥青质较为接近,与典型的寒武系烃源来源的原油差异显著。而储层吸附/包裹烃与已报道的典型寒武系烃源岩来源油的生物标志物及碳同位素特征比较类似,可能形成于较高的热演化阶段。     

A comparison of results from two different flash pyrolysis methods on a solid bitumen sample

A Cambrian solid bitumen from northwestern Sichuan Basin, southern China was analyzed using two different flash pyrolysis methods coupled with gas chromatography–mass spectrometry analysis, including Pyroprobe® and analytical laser micropyrolysis. Results show that pyrolysis products from a Pyroprobe® (model 5000) analysis are dominated by mono-, di- and tricyclic aromatic hydrocarbons, whilst those from laser micropyrolysis are dominated by aliphatic hydrocarbons (n-alk-1-ene/n-alkane doublets), which is consistent with the results from an FT-IR spectrum of the solid bitumen. According to the molecular compositions of the pyrolysates from two types of pyrolysis, results from a 532 nm continuous wave laser may be more suitable for pyrolysis research of bitumen/asphaltene. Thus, differences in pyrolysis techniques must be considered when used to characterize oil asphaltenes or source rock kerogens.     

The influence of biodegradation on resins and asphaltenes in the Liaohe Basin

Asphaltenes are traditionally considered to be recalcitrant to microbial alteration. Resins and asphaltenes of seven biodegraded oils extracted from reservoir cores of two columns (Es3 and Es1) of the Lengdong oilfield in the Liaohe Basin, NE China, were studied to test this hypothesis. Elemental (C, H, N, O, S) and isotopic compositions (δ¹³C and δ¹⁵N) were measured, FT-IR was used to study the oxygenated functionalities of both resins and asphaltenes, and Py–GC–MS was used to elucidate how alkyl side chains of asphaltenes were altered during biodegradation. We conclude that the products of biodegradation, such as carboxylic acids, phenol and alcohols, may not only contribute to the resin fraction of crude oils, but also are linked with functionalities of resins and asphaltenes. The amount of asphaltenes increases because some resin molecules are enlarged and their polarity increased such that they can be precipitated by hexane as newly generated asphaltenes. Thus, the hydrocarbons that are progressively consumed during biodegradation can pull the δ¹³C of asphaltene fraction closer to the δ¹³C of the altered resins and hydrocarbons that were consumed.     

油层不同开采时期原油组分变化特征

运用有机地球化学分离、分析技术对取自塔里木盆地的东河、塔中和轮南油田不同时期开采出的原油样品进行剖析,研究其宏观组成和微观分子在时空上的变化规律。分析结果表明,原油饱 /芳比值随开采时间的推移呈下降的趋势;饱和烃与非烃 +沥青质的含量变化有很好的相关性。随着开采时间的增长,原油中正构烷烃的主峰碳数后移,原油的轻 /重组分比降低。原油碱性氮、有机酸等非烃类化合物随开采时间的推移也呈降低的趋势。这些变化与原油在驱替过程中的自然色层吸附作用机理和原油中各种组分的相互作用关系有关。该项研究对于深入探索驱油机理、预测原油润湿性和提高采收率具有重要的实际意义。