GEOCHEMICAL CORRELATION OF CRUDE OILS IN THE NW NIGER DELTA, NIGERIA

Sixty-six oils from eleven offshore and onshore fields in the NW Niger Delta, Nigeria were analyzed geochemically for their biomarker and isotopic compositions. Multivariate statistical analysis was employed to distinguish generic oil families from the large, complex data set. Biomarker and isotopic source parameter distributions were used to group the oils into three generic families. Family A oils, located in the onshore swamp to transitional area, received charges from predominantly Late Cretaceous or younger marine source rocks laid down in a sub-oxic to oxic depositional environment. Family B oils occur in the near-offshore area and are derived from Tertiary source facies that received an input of mixed terrigenous and marine organic matter. Family C oils, which dominate the offshore area, were derived from Tertiary source rocks typical of those deposited in oxic, nearshore or deltaic settings receiving significant terrestrial organic matter. Biomarker maturity parameters showed that the onshore (swamp) oils were generated at the peak of the oil generating window, while the transitional to offshore oils were expelled at an earlier stage of oil generation.     

GEOCHEMICAL CHARACTERIZATION OF CENTRAL NIGER DELTA OILS

Oil samples from six oilfields in the Central Niger Delta were analyzed for normal alkanes, aliphatic isoprenoid hydrocarbons, aromatic hydrocarbons and biomarkers using gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS). The study was carried out with the aim of assessing the origin and thermal maturity of the oils and the depositional environment of the organic matter from which they were derived, and to evaluate similarities (or differences) among the oils from the Central Niger Delta in comparison to those from other parts of the Delta. The ratios of n-alkanes and aliphatic isoprenoids showed that Pr/nC₁₇ ranged from 0.56 to 0.86 while Ph/nC₁₈ ranged from 0.22 to 0.31 indicating that the oils were derived from organic matter of both terrestrial and marine origin. Pristane/phytane ratios ranged from 2.67 to 3.50, suggesting that the organic matter was deposited in generally oxidizing environments. The oleanane index, which ranged from 0.07 to 1.39, and other biomarker data also indicate contributions from both marine and terrestrial organic matter. Both saturated and aromatic hydrocarbon biomarker data indicate that the oils are thermally mature. The study showed that the Central Niger Delta oils have organic geochemical characteristics similar to those of oils from other parts of the Delta. However, there is a striking difference in the distribution of aromatic hydrocarbons, and this may in future be used as a tool with which oils from different parts of the Delta can be differentiated.     

THE AKAM OILFIELD, NIGER DELTA: GEOCHEMICAL STUDIES OF TERTIARY SHALES

Major, trace and rare-earth elements show significant variations in concentration with depth. Akam Shales are characterised by relatively high contents of alumina, volaties and iron, and low contents of magnesia, lime, soda and potash. The major-element chemistry is significantly different from that of the Akiri-1 onshore oilwell, Niger Delta.
Based on selected major-and trace-element concentrations, the Akam Shales have been discriminated into marine and non-marine groups, and are found to be associated with calcareous sediments. This observation correlates with the Type II source rock, which is oil-and gas-proven in the Niger Delta.
Elemental abundances could be explained on the basis of the terrigenous and authigenic constituents of the samples. Changes in elemental abundances and ratios were observed at a depth of about, 1,733 m; this could be correlated with a change in the source area, a facies change, or a change in depositional setting.     

川西地区二叠系烃源岩发育环境及控制因素

通过对川西地区什邡剖面二叠系进行系统的采样研究,发现不同沉积相对应不同的微量元素地球化学特征。P、Ba、Cr、Ni、Cu、Zn等生物发育的营养型元素富集,反映出具高的生物生产率;V/(V+Ni)值、V/Cr值、Sr/Ba值可以很好地指示有机质保存环境。二叠系烃源岩发育层段有机质丰度、沉积相与海侵―海退旋回的关系、有机质保存环境及生物繁茂程度等反映的地球化学特征相互吻合。微量元素地球化学特征对判别沉积相有重要的指示意义。     

Occurrence and distribution of dibenzofurans and benzo[b]naphthofurans in the crude oils from the Northern and offshore Niger Delta basin,Nigeria

The occurrence and distributions of dibenzofurans (DBFs) and benzo[b]naphthofurans were investigated in crude oils from Niger Delta, Nigeria, by gas chromatography-mass spectrometry-mass spectrometry. The distribution of DBFs was characterized by the predominance of C₂-dibenzofurans. 4-Methyldibenzofuran was the most abundant among the methyldibenzofurans isomers while dimethyldibenzofuran-2 (DMDBF-2), ethyldibenzofuran-1, DMDBF-3, and DMDBF-6 occurred in higher amounts when compared with other DMDBFs. Among the benzonaphthofurans, the abundance of benzo[b]naphtho[2,1-d]furan was higher than other isomers. The DBFs distributions in the oils were not affected by source facies and depositional environments. However, the DBFs concentrations increased with increasing maturity in oils from ADL and MJO oilfields.     

