Geochemical characterization of Cretaceous sandstones from the Southern Benue Trough,Nigeria

Geochemical studies of sandstones from the three lithostratigraphic successions in the southern Benue Trough of Nigeria were undertaken for a geochemical characterization of the sandstones, and to assess their strati-graphic and source evolution.Major and trace elements data were obtained from outcrop sandstone samples. The SIO2/Al2O2, Fe2O3/K2O ra-tios and CaO contents have been used to characterize the Cretaceous sandstones into Al-rich and high and low Fe2O3/K2O ratio sandstones. Results indicate that there are geochemical features that display stratigraphic trends across the succession from the Asu River Group, Eze-Aku Group to the proto-Niger Delta succession which may imply a discontinuous evolution from different source terrains of Precambrian and Mesozoic ages that supplied the sediments. The Asu River Group sandstones have lower SiO2, Al2O3, Fe2O3 and higher MgO; the Eze-Aku sand-stones have higher TiO2, CaO, alkalis and lower MgO while the proto-Niger Delta sandstones have higher SiO2, Al2O3, Fe2O3 and lower alkalis and CaO. These discontinuities signify the influence of tectonic impulses that af-fected the southern Benue Trough during the Cretaceous time. Changes in ratios of TiO2/Al2O3, Fe2O3, Cr and Zr suggest an increasingly mafic contribution to the depositional basin with time. The chemical index of alteration in-creases with time, possibly suggesting that a more intense weathering regime in the hinterland developed with time.     

莺-琼盆地海相烃源岩特征及高温高压环境有机质热演化

莺歌海盆地与琼东南盆地存在渐新统海岸平原-半封闭浅海相和中新统浅海相两套海相烃源岩。其中,渐新统烃源岩发育于琼东南盆地早第三纪半地堑,存在海岸平原和半封闭浅海两类烃源岩组合。海岸平原含煤烃源岩有机质丰度高,富含陆源有机质,具有很好的生气潜力;半封闭浅海相烃源岩TOC含量总体低于1.0%,但其规模大且存在TOC1.5%的较高丰度段,故亦具有较大的生气能力。中新统海相烃源岩主要分布于莺歌海盆地裂后热沉降形成的中央坳陷,其有机质丰度横向变化比较大,位于中央坳陷带的烃源岩有机质丰度较高,TOC大多在0.40%～2.97%之间,有机质以气型干酪根为主。盆地的高地温为有机质向天然气转化提供了有利的条件,尤其是热流体活动使浅层有机质超前熟化,但地层超压对有机质热演化也有一定的抑制作用。盆地模拟结果显示,莺-琼盆地主要凹陷烃源岩大量生气时间较晚,与圈闭形成期的时空配置好,有利于成藏。     

Geochemical Evaluation of the Permian Ecca Shale in Eastern Cape Province,South Africa: Implications for Shale Gas Potential

Shale gas has been the exploration focus for future energy supply in South Africa in recent time. Specifically, the Permian black shales of the Prince Albert, Whitehill, Collingham, Ripon and Fort Brown Formations are considered to be most prospective rocks for shale gas exploration. In this study,outcrop and core samples from the Ecca Group were analyzed to assess their total organic carbon(TOC), organic matter type, thermal maturity and hydrocarbon generation potential. These rocks have TOC ranging from 0.11 to 7.35 wt%. The genetic potential values vary from 0.09 to 0.53 mg HC/g,suggesting poor hydrocarbon generative potential. Most of the samples have Hydrogen Index(HI) values of less than 50 mg HC/g TOC, thus suggesting Type-Ⅳ kerogen. Tmax values range from 318℃ to601℃, perhaps indicating immature to over-maturity of the samples. The vitrinite reflectance values range from 2.22% to 3.93%, indicating over-maturity of samples. Binary plots of HI against Oxygen Index(OI), and HI versus Tmax show that the shales are of Type II and mixed Type Ⅱ-Ⅲ kerogen.Based on the geochemical data, the potential source rocks are inferred as immature to over-matured and having present-day potential to produce gas.     

