Waxes and asphaltenes in crude oils

High molecular weight (HMW) hydrocarbons (>C₄₀) and asphaltenes are important constituents of petroleum and can cause problems related to crystallization and deposition of paraffin waxes during production and transportation as well as in the formation of tar mats. However, traditional methods to isolate asphaltene fractions, by adding 40 volumes in excess of low boiling point solvents such as pentane, hexane or heptane, can produce asphaltene fractions which are contaminated with a significant amount of microcrystalline waxes (>C₄₀). The presence of these microcrystalline waxes in the asphaltene fractions has the potential to provide misleading and ambiguous results in modeling and treatment programs. The sub-surface phase behaviour of an asphaltene fraction will be quite different from that of a wax-contaminated asphaltene fraction. Similarly accurate modelling of wax drop-out requires information on pure wax fractions and not asphaltene-dominated fractions. Hence the goal of this paper is to describe a novel method for the preparation of wax-free asphaltene fractions. In addition, this method provides a quantitative subdivision of the wax fraction into pentane soluble and insoluble waxes which, when correlated with physical properties of crude oil such as viscosity, pour point, cloud point, etc., may help explain causes of wax deposition during production, transportation and storage of petroleum.     

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 Characteristics of Crude Oils from Zao-V Oil Measures in Shenjiapu Oilfield

The geochemical characteristics of crude oils from Zao-V oil measures in the Shen-jiapu oilfield are systematically described in terms of the fractional composition of crude oils, GC characteristics of saturated hydrocarbon fraction of crude oils and the characteristics of their bio-markers. The deposifional environment, type and evolution of the biological source are also discussed. All pieces of evidence such as low saturated hydrocarbon fraction, high resin and asphalt, high isoprenoid alkane, weak odd-carbon number predominance ( CPI ranging from 1.23 to 1,29, OEP ranging from 1.14 to 1.16) and low sterane and terpane maturity parameters show these crude oils are immature oils. Low Pr/Ph ratios (0.66 -0.88) and high gammacer-ante/C31 hopane ratios ( 0.59 - 0.86 ) indicate the source rocks were formed in a slightly saline to brackish reducing lake depositional environment. Gas chromatographic characteristics of the saturated hydrocarbon fraction and the predominance of C30 hopane in terpane series and C29 sterane in sterane series indicate the biological source of the crude oils is composed mainly of bacterial and algal organic matter, and some algae are perhaps the main contributor of organic matter to the source rocks.     

Geochemical characteristics of aromatic hydrocarbons in crude oils from the Linnan Subsag,Shandong Province,China

Abundant aromatic fractions were detected in oils from the Linnan subsag, including aphthalenes, phenanthrenes, triaromatic steroids, biphenyl, fluorenes, dibenzothiophenes, dibenzofuranes, as well as some typical higher plant-sourced compounds, such as pyrene, benzopyrene, fluoranthene, chryaene, benzofluoranthrene, perylene and cadalene. Occurrences of biomarkers indicate that oils from the Linnan subsag are typical terrestrially genetic oils. Developed in the depositional environment is a strong reduction condition typical of brackish-saline lake. Thermal evolution has entered into the mature-high mature stage.     

Geochemical characteristics of crude oils from Well Zheng-1 in the Junggar Basin, Xinjiang, China

Well Zheng-1 is located in the combined area of the central uplift and the north Tianshan piedmont depression in the Junggar Basin. Two oil-bearing beds are recognized at 4788–4797 m of the Lower Cretaceous Tugulu Formation (K1tg) and 4808.5–4812.5 m of the Lower Jurassic Sangonghe Formation (J1s). The geochemical characteristics of family composition, carbon isotopic composition, saturated hydrocarbons, sterane and terpane biomarkers and carotane of two crude oils are described in this paper. The results show that the geochemical characteristics of the two crude oils are basically similar to each other, indicating they were all derived mainly from the high mature, brine, algae-rich lake facies sediments. Oil-source correlation revealed that crude oils of the two beds were derived mainly from the source rocks of Permian and mixed by the oil derived from the source rocks of Jurassic and Triassic. This is consistent with the geological background with several sets of source rocks in the area studied.     