Tricyclic terpane geochemistry of source rocks from Northwestern and Central Niger Delta

This distribution of tricyclic terpanes in source rocks from the northwestern and central Niger Delta was used to evaluate their origin, depositional environment and thermal maturity. The rock samples were extracted using Soxhlet extraction method and the saturated hydrocarbon fraction was analysed for biomarkers using gas chromatography-mass spectrometry (GC-MS). The values of soluble organic matter (SOM) and total hydrocarbons for wells AW and OP source rock samples exceeded the minimum 500 ppm and 300 ppm, respectively with values indicating very good to excellent potential source rocks. The tricyclic terpane source parameters and the complementary distribution of hopanes, regular C₂₇-C₂₉ steranes, n-alkanes and acyclic isoprenoid hydrocarbons showed that samples from well AW consist of mixed marine/terrestrial organic matter while those from well OP consist of organic matter largely from terrestrial origin. The values of Pr/Ph ratio for source rock samples from wells AW and OP indicate deposition of the organic matter under suboxic conditions. Sterane and hopane biomarker maturity parameters indicated that the source rock samples from wells AW and OP are at onset of oil generation and main oil window, Most of the source rock samples from well AW are more thermally mature than those from well OP. The results of tricyclic terpane maturity parameters indicated low thermal maturity for the rock samples from both wells with samples from well AW more thermally mature than those from well OP.     

API gravities,vanadium, nickel,sulfur, and their relation to gross composition: Implications for the origin and maturation of crude oils in Western Desert,Egypt

In the present study, the geochemical analyses of API gravities, vanadium, nickel, sulfur, and bulk composition were performed on eight samples from productive wells in Gindi, South Deep Abu-gharadig, Abu-gharadig, Dahab-Merier, and Faghure basins locates in the North Western Desert. The results were used to describe the source organic matter input, depositional environment, assess the degree of thermal maturity, and to correlate between crude oils to determine the genetic relationship between hydrocarbon generation and their source rock for the studied oil samples. The results showed that a wide range of crude oil parameters exists in this data, indicating that a variety of oil types is represented. Crude oils range from unaltered to altered by vanadium, nickel, and sulfur concentrations, V/Ni and saturate fraction were used to classify the oils. Oils are classified into two groups. Group I contains oil samples from Gindi, South deep Abu-gharadig and Abu-gharadig, and Dahab-Merier that are generated from organic matter input deposited in marine environment under anoxic to suboxic conditions. Group II from Faghur basin possess high Pr/Ph ratios suggesting high contribution of terreginous organic matter deposited under relatively oxic conditions.     

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.     

GEOCHEMICAL CHARACTERIZATION OF LOWER TOARCIAN SOURCE ROCKS FROM NW GERMANY: INTERPRETATION OF AROMATIC AND SATURATED HYDROCARBONS IN RELATION TO DEPOSITIONAL ENVIRONMENT AND MATURATION EFFECTS

The characterization of crude oils in terms of source rock facies and depositional environment, as well as their maturity and alteration stage, is a crucial element in exploration studies. The present contribution has implications for oil‐oil and oil‐source rock correlations. In the past, numerous parameters have been used for this purpose most of which are based on the analysis of saturated and aromatic hydrocarbons (including sulphur aromatics) and also on stable isotope signatures and elemental compositions. Recently, molecular indicators based on dibenzothiophene (DBT), phenanthrene (PHE) and their methyl derivatives methyldibenzothiophene (MDBT) and methylphenanthrene, as well as pristane (PRI) and phytane (PHY), have also been proposed (Hughes et al., 1995). These studies have attempted to infer a crude oil's source rock facies and lithology, and to classify the source rock's depositional environment. In the present study, the above compounds have been quantified by solvent extraction, liquid chromatography and capillary gas chromatography in 98 core samples of the Lower Toarcian Posidonia Shale Formation, a source rock in NW Germany. Most samples, cored between depths of 7m and 70 m, came from the Hils Half‐Graben in the Lower Saxony Basin. With a few exceptions from one borehole, the samples were unweathered marls or calcareous shales. The rocks contained mainly marine organic matter (Type II kerogen), the thermal maturity of which ranged from early mature to postmature (corresponding to 0.48‐1.44% mean vitrinite reflectance), therefore encompassing the range over which effective petroleum generation had occurred. We found that the influences of organic matter type and maturity on the molecular distributions of the above compounds were not obvious when interpreted in terms of a DBT/PHE vs PRI/PHY diagram. However, Principal Component Analysis (PCA) of our data‐set showed that alkylphenanthrene concentrations are strongly controlled by maturity, while the concentrations of PRI, PHY, and 1‐MDBT display a distinct source effect.     