泥岩/页岩：中国元古代－古生代海相沉积盆地主要烃源岩

在中国元古宙—古生代海相沉积体系中,碳酸盐岩是最主要的沉积岩类型,长期以来研究的重点也一直是碳酸盐岩,对海相泥岩/页岩的关注比较少,并且认为碳酸盐岩是海相沉积盆地中主要的烃源岩。对中国南方上、中、下扬子地区、滇黔桂地区、塔里木盆地、鄂尔多斯盆地、华北地区等147条剖面、289口探井及浅井约11200余个样品有机碳含量的分析与统计表明,泥岩/页岩有机质丰度高,是中国元古宙—古生代海相沉积盆地中主要的烃源岩类型,而碳酸盐岩有机质丰度普遍较低,仅仅是次要的烃源岩类型。海相碳酸盐岩中有机质的含量与碳酸盐含量呈现弱的负相关性,泥质输入有利于形成高有机质丰度的碳酸盐岩烃源岩,但并不是高有机质丰度碳酸盐岩烃源岩发育的必要条件,决定碳酸盐岩烃源岩有机质丰度的主要因素是有机质的生产率、有机质的沉积与保存环境。中国元古宙—古生代海相沉积盆地中并不缺乏高有机质丰度泥岩/页岩类好烃源岩,上、中、下扬子地区主要发育于上震旦统陡山沱组、下寒武统、上奥陶统—下志留统、上二叠统;华南地区主要发育于中、下泥盆统;塔里木盆地主要发育于下寒武统、下奥陶统及中上奥陶统;华北地区为中新元古界洪水庄组、下马岭组。泥灰岩类碳酸盐岩烃源岩在塔里木盆地相对比较发育,在中国南方地区只有下二叠统相对发育。     

Fluid inclusion and stable isotope studies of Pb-Zn-fluorite-barite mineralization in the lower and middle Benue Trough,Nigeria

Lead-zinc-fluorite-barite veins in the lower and middle Benue Trough (Nigeria) are located within the Lower Cretaceous (Albian) carbonaceous shales, limestones, and arkosic sandstones of this intracontinental rift structure. The veins in the lower Benue Through consist of sphalerite + galena+marcasite ± chalcopyrite ± barite in a gangue of siderite and quartz hosted by carbonaceous shales, whereas in the middle Benue Trough, fluorite, barite, quartz, and similar sulfide minerals are hosted by limestone and sandstone. Fluid inclusion temperatures in vein minerals range from 95°C to 200°C (without pressure corrections) and salinities range from 14 to 24 equiv. wt% NaCl. Oxygen isotope compositions of limestone wall rocks (middle Benue) have been lowered from premineralization ¹⁸O values of about 25 per mil to approximately 16 per mil. Fluid in equilibrium with vein calcite has a calculated ¹⁸O of +2.6 per mil at 130°C. The ⁸⁷Sr/⁸⁶Sr ratio of this calcite (0.71497) suggests that strontium and calcium had a considerably more radiogenic source than the Cretaceous limestone or evaporite did (⁸⁷Sr/⁸⁶Sr=0.7073–0.7078). Observed strontium data, lead isotope compositions of galena, and REE patterns in fluorite suggest that the Lower Paleozoic basement rocks in the trough or their weathered equivalents are likely sources for the Benue Trough ore components. Sulfur isotope data suggest that the sulfur was probably contributed from the Cretaceous evaporites in the trough.Our data favor a basinal brine source for the ore-forming fluid. Fluid criculation probably resulted from high geothermal gradients accompanying continental rifting. Brine interaction with the clastic, carbonate, and evaporite rocks led to metal and sulfur leaching and later deposition in fractures accompanying the Cenomanian deformation and uplift in the Benue Trough.     

内蒙古拉布达林盆地烃源岩有机地球化学特征

在野外地质勘查、典型剖面实测、样品采集及实验分析基础上,对拉布达林盆地潜在的烃源岩的有机质丰度、类型、成熟度等有机地球化学特征进行了分析评价.结果表明：1）上石炭统新伊根河组有机碳含量和生烃潜力均较低,有机质类型为I型,普遍达到到高成熟-过成熟阶段,为差烃源岩;2）中侏罗统万宝组有机碳含量中等,生烃潜力偏低,有机质类型为I型和II₁型,处于低成熟阶段,为中等-差烃源岩;3）上侏罗统满克头鄂博组有机碳含量中等,生烃潜力较好,有机质类型为II₁型,处于低成熟阶段,为较好烃源岩;4）下白垩统大磨拐河组有机碳含量较高,生烃潜力较好,有机质类型为为II₁型和II₂型,处于低成熟-成熟阶段,为好烃源岩.     

