Characterization and correlation of crude oils from some wells in the North Western Desert,Egypt

Characterization and correlation of crude oils from some wells in the North Western Desert, based on six crude oil samples, were studied by different analytical techniques, including API gravity, sulfur content, nickel and vanadium, bulk compositions and saturated fraction obtained from gas chromatography have been studied. The results show that the crude oils are normal to medium aromatic oils, with high API gravity and high sulfur content. V, Ni, V/Ni and V/(V + Ni) reflecting oils might be sourced from non-clastic source rocks, possibly carbonates, deposited under anoxic-suboxic conditions. Bulk compositions revealing that the crude oils were derived from marine organic sources. While, the paraffins and naphthenes percent indicates that the oils belong to paraffinic to naphthenic oil types, deposited in slightly anoxic to suboxic conditions and contained marine organic matter. Thermal maturity data showed that the oil samples were generated from mature source rocks. This indicates the studied oil samples are well correlated with each other, where they are similar in their oil type maturation and source depositional environments.     

BIOMARKER GEOCHEMISTRY OF CRUDE OILS FROM THE QAIDAM BASIN, NW CHINA

A suite of 16 crude oil samples from 13 oilfields in the Qaidam Basin were analyzed using techniques including gas chromatography and gas chromatography - mass spectrometry. Biomarker compositions and parameters were used to investigate the palaeoenvironmental and depositional conditions and to correlate the oils with eachother. Oils from the western Qaidam Basin have pristane/phytane (Pr/Ph) ratios of less than 0.7, and contain abundant gammacerane, C₂₇ steranes, 4-methyl steranes and long-chain tricyclic terpanes. C₂₉ sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios show that the western Qaidam oils have variable maturities ranging from immature to mature. Oils from the northern Qaidam Basin, by contrast, have Pr/Ph ratios greater than 3, low gammacerane contents, and relatively abundant C₂₉ steranes, bicyclic terpanes and alkylcyclohexanes. C₂₉ sterane 20S/(20S+20R) and ββ/(ββ+αα) ratios indicate that the northern Qaidam oils are mature.
δ¹³C values, which range from -25.4‰ to -28.3‰ with the exception of one oil from the north (-3l.6‰), are similar for oils from both the northern and western parts of the Qaidam Basin. The oils'carbon isotope compositions are similar to those of the organic matter in potential source rocks.
The western Qaidam oils are inferred to have originated from Tertiary source rocks deposited under anoxic and saline-hypersaline lacustrine conditions with dominant algal organic matter. The northern Qaidam oils are interpreted to be derived from Jurassic source rocks which were deposited in a freshwater lacustrine environment and which are dominated by terrigenous organic matter.     

GEOCHEMICAL CHARACTERISTICS OF CRUDE OILS FROM THE SHARARA-C OIL FIELD,MURZUQ BASIN,SOUTHWESTERN LIBYA

Crude oil samples from the Sharara-C oil field (Concession NC-115, Murzuq Basin, SW Libya) were analysed by organic geochemical methods in order to infer the geochemical characteristics of their respective source rocks. Aromatic hydrocarbons were analysed by gas chromatography – mass spectrometry (GC-MS), and gas chromatography – tandem mass spectrometry (GC-MS-MS) was used to analyse saturated biomarkers. The Sharara-C oils are interpreted to have been generated by marine shales containing mixed terrigenous and marine organic materials deposited in an intermediate (suboxic) environment. Age-specific biomarker ratios indicated that the oils are older than Cretaceous, and maturation-related parameters pointed to their high thermal maturity. Consistent with previous studies, source rocks are inferred to be “hot” shales in the Lower Silurian Tanezzuft Formation. Almost all the parameter ratios calculated varied over a very narrow range, indicating that the investigated oils were compositionally similar. The only significant difference that was noted concerned the sterane/hopane ratios whose variation suggested that there was some variability in the composition of the source organic material. The organic geochemical parameters determined for the Sharara-C crude oils were compared with published data on other crude oils from Concession NC-115. Almost all the parameters agreed well with previously published data on oils from this part of the Murzuq Basin. The greatest deviation concerned the values of some of the maturity parameters. This tended to confirm the conclusions of previous studies concerning the presence of a number of distinct oil families and sub-families in the Sharara oil field area which are genetically related but which have different maturities.     