咔唑、酚类和二苯并噻吩类化合物在油气运移研究中的应用

原油和源岩中咔唑、酚类和二苯并噻吩类化合物为油气运移研究提供了理想的工具.烷基咔唑浓度、屏蔽和半屏蔽异构体-暴露异构体比值和苯并咔唑比值是很好的运移指标,但源岩有机质输入、沉积环境、热成熟度、初次和二次运移及生物降解作用都可能对这些化合物分布产生影响.不同沉积环境的原油中咔唑类化合物分布差异很大,咔唑类化合物运移参数在成熟度较低时随成熟度增加而增加,在成熟度较高时随成熟度增加而降低,限制了其作为油气运移指标的适用性.烷基酚浓度可以指示油气运移方向,但同样受相和沉积环境、成熟度和生物降解等多种因素影响.同源石油的烷基二苯并噻吩类参数(如4-/1-MDBT,2,4-/1,4-DMDBT和4,6-/1,4-DMDBT)可应用于不同成熟度的原油、生物降解原油和同源石油两期充注的混合油的油气运移示踪,是很好的油气运移评价工具.断陷盆地生成的油气成熟度差异大,相和沉积环境变化快,因此利用分子地球化学指标进行油气运移示踪时,需要综合考虑各种影响因素.     

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.     

准噶尔盆地东部三台—北三台地区原油的亲缘性及生油层热演化史

通过原油常规物性和地化性质分析与对比,认为三台、北三台地区原油物性的分布受控于该区构造运动;三台和北三台南斜坡的重质油是由储层内发生次生氧化、生物降解等蚀变作用所致。由生源物和生源环境探讨得知:三台、北三台地区原油具有相似的生源环境,生源物具有湖相混合型有机质的特征,但北三台北部 (北16井区)原油生源物中含有相对较多的低等生物;北三台西南斜坡的北27井侏罗系原油具有弱氧化—还原的生源环境,生源物为湖沼相的腐殖型有机质。生油层的热演化史证实,山前凹陷二叠系和阜康凹陷二叠系、侏罗系可能是三台—北三台原油的主要油源区。     

柴达木盆地北缘微量元素含量及油气地质意义

对柴北缘微量元素测试结果表明,Sc、Ti、Cr、Ni、Zn、Cs、La、Tb、Ho、Lu 等元素在半深湖相中的含量明显高于其他沉积环境中的;微量元素同常量元素具有较强的相关性,Cr、Co、Cu、Ni、Cs、La 以及 La 系元素与 Ti、Fe 呈正相关,而与 K、Si、Al 呈负相关;鱼 25 井 Ni/Co 比值平均为 4.22,δCe 值平均为 3.9,均显示该地区中侏罗世为还原环境。微量元素垂向上的变化反映了鱼 25 井中侏罗世经历了 2 个完整的基准面上升-下降的旋回。经测试样品与标样的微量元素对比认为,柴北缘地区的砂岩为基性-中性物源。微量元素对沉积环境分析、找寻烃源岩和油气勘探具有重要意义。     

Upper Cretaceous Duwi Formation shale’s oil potentiality Safaga-Quseir,Red Sea,Egypt

Duwi Formation shale deposits at Safaga – Quseir area were assessed by rock eval pyrolysis for its shale oil potentiality. The studied shale’s organic matter are mainly of type I, type II, mixed type I/II in addition to some of type III. The studied shale is described as very good source rock with total organic carbon (TOC?>?2?wt%). The kerogen maturation in the studied samples is immature according to PI and Tmax. Extracted shale oil from the studied samples as bitumen was fractionated by liquid chromatography (LG) into saturated hydrocarbon, aromatic hydrocarbon, resin and asphaltene. The saturated hydrocarbon was described by gas chromatography (GC), the pristane/phytane, isoprenoids/n-alkane and CPI ratios show that, samples were deposited in marine environment with some input of terrestrial environment and are characterized by immature level.The X-Ray Diffraction (XRD) analysis for the studied shale samples show their bulk minerals are mainly quartz, calcite, dolomite, gypsum, hematite and pyrite in addition to apatite and fluorapatite. While their clay minerals are mainly smectite and kaolinite. The chemical composition of the studied samples showed high concentration of trace elements such as Co, Cr, Cu, Ni, V and Zn which in turn through more lights on the depositional environment of the organic matter.     