辽宁秀水盆地白垩系义县组烃源岩地球化学特征 ——以辽法D1井为例

对辽宁北部秀水盆地辽法D1井白垩系义县组上部烃源岩的有机质丰度、类型与成熟度及烃源岩厚度等特征分析表明,义县组湖相泥岩在盆地内分布广泛,厚度较大,为秀水盆地的主力烃源岩.义县组烃源岩TOC在1.37%~8.61%之间,平均值为3.70%,为中等-好的烃源岩,有机质类型为Ⅱ₁-Ⅱ₂型,R_o值较高,分布范围1.08~1.20,指示热演化达到成熟-生油高峰阶段,具有较好的资源潜力.     

Maturity studies and source-rock potential in the southern Sverdrup Basin, Arctic Canada

The purpose of the study is to better understand the relationship between organic matter optical properties and the presence of potentially large oil and gas accumulations in Arctic Canada. The type and thermal maturity of the dispersed organic matter of the Mesozoic formations in the southern Sverdrup Basin, Melville Island, have been studied using organic petrology and Rock-Eval pyrolysis.All types of organic matter are present in the strata of Mesozoic age. Hydrogen-rich liptinite is dominated by alginite (Botryococcus and Tasmanites), dinoflagellate cysts and amorphous fluorescing matrix. Sporinite, cutinite, resinite and liptodetrinite made up the lesser hydrogen-rich exinite. Vitrinite reflectance in Cretaceous sediments ranges from 0.36 to 0.65% R_o; in Jurassic sediments it ranges from 0.40 to 1.0% R_o and in the Triassic from 0.45 to 1.30% R_o, showing an overall increase with depth of burial.Cretaceous sediments of the Deer Bay Formation are thermally immature and contain organic matter of terrestrial origin. The Upper Jurassic shales of the Ringnes Formation contain predominantly organic matter of liptinitic and exinitic origin with a considerable vitrinitic input. At optimum maturation levels, potential source beds of this formation would have a good hydrocarbon-generating potential. The hydrocarbon potential, however, would be limited to the generation of gases due to the leanness of the source rocks. Parts of the Lower Jurassic Jameson Bay Formation are organic-rich and contain a mixed exinitic/vitrinitic organic matter, Botryococcus colonial algae but visible organic matter is dominated by high plant remains (mainly spores). The Schei Point Group shales and siltstones contain organic matter of almost purely marine origin, whereas the predominantly higher plant-derived organic matter found in the Deer Bay, Jameson Bay and partly in the Ringnes formations have higher TOC. Among the Schei Point Group samples, the Cape Richards and Eden Bay members of the Hoyle Bay Formation are richer in TOC (>2.0%) than the Murray Harbour Formation (Cape Caledonia Member). This may reflect differences in the level of maturity or in the depositional environment (more anoxic conditions for the former).Regional variations in the level of thermal maturity of Mesozoic sediments in Sverdrup Basin appear to be a function of burial depth. The Mesozoic formations thicken towards the basin centre (NNE direction), reflecting the general pattern of increasing thermal maturity north of Sabine Peninsula. However, the regional thermal-maturation pattern of the Mesozoic is not solely a reflection of the present-day geothermal gradient, which indicates that anomalous zones of high geothermal gradient may have existed in the past, at least since when the Mesozoic sediments attained maximum burial depth. The contour pattern of the regional variation of maturity at the base of numerous Triassic formations is similar to that of the structural contours of the Sverdrup Basin, indicating that present-day maturation levels are largely controlled by basin subsidence.     