BIOMARKER GEOCHEMISTRY OF PALEOGENE CRUDE OILS AND SOURCE ROCKS FROM THE RAOYANG SAG,BOHAI BAY BASIN,NE CHINA: AN OIL – SOURCE ROCK CORRELATION STUDY

This study presents a systematic geochemical analysis of Paleogene crude oils and source rocks from the Raoyang Sag in the Jizhong sub-basin of the Bohai Bay Basin (NE China). The geochemical characteristics of fifty-three oil samples from wells in four sub-sags were analysed using gas chromatography (GC) and gas chromatography – mass spectrometry (GC-MS). Twenty core samples of mudstones from Members 1 and 3 of the Eocene-Oligocene Shahejie Formation were investigated for total organic carbon (TOC) content and by Rock-Eval pyrolysis and GC-MS to study their geochemistry and hydrocarbon generation potential. The oils were tentatively correlated to the source rocks. The results show that three groups of crude oils can be identified. Group I oils are characterized by high values of the gammacerane index and low values of the ratios of Pr/Ph, Ts/Tm, 20S/(20S+20R) C₂₉ steranes, ββ/(ββ+αα) C₂₉ steranes, C₂₇ diasteranes/ C₂₇ regular steranes and C₂₇/C₂₉ steranes. These oils have the lowest maturity and are interpreted to have originated from a source rock containing mixed organic matter deposited in an anoxic saline lacustrine environment. The biomarker parameter values of Group III oils are the opposite to those in Group I, and are interpreted to indicate a highly mature, terrigenous organic matter input into source rocks which were deposited in suboxic to anoxic freshwater lacustrine conditions. The parameter values of Group II oils are between those of the oils in Groups I and III, and are interpreted to indicate that the oils were generated from mixed organic matter in source rocks deposited in an anoxic brackish–saline or saline lacustrine environment. The results of the source rock analyses show that samples from Member 1 of the Shahejie Formation were deposited in an anoxic, brackish – saline or saline lacustrine environment with mixed organic matter input and are of low maturity. Source rocks in Member 3 of the Shahejie Formation were deposited in a suboxic to anoxic, brackish – saline or freshwater lacustrine environment with a terrigenous organic matter input and are of higher maturity. Correlation between rock samples and crude oils indicates that Group I oils were probably derived from Member 1 source rocks, while Group III oils were more likely generated by Member 3 source rocks. The Group II oils with transitional characteristics are likely to have a mixed source from both sets of source rocks.     

APPLICATION OF BIOMARKERS AND STABLE CARBON ISOTOPES TO ASSESS THE DEPOSITIONAL ENVIRONMENT OF SOURCE ROCKS AND THE MATURATION OF CRUDE OILS,EAST ZEIT FIELD,SOUTHERN GULF OF SUEZ,EGYPT

Four crude oil samples were collected from the producing wells EZ A-11, EZ A-1, EZ A-14 and EZ A-7 of East Zeit Field, offshore southern Gulf of Suez, Egypt. These crude oil samples represent the producing Kareem, Rudeis, Nukhul and Nubia reservoirs respectively. The crude oils were subjected to a variety of organic geochemical analyses including Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS). In addition to stable carbon isotopes were done for the saturate and aromatic fractions and trace elements analysis. The organic geochemical results suggest the presence of two different types of oils that were originated from two different source environments. The Lower Miocene Kareem, Rudeis and Nukhul oils have a V/Ni ratios of 2, Ts/Tm ratio less than 1, Oleanane index of more than 20% and a gammacerane index of around 10%. Such results suggest that the Miocene crude oils were generated from an angiosperm-rich, Tertiary source rocks with high terrestrial input. The Nubian crude oil has a V/Ni ratio equal 3.2, Ts/Tm ratio less than 1, Oleanane index less than 20% and high gammacerane index of more than 30% suggesting a marine saline-source depositional environment of Late Cretceous or younger rich in type-II kerogen with minor terrestrial influence. The maturation parameters of the Miocene and Nubian crude oils obtained from aromatic and fraction imply marginally mature to mature oils. The Nubian crude oil however, was generated at a relatively higher maturation level than that of the Miocene oils. Also, the maturation level of the Miocene oils is in accordance with their relative stratigraphic position with the Nukhul reservoired oil being the most mature.     

