乍得Bongor强反转裂谷盆地高酸值原油成因

乍得Bongor盆地是受中非剪切带影响发育起来的中、新生代陆内强反转裂谷盆地,反转和走滑构造是盆地最显著的构造特征。所发现的原油主要为中质油(重度为20°～34°API),其次为重质油(重度小于20°API),普遍高含沥青质、高含蜡、高酸值、低含硫。为了探讨高酸值原油的成因,作者选择了该盆地15个不同酸值的原油样品,尝试应用高分辨率质谱分析原油有机酸的组成。分析结果表明,高酸值原油的有机酸主要由环烷酸组成;环烷酸碳原子数分布范围较宽,且以一环、二环、三环环烷酸为主。生物降解作用是形成高酸值原油的主要原因,而构造反转造成盆地抬升,则加速了生物降解作用的发生。     

Les rhyolites hyperalcalines (pantellérites) du lac Tchad. Composition et signification tectonomagmatiqueThe peralkaline rhyolites (pantellerites) of the Lake Chad. Composition and tectonomagmatic significance

Remnants of rhyolite lava-dome and of alkaline microgranite extrusions emerge, as inselbergs, in the alluvial plain, south of Lake Chad. The peralkaline, arfvedsonite- and acmite-bearing rhyolites are determined as pantellerites. They can be related to a tectonomagmatic stage, dated of the Late Cretaceous, of the western central Africa rift system. They may constitute the NNE extension of a N50° to 30° elongated succession of small alkaline complexes, from the Gulf of Guinea to the North-Cameroon, which initiated the magmatic activity of the Cameroon Line. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 885–891.     

非洲热带草原景观区金的次生晕特征及分散模式

乍得共和国Mayokebbi省Pala地区是该国最重要的砂金矿区,多家公司先后致力于在该区域寻找岩金,然而经过大量的槽探工程施工与钻孔验证,均未发现好的矿体。为了解释地表Au异常的形成过程及原因,笔者在Pala地区针对金矿开展了地球化学勘查,对热带草原景观区Au的次生晕特征进行了讨论与总结,通过土壤垂直剖面测量分析了Au在准平原化过程中的地球化学行为,并最终解释了该区域金异常的成因。本次研究对国内企业赴非洲相同景观条件下进行金矿勘查具有指导意义。     

Modèle hydrologique du Lac Tchad

Résumé

Le Lac Tchad est modélisé par un bilan de volume donnant ses niveaux et superficies de plans d'eau en fonction d'apports et pertes: débit des tributaires; pluie; évaporation; infiltration. Il est représenté par trois réservoirs (cuvette nord et archipel connectés à la cuvette sud) dont les niveaux de plan d'eau, distincts en période d'apports déficitaires (situation actuelle), sont confondus en période excédentaire (situation avant 1973). Suite aux observations de terrain, la cuvette nord est représentée avec un “réservoir sol” devant être rempli pour que l'eau puisse apparaître en surface. Le modèle utilise sept paramètres optimisés: intensité d'infiltration; profondeur du “réservoir sol”; quatre paramètres d’échange entre réservoirs; niveau de fond de l'archipel. Grâce aux données mesurées au sol ou par satellite (débits, précipitations, évaporation, niveaux et superficie de plans d'eau) il est calé sur la période 1970–1996 et validé sur la période 1956–2008.

Citation Bader, J.-C., Lemoalle, J. & Leblanc, M. (2011) Modèle hydrologique du Lac Tchad. Hydrol. Sci. J. 56(3), 411–425.     

Pioneering assessment of carbon stocks in polder soils developed in inter-dune landscapes in a semiarid climate,Lake Chad

In semiarid Sahelian region, the dynamics of soil organic carbon (SOC) and water are key to sustainable land management. This work focuses on the behaviour of carbon. A total of 33 soil profiles in four polders, ranging from 10 to 65 years in age, were sampled, analysed (0–1 m), and matched with marsh soil profiles in recent sediments considered as reference (t₀) for carbon stocks determination. SOC and soil inorganic carbon (SIC) stocks show a spatial variability between polders. SOC stocks were t₀ 200 ± 0.8; t₆₀ 183 ± 34; and t₆₅ 189 ± 1.1 MgC·ha^?1, whereas the SIC stocks were negligible. These results show the highest stocks of soil carbon observed for this climatic region. The SOC stocks were also calculated for the equivalent soil mass at a defined depth (0–0.3 m); the corrected calculation of SOC stocks (S_corr) for 2450 Mg·ha^?1 of equivalent soil mass is t₀ 64 ± 1.9, t₆₀ 59 ± 9.8, and t₆₅ 53 ± 2.2 MgC·ha^?1; the stocks decrease by ?7.8% and ?17.2% from t₀ to t₆₀ and t₆₅. Carbon was inherited from the pre-existing·marsh and the polders have conserved high stock values.     