Modelling of hydrocarbon generation in the Cenozoic Song Hong Basin,Vietnam: a highly prospective basin

The Cenozoic Song Hong Basin, situated on the northern part of the Vietnamese shelf, has been only sporadically explored for hydrocarbons. A review of the results of the exploration efforts so far shows that the distribution of potential source rocks and their time of hydrocarbon generation are the critical risks for finding commercial amounts of hydrocarbons. In the Song Hong Basin, including the Hanoi Trough, the rocks most likely to have source potential are: (1) oil-prone Eocene–Lower Oligocene lacustrine mudstones and coals, (2) oil- and gas-prone Middle Miocene coal beds, (3) gas-prone Upper Oligocene–Lower Miocene coals, and (4) gas- and oil-prone Miocene marine mudstones. To assess the time of hydrocarbon generation from these units, relative to the formation of traps, the generation history was modelled at 32 well and pseudo-well locations. The modelling demonstrates that the two first-mentioned source rock units are especially important. In the northern and northeastern part of the basin and along its western margin traps may have been charged by Eocene–Lower Oligocene source rocks. In the Hanoi Trough, the excellent Middle Miocene coal beds have probably generated hydrocarbons within the last few million years. Thus the huge and still underexplored Song Hong Basin provides attractive areas for further exploration.     

新疆准噶尔盆地东北缘石炭系姜巴斯套组烃源岩评价

准噶尔盆地东北缘石炭系烃源岩主要分布在准东和腹部地区,根据野外地表调查,针对准噶尔东北缘石炭系烃源岩有机质丰度、类型、成熟度特征进行了综合研究和评价,研究区烃源岩主要为灰黑色碳质粉砂质泥岩、泥岩。结果表明:该区石炭系烃源岩有机质丰度总体上属差-中等烃源岩。姜巴斯套组下段烃源岩有机质类型主要为Ⅰ型;中段烃源岩有机质类型以Ⅱ1、Ⅲ为主;上段有机碳含量较低,据相关变化趋势推测其有机碳类型为以Ⅲ型为主。因此总体上有机质类型为Ⅱ2-Ⅲ型。有机质成熟度总体上属成熟-高熟,主要为高熟。综上所述,准噶尔东北缘的石炭系是一套较有效的烃源岩,在该区寻找油气具重要意义。     

Source rock potential of the Middle Jurassic to Middle Pliocene,onshore Nile Delta Basin,Egypt

Organic geochemical characterization of cutting samples from the Abu Hammad-1 and Matariya-1 wells elucidates the depositional environment and source rock potential of the Jurassic and Lower Cretaceous successions and the Middle Miocene to Pleistocene section in the southern and eastern Nile Delta Basin. The burial and thermal histories of the Mesozoic and Miocene sections were modeled using 1D basin modeling based on input data from the two wells. This study reveals fair to good gas-prone source rocks within the Upper Jurassic and Lower Cretaceous sections with total organic carbon (TOC) averaging 2.7% and hydrogen index (HI) up to 130 mg HC/g TOC. The pristane/n-C₁₇ versus phytane/n-C₁₈ correlation suggests mixed marine and terrestrial organic matter with predominant marine input. Burial and thermal history modeling reveals low thermal maturity due to low heat flow and thin overburden. These source rocks can generate gas in the western and northern parts of the basin where they are situated at deeper settings. In contrast, the thick Middle Miocene shows fair source rock quality (TOC averaging at 1.4%; HI maximizing at 183 mg HC/g TOC). The quality decreases towards the younger section where terrestrial organic matter is abundant. This section is similar to previously studied intervals in the eastern Nile Delta Basin but differs from equivalents in the central parts where the quality is better. Based on 1D modeling, the thick Middle Miocene source rocks just reached the oil generation stage, but microbial gas, however, is possible.     

Brittle deformation in the Afikpo Basin （Southeast Nigeria）： Evidence for a terminal Cretaceous extensional regime in the Lower Benue Trough, Nigeria

This paper deals with the regional and structural framework of the Cretaceous rocks in the Afikpo Basin located in the southeastern part of the Lower Benue Trough. Results from regional tectonics are presented together with those of the microtectonic analysis of microfaults in the Owutu-Afikpo-Adadama area in the basin. The Owutu-Afikpo-Adadama ridge at the north-central part of the basin marks the boundary between the Late Cenomanian-Turonian-Conianian sediments and the Campanian-Maastrichtian sandstones. This ridge trends N45°E on average and is faulted in three main directions, namely: (1) N-S normal faults; (2) NE-SW strike-slip faults; and (3) NW-SE strike-slip faults. The faulted rocks along these brittle discontinuities are mainly cataclastics with internal fracture cleavage and sigmoidal quartz mosaics that are reminiscent of extensional deformation. The cataclasites often bear slickenside striations.     