APPLICATION OF BIOMARKERS AND STABLE CARBON ISOTOPES TO ASSESS THE DEPOSITIONAL ENVIRONMENT OF SOURCE ROCKS AND THE MATURATION OF CRUDE OILS, EAST ZEIT FIELD, SOUTHERN GULF OF SUEZ, EGYPT

Four crude oil samples were collected from the producing wells EZ A-11, EZ A-1, EZ A-14 and EZ A-7 of East Zeit Field, offshore southern Gulf of Suez, Egypt. These crude oil samples represent the producing Kareem, Rudeis, Nukhul and Nubia reservoirs respectively. The crude oils were subjected to a variety of organic geochemical analyses including Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS). In addition to stable carbon isotopes were done for the saturate and aromatic fractions and trace elements analysis. The organic geochemical results suggest the presence of two different types of oils that were originated from two different source environments. The Lower Miocene Kareem, Rudeis and Nukhul oils have a V/Ni ratios of 2, Ts/Tm ratio less than 1, Oleanane index of more than 20% and a gammacerane index of around 10%. Such results suggest that the Miocene crude oils were generated from an angiosperm-rich, Tertiary source rocks with high terrestrial input. The Nubian crude oil has a V/Ni ratio equal 3.2, Ts/Tm ratio less than 1, Oleanane index less than 20% and high gammacerane index of more than 30% suggesting a marine saline-source depositional environment of Late Cretceous or younger rich in type-II kerogen with minor terrestrial influence. The maturation parameters of the Miocene and Nubian crude oils obtained from aromatic and fraction imply marginally mature to mature oils. The Nubian crude oil however, was generated at a relatively higher maturation level than that of the Miocene oils. Also, the maturation level of the Miocene oils is in accordance with their relative stratigraphic position with the Nukhul reservoired oil being the most mature.     

BULK COMPOSITION OF SOME SAUDI ARABIAN CRUDE OILS IN RELATION TO MATURATION,ORIGIN AND DEPOSITIONAL ENVIRONMENTS

ABSTRACT

Ten crude oil samples, covering wide range of maturity (API = 18.5?36.1), were assembled from Safaniya, Abqiq, Ain-Dar, Wafra, Marjan, and Zulf oil fields in the area of Arabian Gulf. Different analytical techniques, including liquid chromatographic separation, gas chromatography, and nuclear magnetic resonance spectroscopy, were used to characterize the oil samples. Some bulk and specific compositional parameters were used for oil-oil correlations in order to evaluate thermal maturation, origin, biodegradation and depositional environments of the crude oils. Considering the maturity relationships, it was suggested that the bulk composition of Wafra/Iucene crude oil was altered due to biodegradation and vertical migration. Correlation between API gravity and PAP of aromatic fraction indicated two maturity levels of onshore and offshore oil fields. The distribution of n-alkanes reflected the marine origin of the crude oils. Isoprenoids/n-alkanes and pristane/phytane ratios were determined by capillary GC of a middle distillate of the crude oil samples. The correlations showed that the oil samples, except for those collected from Wafra oil field, are normal mature crude oils which were deposited under oxidizing environments. The much lower Pr/Ph ratio of Wafra/Iucene oil field could be considered as an indicator of reducing depositional environments.     

BULK COMPOSITION OF SOME SAUDI ARABIAN CRUDE OILS IN RELATION TO MATURATION, ORIGIN AND DEPOSITIONAL ENVIRONMENTS

Ten crude oil samples, covering wide range of maturity (API = 18.5-36.1), were assembled from Safaniya, Abqiq, Ain-Dar, Wafra, Marjan, and Zulf oil fields in the area of Arabian Gulf. Different analytical techniques, including liquid chromatographic separation, gas chromatography, and nuclear magnetic resonance spectroscopy, were used to characterize the oil samples. Some bulk and specific compositional parameters were used for oil-oil correlations in order to evaluate thermal maturation, origin, biodegradation and depositional environments of the crude oils. Considering the maturity relationships, it was suggested that the bulk composition of Wafra/Iucene crude oil was altered due to biodegradation and vertical migration. Correlation between API gravity and PAP of aromatic fraction indicated two maturity levels of onshore and offshore oil fields. The distribution of n-alkanes reflected the marine origin of the crude oils. Isoprenoids/n-alkanes and pristane/phytane ratios were determined by capillary GC of a middle distillate of the crude oil samples. The correlations showed that the oil samples, except for those collected from Wafra oil field, are normal mature crude oils which were deposited under oxidizing environments. The much lower Pr/Ph ratio of Wafra/Iucene oil field could be considered as an indicator of reducing depositional environments.     