烃源岩的非均质性研究——以东营凹陷牛38井为例

通过对东营凹陷沙河街组烃源岩的研究,发现厚度大、分布广的沙三段主力烃源岩在有机质和岩性等方面存在多种尺度的非均质性.用牛38井资料证实,烃源岩受沉积环境、有机质保存条件、生烃和排烃作用的影响,可以形成岩性和有机质的不均匀分布状况.烃源岩非均质性出现在同一岩层、相带内,也可出现在毫米级的纹层间,甚至出现在分子组成(生物标志物)上.从宏观到微观系统地分析了烃源岩的非均质性,并从古生产力、缺氧程度、沉积速率等方面讨论了它的控制因素,提出优质烃源岩的形成环境为高生产力和缺氧条件的叠合区.     

渤海湾盆地惠民凹陷临南洼陷沙河街组原油地球化学特征及油源对比

惠民凹陷是渤海湾盆地济阳坳陷中的一个富油气单元。系统研究惠民凹陷临南洼陷20个原油样品和4个烃源岩样品地球化学特征,分析其成熟度、沉积环境和有机质来源,利用烃源岩和原油生物标志化合物参数建立油-岩关系,筛选有效参数运用交会图和聚类统计划分原油类型和进行油源对比。研究区原油已进入成熟阶段,生油母质的沉积环境为弱氧化-弱还原的湖沼相-淡水湖相,具有陆相湖盆混合型母质特征。总体上,研究区原油有较高的饱和烃含量和较低的胶质及沥青质含量,正构烷烃为"平台式"双峰形态分布,伽马蜡烷含量较低,4-甲基甾烷含量较高。研究区内不同地区原油的地球化学特征存在一定差别,可按地区分为南带和北带。相比南带的原油,北带原油样品中4-甲基甾烷/C₂₉R、Ts/Tm比值较低,而C₂₉降藿烷/C₂₉降新藿烷、1,2,7-/1,2,6-三甲基萘、菲/∑甲基菲较高。研究区沙四段烃源岩有机质类型为Ⅲ型,有机质丰度较低;沙三段烃源岩有机质以Ⅱ型为主,有机质丰度较高,其中,沙三下亚段和中亚段是临南洼陷的主力烃源岩。通过系统对比研究区原油和烃源岩的地球化学特征,认为临南洼陷沙三、沙四段原油来自沙三下、中亚段烃源岩。      

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.     

Oil-source Rock Correlation and Distributions of Pyrrolic Nitrogen Compounds in Oils From the Niger Delta,Nigeria

The occurrence and distributions of carbazole compounds were investigated in crude oils from Niger Delta, Nigeria, by gas chromatography-mass spectrometry-mass spectrometry. Geochemical characterization of the oils and the available source rocks based on saturate and aromatic biomarkers showed that the oils have higher thermal maturity than the source rocks. All the oils are isotopically related and might have originated from the same source rocks. However, subtle maturity differences exist among the oils from the same field. C₂- carbazoles are the dominant components of the crude oils. 4-methylcarbazole is the most abundant of the methylcarbazoles isomers while 1,5-dimethylcarbazole occurs as the most abundant among the dimethylcarbazoles. The abundance of benzocarbazoles are low compared to the alkylcarbazoles. Cross plots of the methylcarbazoles and dimethylcarbazoles against Pr/Ph ratio showed that carbazoles distributions in the oils are not affected by source facies. However, the cross plots of C₁- carbazoles and C₂- carbazoles against the calculated vitrinite reflectance indicated that the carbazoles concentrations increase with increasing maturity.     

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.     

Oil Families and Their Potential Sources in the Southern Gulf of Suez, Egypt

Four oil families are identified in the southern Gulf of Suez, through high-resolution geochemical studies including gas chromatography, gas chromatography-mass spectrometry, and carbon isotope analyses. Biological features characterize oils in family 1a, suggesting tertiary carbonate source rocks for these oils, rich in type II organic matter and deposited under anoxic depositional environment. Family 1b oil shows minor variations in the source of organic matter and the depositional environment, as it was derived from carbonate source rock with more algal and bacterial contribution and minor input of terrestrial organic sources, deposited under less saline condition compared to family 1a oil. Family 2 oil, although genetically related to family 1a oil, has some distinctive features, such as diasterane to sterane and pristane to phytane ratios, which suggest clay-rich source rocks and a more oxic depositional environment. Also, the lack of oleanane indicates pre-tertiary source rocks for this oil. In contrast, family 3 oil is of mixed sources (marine and non-marine), generated from low sulfur and clay-rich source rock of tertiary and/or younger age. Family 4 oil seems to be mixed from family 1b and family 3 oils, sourced mainly from carbonate source rocks rich in clay minerals with algal and bacterial contributions. Family 4 oil is highly mature, family 1b oil lies within equilibrium values (peak oil generation stage), while the other families are more or less near equilibrium.