An organic geochemical investigation on organic rich sediments from two Neogene formations in the Klias Peninsula area,West Sabah,Malaysia

Twenty organic rich outcrop samples from the Belait and Setap Shale formations in the Klias Peninsula area, West Sabah, were analysed by means of organic petrology and geochemical techniques. The aims of this study are to assess the type of organic matter, thermal maturity and established source rock characterization based primarily on Rock-Eval pyrolysis data. The shales of the Setap Shale Formation have TOC values varying from 0.6 wt%–1.54 wt% with a mean hydrogen index (HI) of 60.1 mg/g, whereas the shal...     

Source Rock Characterization and Petroleum Systems in North Ghadames Basin,Southern Tunisia

This paper presents geochemical analysis of drilled cutting samples from the OMZ‐2 oil well located in southern Tunisia. A total of 35 drill‐cutting samples were analyzed for Rock‐Eval pyrolysis, total organic carbon (TOC), bitumens extraction and liquid chromatography. Most of the Ordovician, Silurian and Triassic samples contained high TOC contents, ranging from 1.00 to 4.75% with an average value of 2.07%. The amount of hydrocarbon yield (pyrolysable hydrocarbon: S2b) expelled during pyrolysis indicates a good generative potential of the source rocks. The plot of TOC versus S2b, indicates a good to very good generative potential for organic matter in the Ordovician, Silurian and Lower Triassic. However, the Upper Triassic and the Lower Jurassic samples indicate fair to good generative potential. From the Vankrevelen diagram, the organic matter in the Ordovician, Silurian and Lower Triassic samples is mainly of type II kerogen and the organic matter from the Upper Triassic and the Lower Jurassic is dominantly type III kerogen with minor contributions from Type I. The thermal maturity of the organic matter in the analyzed samples is also evaluated based on the T_max of the S2b peak. The Ordovician and Lower Silurian formations are thermally matured. The Upper Silurian and Triassic deposits are early matured to matured. However, Jurassic formations are low in thermal maturity. The total bitumen extracts increase with depth from the interval 1800–3000 m. This enrichment indicates that the trapping in situ in the source rocks and relatively short distance vertical migration can be envisaged in the overlying reservoirs. During the vertical migration from source rocks to the reservoirs, these hydrocarbons are probably affected by natural choromatography and in lower proportion by biodegradation.     

柴达木盆地西部第三系盐湖相有效生油岩的识别

柴达木盆地西部第三系发现了储量可观的油气资源,但是总体上讲这里的生油层钙质含量高、有机质丰度低;如何识别有效生油岩,正确评价油气资源潜量,成为这里油气勘探和地球化学的首要问题。通过上、下干柴沟组生油岩的沉积特征和地球化学分析,可发现盐湖相存在许多有机质丰度较高的生油岩;利用热解和模拟实验等方法确定出有效生油岩的有机碳含量下限为 0.4%。这样既为该区生油岩提供了评价标准,又找到大量有效生油岩,解决了研究区油气资源预测的基本问题.     

鸡西含煤沉积盆地特征及早期油气勘探

鸡西盆地位于黑龙江省东部,1985年见天然气显示,有效勘探面积只占盆地面积的50%,而且生油坳陷（沉积坳陷）局部遭到破坏,地质条件复杂,勘探风险大。盆地内主要充填了白垩系及第三系,是一个中、新生代盆地,其中下白垩统分为鸡西群和桦山群,鸡西群是主要含煤地层,也是油气勘探的目的层。下白垩统东荣组含有海湾湖相暗色泥岩,而城子河组与穆棱组沼泽相煤和高碳泥岩发育。老第三系永庆组深湖相暗色泥岩丰富,生油岩类型及纵向分布层位差异受沉积充填序列控制。孔隙型储层物性较低,裂隙型储层具有实际意义。生储盖组合匹配,发现的天然气源于下伏的鸡西群,气层之上的盖层是厚层泥岩与4层凝灰岩。研究表明,鸡西盆地含有较丰富的天然气资源,大部分为煤层气,应以天然气勘探为目标。建议在梨树镇坳陷下白垩统找气,在平阳镇坳陷老第三系找油。     