Bulk Compositions,Genetic Origin,Classifications, and Maturation of Crude Oils in the Gulf of Suez,Egypt

Abstract

API gravity, sulfur content, gas chromatography, and gas chromatography-mass spectrometry analyses were carried out for eight oil samples collected from different wells in the Gulf of Suez. The results showed that two types of oils could be recognized: (a) heavy oils, which are oils from Zafarana, Rahmi, West Bakr, and Ras Gharib wells, are of low maturation and originated mainly from terrestrial organic sources; and (b) light oils, which are oils from Um El Yuser, Ras El Ush, Gemsa-SE, and Hurghada wells, have a high level of maturation and orginate mainly from marine organic sources.     

鄂尔多斯盆地杏子川油田延长组原油地球化学特征与油源探讨

陕北斜坡中部杏子川油田延长组原油物性、族组成、饱和烃气相色谱、萜甾烷生物标志化合物以及地质综合分析显示,不同层位延长组原油地球化学特征具有显著的差异。长4+5、长6原油性质基本一致,具有同源属性,其差异性可能主要与运移分馏效应以及原油成熟度有关。长9、长10原油与长4+5、长6原油性质具有明显的差异,反映了其具有不同的母源属性。Ts/Tm、C₂₉ββ/（ββ+αα）、αααC₂₉20S/（20S+20R）等反映原油成熟度的生物标志化合物显示长9、长10原油具有较高的成熟度。长9和长10原油规则甾烷含量显示具有不同的成因。地质综合分析和油源对比表明:长4+5、长6原油均来自于长7生油岩,长9原油来自于其顶部的李家畔页岩,长10原油可能主要来源于长92有效烃源岩。      

Organic and Carbon Isotope Geochemistry of Crude Oils from Ashrafi Field,Southern Gulf of Suez Province,Egypt: Implications for the Processes of Hydrocarbon Generation and Maturation

Abstract

Four crude oil samples representing the Miocene formations of Belayim, Kareem, and Nukhul, and the Pre-Miocene Nubia Sandstone of Ashrafi Field, offshore southern Gulf of Suez, Egypt. These crude oil samples were analyzed using a variety of organic geochemical techniques including both C₇ and whole oil Gas Chromatography (GC), Gas Chromatography–Mass Spectrometry (GC–MS) in addition to stable carbon isotopes to the saturate and aromatic fractions. The organic geochemical results revealed that the Miocene and Pre-Miocene Nubia Sandstone oils generated from two different source rocks subjected to different maturation levels. The Miocene crude oil possesses geochemical properties rich in tricyclic terpanes and extended hopanes typical characteristics of oils generated from marine siliciclastic source rocks with angiosperm land plants input like Rudeis Formation as indicated from the oleanane index which exceeds 30% and low gammacerane index around 10%. Meanwhile the Pre-Miocene crude oil correlates with the marine carbonate sources of high salinity waters like Brown Limestone of Upper Cretaceous age as indicated from the oleanane index <10% and high gammacerane index >30%. The maturity parameters based on both ratios of 20S/(20S + 20R)-C₂₉ ααα cholestane and TAS/(MAS + TAS) were found to be around 0.5 for the Miocene crude oils, meanwhile both the ratios for the Pre-Miocene crude oil were >0.5. The concluded results further support the very high maturation level of the Pre-Miocene crude oil than the Miocene crude of Ashrafi Field which concordant with the reservoir stratigraphic depths.     

巴楚—麦盖提地区主要油气藏原油地球化学特征及油源探讨

巴楚—麦盖提地区是重要的油气聚集区。玉北地区玉北1井在奥陶系鹰山组测试获25.2 m3/d的海相工业油气流。玉北奥陶系油藏与巴什托（普）油气田的原油是否同源是困扰该地区油气勘探并急需解决的问题之一。通过玉北1井、巴什托（普）主要油气藏（田）的原油地球化学特征研究、油—油对比、油源对比等综合分析,并类比塔河奥陶系原油进行了巴楚—麦盖提地区主要油气藏原油地球化学特征及油源探讨分析。综合研究认为玉北地区奥陶系的原油地球化学特征与塔河奥陶系原油相近。通过烃源有机相类比,认为玉北地区奥陶系原油可能来自塔西南地区中下寒武统斜坡相烃源岩;巴什托轻质油推测主要来自盆地—陆棚相寒武系烃源岩,部分来源于海陆交互相石炭系。      