塔里木盆地原油轻烃地球化学特征

通过塔里木盆地５４０个原油要品中轻烃,尤其是Ｃ７烃类异构烷烃、环烷烃和正构烷烃的研究,认识到２－ＭＨ与３－ＭＨ化合物之间的相关性要高于２－ＭＨ与２－ＭＰ化合物之间的相关性,而ＭＣＰ与ＭＣＨ的化合物之间的相关性最差。原油Ｋ１值分布在０．７８－１．５４之间,其中８０％原油样品的Ｋ１值为０．９０－１．２０。原油中（２－ＭＨ＋２,３－ＤＭＰ）与（３－ＭＨ＋２,４－ＤＭＰ）占总径（Ｃ４－Ｃ７）的百分比的变化     

鄂尔多斯盆地姬塬-西峰地区原油地球化学特征及油源分析

采用色谱-质谱(GC-MS)、裂解-色谱-质谱(Py-GC-MS)、色谱-同位素比值质谱(GC-IRMS)等方法,系统地分析了鄂尔多斯盆地南部姬塬与西峰地区原油的地球化学特征,重新确定了原油成熟度及对应源岩的形成环境.分析表明,单体碳同位素组成偏轻,介于-30‰～-33‰之间.原油饱和烃馏分中五环三萜烷含量较高;藿烷以C30藿烷占优势,C29藿烷次之;含较高的18α(H)-30-降新藿烷(C29Ts);C30重排藿烷和伽马蜡烷含量较低;规则甾烷含量高并以C29甾烷为主,含一定的孕甾烷、升孕甾烷和重排甾烷;芳烃馏分中富含萘、菲系列化合物;沥青质裂解产物中芳基类异戊二烯烃含量很低.原油成熟度相当于镜质组反射率0.82%左右,是烃源岩在生油高峰阶段的产物.油源分析表明,该区原油主要来源于三叠系长7段泥岩,有机质形成于水体较深、淡水湖相沉积环境.     

Chemical composition and stable carbon isotopic ratios of crude oils from Southern Germany

Thirty crude oils were sampled in the South German Molasse Basin. The crude oils were chemically separated into five component groups and their stable carbon isotopic ratios were determined by mass spectrometry. A distinct differentiation of four regionally connected groups can be inferred from the results of the δ¹³C values and chemical analyses. This grouping is in accordance with the present geologic information on the origin of crude oils in the Molasse Basin.The chemistry of crude oils seems to have been influenced by secondary processes in the eastern part of the Molasse Basin.     

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.     

New data on composition and properties of Lower Cambrian crude oils in Lena region,Irkutsk oil and gas basin

Two groups of oils are considered: condensates and light oils from the terrigenous Lower Cambrian section, and oils occurring in the halogen-carbonate section. Preliminary conclusions on different cycles of oil formation in the terrigenous and carbonate complexes are drawn from the study of chemical composition, optical activity, and distribution of microelernents in the oils. The Parfenovo horizon of Markovo field is considered as a differentiated oil and gas deposit. -- Author.     

Geochemical Characteristics of Different Kinds of Crude Oils in the Tarim Basin,Northwest China

Based on the compositions and distributions of biomarkers in thirty-five representative oil samples, oils from the Tarim Basin of northwestern China are mainly divided into two oil families. One oil family contains relatively low amounts of C₁₅-C₂₀ isoprenoid hydrocarbons and shows pristane predominance with Pr/Ph ratios ranging from 1.50 to 3.00. The GC/MS analytical data of these oils show the occurrence of abundant hopanes, and low concentrations of steranes and tricyclic terpanes with hopanes/steranes ratios from 6.25 to 12.24 and tricyclic terpanes/hopanes ratios from 0.03 to 0.24. These oils contain low drimane relative to homodrimane (C₁₅/C₁₆ < 1.0) and abundant rearranged bicyclanes in bicyclic sesquiterpanes. They are dominated by low carbon number (C₁₉-C₂₁) compounds in the tricyclic terpanes, and are rich in rearranged hopanes, C₂₉Ts and an unknown C₃₀ compound in pentacyclic triterpanes. These geochemical characteristics suggest that the oils were generated mainly from terrigenous organic matter. The other oil family shows remarkably different biomarker compositions and distributions. The oils revealed Pr/Ph ratios of about 1.0, high drimane/homodrimane ratios (>1.0), low hopanes/steranes ratios (0.65–2.50), high tricyclic terpanes/hopanes ratios (0.30–2.00) and a dominant peak at C₂₃ in tricyclic tepanes, suggesting a marine organic origin. Oil-source rock correlation indicates that these two oil families seem to have been derived from Mesozoic Jurassic-Triassic terrestrial source rocks (shales and coal seams) and Lower Paleozoic Ordovician-Cambrian marine source rocks, respectively.     