可可西里卓乃湖地区五道梁群油页岩石油地质意义

对可可西里中新统五道梁群油页岩研究表明：油页岩有机质丰度中等一高;有机质类型以腐泥型（Ⅰ）-腐殖腐泥型（Ⅱ,）为主。有机质的热演化以低成熟为主;总体上为较好的烃源岩,中新世时可可西里盆地为封闭、干热的以碳酸盐岩沉积为主的盐湖盆地。封闭与还原性静水环境为有机质的堆积和转化形成油页岩提供了条件。     

Quantitative Assessment of Hydrocarbon Expulsion of Petroleum Systems in the Niuzhuang Sag, Bohai Bay Basin, East China

Based on a detailed survey of the distribution and organic geochemical characteristics of potential source rocks in the South Slope of the Niuzhuang Sag, Bohai Bay Basin, eastern China, a new approach to assess the amount of hydrocarbons generated and expelled has been developed. The approach is applicable to evaluate hydrocarbons with different genetic mechanisms. The results show that the models for hydrocarbon generation and expulsion vary with potential source rocks, depending on thermal maturity, types of organic matter and paleoenvironment. Hydrocarbons are mostly generated and expelled from source rocks within the normal oil window. It was calculated that the special interval (algal-rich shales of the ES4 member formed in brackish environments) in     

藏北羌塘盆地QK-1井中侏罗统烃源岩地球化学特征

QK-1天然气水合物钻探试验井位于西藏羌塘盆地毕洛错地区,主要钻遇中侏罗统夏里组粉砂质泥岩和布曲组灰岩。其有机质丰度、有机质类型、有机质成熟度等有机地球化学特征表明,南羌塘地区中侏罗统烃源岩的有机质丰度偏低,夏里组泥岩TOC介于0.26%~0.38%之间,达到差-中烃源岩标准,而布曲组灰岩TOC介于0.02%~0.08%之间,仅局部达到差烃源岩标准;两个组的烃源岩有机质类型主要为Ⅱ₂型及Ⅲ型,较利于天然气的生成,其中夏里组的生气能力优于布曲组;该区烃源岩热演化程度较高,Ro为1.31%~1.63%之间,均达到高成熟阶段,而夏里组有可能已进入干气阶段。基于冻土区天然气水合物成藏条件分析,综合考虑夏里组烃源岩及沉积厚度特征,认为中侏罗统夏里组具备良好的天然气水合物勘探前景。     

上扬子西南缘下志留统细粒沉积岩有机质富集机理研究——以盐津牛寨剖面为例

利用化学元素行为特征的差异性,以总有机碳（TOC）、主量元素和微量元素的含量或比值判别四川盆地西南缘盐津牛寨剖面下志留统龙马溪组细粒沉积岩形成时的初级生产力、氧化还原条件、陆源供应的输入和盆地水体滞留程度等环境特征,并对比川南沉降中心（长宁剖面）和南川非沉降中心（南川剖面）龙马溪组的古环境参数,综合探讨了早志留世上扬子海滞留海盆细粒沉积岩有机质富集的差异性。结果表明,盐津牛寨地区下志留统细粒沉积岩以黑色—灰黑色碳质页岩、页岩和泥岩为主,富有机质主要集中在龙马溪组下段,TOC平均含量为2.73%;Ba_bio显示盆地西南缘龙马溪组早期具有较高的生产力;Ni/Co、V/（V+Ni）,U/Th和V/Cr揭示龙马溪组从底部缺氧环境逐步过渡到顶部的氧化环境;TiO₂、Al₂O₃、Zr表明龙马溪组下段陆源碎屑供应量明显增加;Mo/TOC值和U-Mo协变模式图均说明四川盆地西南缘在早志留世水体呈中等滞留环境。与长宁剖面和南川剖面对比分析,发现盆地西南缘TOC与氧化还原条件呈现显著的正向相关关系;与盆地水体的滞留程度呈现微弱的正向相关关系;与陆源碎屑物质的注入和初级生产力无明显相关性,表明缺氧还原的沉积环境是影响该地区龙马溪组黑色页岩有机质富集的主要因素。