Distribution of triterpanes and steranes biomarkers as indication of organic matters input and depositional environments of crude oils of oilfields in Gulf of Suez,Egypt

Eight crude oils collected from different oilfields distributed within the northern, central and southern Gulf of Suez basin to detect the distribution of triterpanes and steranes biomarkers as indication of organic matter input and depositional environments of crude oils and lithology of organic matters. This achieved throughout the application of gas chromatography mass spectrometry analysis. The results revealed that the Gulf of Suez samples are believed to be of marine organic matter input deposited under anoxic depositional environment. Off shore samples S3 and S4 from Central Province of Gulf of Suez basin show low maturity levels, while the other samples which were of higher maturity levels.     

Biomarkers and isotopic characteristics of crude oils from a central Niger delta depobelt field,Nigeria

Twelve crude oils samples from a field in the central depobelt in the Niger delta, Nigeria were analyzed for their biomarkers and isotopic composition by Gas chromatography–Mass spectrometry and Isotope mass spectrometry. The percentage C_27, C₂₈ and C₂₉ steranes in the oils ranged from 35.80 to 39.9, 28.1 to 30.8 and 29.9 to 35.0, respectively. The distribution of molecular biomarkers and isotopic composition in the oils indicated that they were formed from source rocks of a mixed source (marine and terrestrial kerogen) but with greater input from marine organic matter. The Pr/Ph ratios of the oil samples ranged from 1.2 to 2.3 and this indicated organic matter deposited under suboxic conditions. The vitrinite reflectance (%VRc) values calculated from methylphenanthrene index-1 (MPI-1) parameter ranged from 0.89 to 1.07 indicating oils generated at the peak of oil window.     

Fingerprinting of biomarker characteristics of some Egyptian crude oils in Northern Western Desert as evidence for organic matter input and maturity level assessment

This study describes the fingerprinting of crude oils from different Egyptian oil formations using gas chromatography (GC) and gas chromatography mass spectrometry (GC–MS). The samples were obtained from Gindi, Abu El gharadig, south deep Abu El gharadig, Dahab- Merier and Faghur basins from Western Desert. Diagnostic biomarkers parameters applied in this study provide evidences about the source of organic matter, the depositional environment and maturity of the studied oils. The results showed that the crude oils of Faghur basin are believed to be originated from mixed source predominately terrestrial with chief contribution of clastic rocks deposited under oxic conditions. However, the crude oils from Gindi, Abu El gharadig, South deep Abu El gharadig and Dahab- Merier basins were generated from marine carbonate source rock deposited under anoxic depositional environment.     

渤海湾盆地沧东凹陷古近系孔二段页岩层系原油地球化学特征

利用原油物性、族组分分离与定量、饱和烃色谱—质谱、芳烃色谱—质谱、稳定碳同位素等分析测试资料,系统分析了渤海湾盆地沧东凹陷古近系孔二段页岩层系原油地球化学特征。研究表明,原油属于中质、中凝稠油,流动性较差;原油中烃类含量偏低,饱芳比和非沥比均较低。不同构造单元上的原油生物标志物及稳定碳同位素分布特征相似,反映其母质来源及生烃演化过程基本一致。饱和烃色谱以正态分布的单峰型为主,轻碳优势不明显,C27、C28、C29规则甾烷呈"上升型"分布,三环萜烷含量低,反映原油有机母质来源具有湖相水生生物和陆源高等植物的双重贡献,且以陆源高等植物为主。伽马蜡烷指数、重排霍烷含量及分布、Pr/Ph、三芴系列化合物、原油稳定碳同位素分布等指标反映了原油形成于弱氧化—还原条件下的淡水—微咸水沉积环境。正构烷烃分布特征和奇偶优势比、αββ/(ααα+αββ)C29与20S/(20S+20R)C29甾烷异构化指数以及霍烷异构化指数均指示原油成熟度偏低。     

Bulk Geochemical Characteristics of Crude Oils From Wells in the North Western Desert,Egypt

Abstract

Different oil samples were collected from oil fields in the North Western Desert; namely, Meleiha, Razaak, and North Qarun fields. These oils were analyzed geochemically, including sulfur content, American Petroleum Institute (API) gravity, and gas chromatography The results showed that API gravity and sulfur content indicate that the crude oils have high mature level of marine origin. Distribution of n-alkanes (Carbon Preference Index [CPI], pr/ph, and isoprenoide/n-alkanes ratios) reflects that the oil samples originated mainly from marine organic sources deposited in a reducing environment. This indicates that the oil samples under investigation are well correlated with each other in the western part of the North Western Desert.     