Transport properties and model-based dynamical properties of Omani crude oils

We have measured the density and viscosity of five crude oil samples, collected from various hydrocarbon reservoirs in Oman, as functions of temperature. The measured quantities are expressed in terms of fitted formulae allowing their easy usage for different computational and simulation works. As an application, these thermo-physical data have been utilized to investigate the flow dynamics of hydrocarbon films under gravity at various temperatures. We have modeled the flow of these crude oils to study the dynamics of falling films in an open-top rectangular pipe set at various angular alignments under the assumption of Newtonian fluid describing a laminar flow. The adopted model of the investigation is not entirely novel, but the calculations aim to apply the model to various Omani crude oil samples with various American Petroleum Institute (API) values; the calculated results shed light on the dynamics of these crude oil films, which might be correlated to crude oil purification mechanism and open-top transportation.     

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.     

Origin and significance of mud-filled incised valleys (Upper Cretaceous) in southern Alberta, Canada

Seven mud-filled incised valleys (MFIVs) in the paralic facies of the Dinosaur Park and Horseshoe Canyon formations (Upper Cretaceous) of southern Alberta were studied to better understand their morphology, geometry and depositional histories in an estuarine context. Two preservational geometries occur: simple, U-shaped forms; and internally complex forms. Both types of MFIV record deposition in the central zone of low energy (turbidity) in an estuarine setting. Simple, U-shaped MFIVs have sharp basal erosional surfaces and consist of mudstone-dominated heterolithic fills of channel-wide, concave-up laminae. Associated fossil assemblages are marine to brackish. Each simple MFIV records a cut-and-fill history associated with a cycle of relative sea-level drop and rise. Low-energy depositional settings, loss of channel form during infilling, and associated shoreface deposits, as well as the absence of clear tidal indicators suggest a coastal plain estuarine setting, along a wave-dominated, barred coastline. Complex MFIVs are rarer, and consist of imbricated, wedge-shaped sets of inclined-to-horizontal heterolithic strata. Tidal deposits and/or nonmarine-to-marine macrofossils occur locally. Complex MFIVs were infilled in meandering reaches of the central zone of low energy in tide-dominated estuaries. Their rarity compared to simple MFIVs and their freshwater palaeontological content suggest that they were contiguous landward with extensive fluvial channels. A complex MFIV near Onefour comprises three in-channel depositional cycles. Each cycle consists of an erosional surface overlain by lateral accretion bedding and a conformable transition to vertically aggraded strata. Each cycle reflects a cut-and-fill event under the control of changes in relative sea-level that culminated in overbank flooding. All MFIVs formed in low-gradient settings (≤0.03%) where estuarine zones were stretched out over many tens of kilometres. Tide-dominated estuaries apparently exhibited simple, straight-to-meandering upstream transitions and extensive landward penetration (≥200 km) of tidal backwater effects. Few modern estuaries serve as adequate modern analogues to these ancient, tide-dominated estuaries. Radiometric data indicate that MFIV cut-and-fill cycles were 100 000-400 000 years in maximum duration and thus, equivalent to 4th order sea-level cycles. However, negative evidence tentatively suggests that these cycles took place over time intervals 1-2 orders of magnitude smaller (5th order or higher sea-level cycles).     

Geochemical characteristics of Ordovician crude oils in the northwest of the Tahe oil field,Tarim basin

Forty-six crude oil samples were selected from the Ordovician in the northwestern part of the Tahe oilfield for detailed molecular geochemical and isotopic analysis, including group compositions, carbonhydrogen isotopes and gas chroma-tograms of saturated hydrocarbons, as well as the characteristics of terpane, sterane and other biomarkers, indicating that crude oils are of the same origin from different districts in the Tahe oilfield and were derived from the same source kitchen (or oil source formation), i.e., mainly stemming from marine hydrocarbons. Detailed studies of oil physical properties of 25-honpane revealed that such oils have heavy or thick oil qualities due to biodegradation. Comprehensive assessment in terms of five maturity parameters shows that the oils from the Ordovician with Ro values varying from 0.80% to 1.59% are widely distributed in the northwest of the Tahe oilfield.     