乍得南部Bongor盆地下白垩统碎屑岩储层成岩作用及其对物性的影响

综合利用铸体薄片、X射线衍射、阴极发光、扫描电镜、物性分析及镜质体反射率和包裹体温度等资料,结合岩心观察法,对Bongor盆地下白垩统碎屑岩储层基本特征及成岩作用进行了研究。依据胶结物的体积分数及形态、黏土矿物组分的分布、岩石结构镜下特征及孔隙演化、有机质热演化等证据划分了岩石成岩阶段,并分析了成岩作用对物性的影响。结果显示：该区储层岩石类型以长石砂岩为主,胶结物以碳酸盐和黏土矿物为主;溶孔-粒间孔组合型储集性最好,粒间孔-溶孔组合型次之;成岩作用是控制储层物性的主要因素,成岩作用主要包括压实作用、胶结作用、交代作用和溶蚀作用;压实作用和胶结作用是破坏性成岩作用,二者分别使原始孔隙度损失54.93%和31.30%,岩石颗粒中长石、碳酸盐胶结物等易溶物质发育,溶蚀作用是主要建设性成岩作用;目的层成岩阶段以中成岩A期为主,成岩作用弱-中等。     

Late Quaternary river development in the southwest Chad Basin: OSL dating of sediment from the Komadugu palaeofloodplain (northeast Nigeria)

The Komadugu river system is the major Nigerian tributary to Lake Chad. Its large alluvial complex represents an important archive of the late Quaternary environmental history of the Chad Basin. Modern floodplains are incised into an older fluvial terrace that extends over 5000 km². Evidence from satellite images suggests that the ancient river system was dammed up by the Bama Beach Ridge during high stands of mega Lake Chad. This caused the formation of extensive wetlands with a chaotic network of relic channels. The first set of luminescence ages presented here fits well into the previously established environmental history of the Chad Basin. Both the early and mid‐Holocene pluvial periods as well as a final relatively humid period during the late Holocene are reflected by point bar to overbank deposits or channel fills. The onset of the Holocene was characterised by erosion of older dune fields as indicated by grain‐size distributions of the alluvium. Satellite images and sedimentological observations show that the floodplain was dominated by meandering channels shifted by frequent avulsion, reflecting a high variability in precipitation and discharge patterns at the beginning and end of Holocene humid periods. Copyright © 2007 John Wiley & Sons, Ltd.     

Evolution of the Mayo Kebbi region as revealed by zircon dating: An early (ca. 740 Ma) Pan-African magmatic arc in southwestern Chad

The Mayo Kebbi region in SW Chad is part of the NNE-SSW trending Neoproterozoic Central African Fold Belt (CAFB) and is made up of three calc-alkaline granitoid suites emplaced into a metavolcanic–metasedimentary sequence. The first suite is represented by mafic to intermediate rocks (gabbro-diorite and metadiorite) emplaced between 737 and 723 Ma during early Pan-African convergence. The second consists of the Mayo Kebbi batholith and includes tonalites, trondhjemites and granodiorites, emplaced during several magmatic pulses between 665 and 640 Ma. The third suite includes porphyritic granodiorite and hypersthene monzodiorite dated at ca. 570 Ma. The Mayo Kebbi domain extends southward into Cameroon and is interpreted as a middle Neoproterozoic arc stabilized at ca. 650 Ma. This study also revealed a diachronous evolution between Mayo Kebbi and western Cameroon (e.g., the Poli region). The overall evolution of this part of the CAFB is interpreted as the result of successive development of magmatic arcs, since ca. 740 Ma, and tectonic collage of three different domains (Adamawa-Yade, Mayo Kebbi, and West Cameroon) which, after suturing, were intruded by post-collisional granitoids (<600 Ma).     

Tracing stable isotope values from meteoric water to groundwater in the southwestern part of the Chad basin

The southwestern Chad basin is a semi-arid region with annual rainfall that is generally less than 500 mm and over 2,000 mm of evapotranspiration. Surface water in rivers is seasonal, and therefore groundwater is the perennial source of water supply for domestic and other purposes. Stable isotope has been measured for rainwater, surface water and groundwater samples in this region. The stable isotope data have been used to understand the inter-relationships between the rainwater, surface water, shallow and deep groundwater of this region. This is being used in a qualitative sense to demonstrate present day recharge to the groundwater. Stable isotope in rainwater for the region has an average value of –4‰ δ¹⁸O and –20‰ δ²H. Surface water samples from rivers and Lake Chad fall on the evaporation line of this average value. The Upper Zone aquifer water samples show stable isotope signal with a wide range of values indicating the complex character of the aquifer Zone with three distinguishable units. The wide range of values is attributable to waters from individual unit and/or mixture of waters of different units. The Middle and Lower aquifers Zones’ waters show similar stable isotopes values, probably indicating similarity in timing and/or mechanism of recharge. These are palaeowaters probably recharged under a climate that is different from today. The Upper Zone aquifer is presently being recharged as some of its waters show stable isotope compositions similar to those of average rainfall waters of the region.     

乍得湖盆地构造特征与油气成藏规律初探

以前人的研究为基础,结合研究区及邻区地球物理资料、岩矿特征、钻井资料,分析了盆地结构特征、构造样式、演化序列以及油气成藏规律。研究表明,乍得湖盆地为伸展断-坳型裂谷盆地,断裂、褶皱构造发育;经历了前裂谷(侏罗纪以前)、同裂谷(135~23.5Ma)、后裂谷(23.5Ma以后)三个演化阶段;主要发育白垩纪和古近纪两套含油气系统。