黄河口凹陷BZ35/36构造高硫原油特征及运聚方向

渤海湾盆地黄河口凹陷BZ36构造沙河街组、东营组及BZ35构造东营组产出中-重质高硫原油,其物理性质及地球化学特征与黄河口凹陷其他构造所产原油有显著差别。通过姥/植比、伽马蜡烷、C₂₄Te/C₂₆TT、C₂₉20S/(20S+20R)、C₂₉ββ/(ββ+αα)等生物标志化合物的对比分析,认为上述2个构造所产高硫原油与紧邻黄河口凹陷东部的庙西凹陷高硫低熟原油具有亲缘性。庙西凹陷的高硫低熟原油由NE向SW方向充注,黄河口凹陷的低硫成熟原油则向相反方向充注,由于2个凹陷均有较强的生烃潜力,处于2个凹陷油气充注路径上的BZ36、BZ35构造聚集了来自黄河口凹陷与庙西凹陷烃源岩生成的原油。通过与同类型高硫原油的类比,初步认为该类型高硫低熟原油的母岩形成于干旱强还原及生物繁茂的盐湖。     

塔里木盆地原油碳硫同位素特征及油源对比

以碳、硫同位素在石油生成演化过程中的分馏作用为基础,对塔里木盆地原油、油源对比进行了探讨。原油的δ13C继承了生物母质的δ13C值,海相原油的δ13C小于-32‰或大于-24‰,陆相原油的δ13C为-32‰~-24‰。原油中有机硫来源于源岩中的硫酸盐,温度高于成熟原油(80~120℃)的δ34S与油源岩中硫酸盐的δ34S接近,但原油δ34S似要比相应的硫酸盐δ34S轻3‰~4‰。根据碳、硫同位素特征和原油其它物理化学特性可以将塔里木盆地原油分为4种。第1种为寒武—奥陶纪原油,其δ34S为21‰~26‰,δ13C小于-32‰,为典型的海相原油,并可细分为寒武纪原油(δ34S在24‰~26‰)和奥陶纪原油(δ34S为21‰~22‰)。第2种为石炭—二叠纪原油,其δ34S为5‰~7‰,δ13C小于-32‰,亦为海相原油,如沙3井原油,但母质类型较差。第3种为三叠—侏罗纪原油,δ34S在10‰~14‰,δ13C在-26‰左右,为典型的陆相原油。第4种则是前3种原油的混合相原油,δ34S介于海相原油与陆相原油之间,-C—O和C—P两种海相原油混合,其物理化学性质仍为海相原油特征,而-C—O与T—J原油混合则具有混合相原油的性质,没有发现C—P原油与T—J原油的混合油。      

OCCURRENCE AND DISTRIBUTION OF n-ALKANESAND n-FATTY ACIDS IN SAUDI ARABIAN CRUDE OILS

Ten crude oil samples, covering wide range of maturity (API gravity = 18·5-36·1), were assembled from Safaniya, Abqiq, Ain Dar, Wafra. Marjan and Zuluf oil fields in the area of Arabian Gulf. n-Alkanes of were separated from the petroleum distillate ((150°C-450°C) of the crude oils by urea adduction. n-Fatty acids were separated from the fraction of n-alkanes by treatment with aqueous solution of KOH. Distribution of n-alkanes and n-fatty acids has been investigated by means of gas chromatography. The studied crude oils showed symmetrical distribution curves of n-paraffins and fatty acids of low molecular weight were abundant as compared with n-paraffins. The n-paraffins distribution curve of Wafra/Iucene biodegraded immature crude oil showed three maxima at C₁₇, C₁₉, and C₃₁ whereas the maxima of n-fatty acids are located at C₁₄,C₂₂and C₂₄. The results were interpreted in terms of origin, maturation and depositional environments of the crude oils.