Geochemical characteristics and genetic types of crude oils from Qinjiatun and Qikeshu oilfields in the Lishu Fault Depression,Songliao Basin,northeastern China

The Qinjiatun and Qikeshu oilfields are new Mesozoic petroleum exploration targets in Lishu Fault Depression of Songliao Basin, northeastern China. Currently, researches on geochemistry of crude oils from Qinjiatun and Qikeshu oilfields have not been performed and the genesis of oils is still uncertain. Based on bulk analyses, the crude oils in the Qinjiatun and Qikeshu oilfields of Lishu Fault Depression from the Lower Cretaceous can be classified as three types. TypeⅠoils, from Quantou and Denglouku formations of Qikeshu oilfield, are characterized by high C24tetracyclic terpane/C26tricyclic terpanes ratios, low gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29norhopane ratios and 17α(H)-diahopane/17α(H)-hopane ratios, indicating a brackish lacustrine facies. TypeⅡoils, from Shahezi Formation of Qikeshu oilfield show low C24tetracyclic terpane/C26tricyclic terpanes, high gammacerance/C30hopane ratios, tricyclic terpanes/hopanes ratios, C29Ts/C29 norhopane and C30diahopane/C30hopane ratios, thus suggesting that they originated from source rocks deposited in a weak reducing brackish lacustrine environment, or clay-rich sediments. Type oilsⅢ, from some wells of Qikeshu oilfield have geochemical characteristics intermediate between those two types and may be mixture of typeⅠand Ⅱoils.     

Geochemical characteristics of aromatic hydrocarbons in saline lacustrine crude oils and their significance as exemplified by the south area of western Qaidam Basin

Based on quantitative GC-MS analysis of 40 crude oil samples collected from the south area of western Qaidam Basin,one of the largest saline lacustrine basins in China,the geochemical characteristics of aromatic hydrocarbons in oils were studied systematically in this paper.Among those constitutes,naphthalene(43% 59%),phenanthrene(12% 21%) and taromatic-sterane series(6% 28%) were the main ones of aromatic hydrocarbons.The ratio of aromatic hydrocarbon maturity parameter vs.saturated hydrocarbon maturity parameter C 29 20S/(20S+20R) shows that some aromatic hydrocarbon maturity parameters are not suitable for low-mature oils,including MPI,MNR,DNR,etc.Meanwhile,maturity parameters for dibenzothiophene and taromatic-sterane series are more appropriate for low maturity saline lacustrine crude oils.Based on the ratio of 4,6-DMDBT/1,4-DMDBT,the R c values are within the range of 0.59% 0.72%.However,the abundance of dibenzothiophene(DBT) is low,and the dibenzofuran(DBF) content is even lower,suggesting that the crude oils were formed in a saline lacustrine anaerobic environment.The high abundance of C 26 triaromatic steroid also indicates that the source material is brackish water-saline water with strong reducibility.     

Compositional characteristics and geochemical significance of light hydrocarbons for crude oils in Tarim Basin, China

Light hydrocarbons, especially C7 isoalkanes, cyclopentanes, cyclohexanes and n-heptane in 540 oils from the Tarim Basin were studied systematically. The results suggested the K1 values [ （2-MH＋ 2,3-DMP）/（3-MH ＋ 2,4-DMP） ] of crude oils vary from a lowest value of 0.78, to a highest value of 1.54, but the K1 values of the majority of oil samples （80%） range from 0.90 to 1.20. However, the absolute contents of （ 2-methylhexane ＋ 2,3-dimethylpentane） and （ 3-methylhexane ＋ 2,4-dimethylpentane） in the total light hydrocarbons （ C4-C7 ） show a similar variation trend. The variation of K1 values of crude oils can suggest a typical sedimentary environment for source rocks. Therefore, according to the compositional characteristics of C7 light hydrocarbons in light of the steady-state catalytic hypothesis, nonmarine oils from the Yingmaili area and marine oils from the Tazhong uplift in the Tarim Basin were genetically classified.     

Geochemical characteristics and genetic types of crude oils from the Tertiary system in the southern part of western Qaidam Basin, Northwest China

Crude oil samples taken from the southern part of western Qaidam Basin were analyzed with GC/MS in order to understand the geochemical characteristics of crude oils. The results reveal that most crude oils are characterized by high abundance of gammacerane and C35 homohopane, which are the representative characteristics of saline lacustrine crude oils. Based on the variation of the ratios of gammacerane/C30 hopane （G/H） and C35 homoho-pane/C34 homohopane （C35/C34H）, two crude oil groups, A and B, are identified. Group-A crude oils mainly occurr in the north of the study area, with higher ratios of G/H （〉0.8） and C35/C34H （〉1.2）, whereas group-B crude oils, selected from the south of the study area, show lower ratios of G/H （〈0.8） and C35/C34H （〈1.0）. In addition, group-A crude oils are distinguished into three subgroups in accordance with their different ratios of G/H and C35/C34H and different distribution characteristics of n-alkanes, isoprenoids and steranes. These may be helpful for understanding the distribution characteristics of crude oils in the southern part of western Qaidam Basin and providing clues to the forthcoming exploration of crude oils and gas.