Central African Rift System Research Articles

Thermochronological constraints on the evolution of the Bongor Basin, Chad

The Bongor Basin is an important petroliferous basin in the Western and Central African rift system. The basin's evolution history is featured with a strong tectonic inversion during the Late Cretaceous, which resulted in its unique basin structure and hydrocarbon accumulation pattern. However, due to the complex process of repeated cooling and heating, single sample bedrock thermochronology can hardly provide accurate constraints to its thermal evolution history. In this paper, nine granitic core samples from the crystalline basement in five wells on the northern slope of the Bongor Basin were analyzed using multiple thermochronological methods (apatite U-Th/He, apatite fission tracks and length distribution, apatite U-Pb dating) as well as vertical profiles to obtain a more accurate thermal history. The results show that the samples from all five wells underwent four stages of thermal history: from ∼600 Ma to 135 Ma, the samples cooled continuously from 600 °C to near-surface temperatures; from ∼135 Ma to 100 Ma, the samples were heated rapidly; from 100 Ma to 60–80 Ma, the samples cooled rapidly; and after that, the samples experienced slow differential heating and cooling. The thermal history results show that the key time for the strong inversion of the Bongor Basin was between 80 and 90 Ma when the basin was uplifted and exhumed as a whole, while samples in different fault blocks underwent differential uplift and subsidence since the Paleogene.

Paleoenvironmental factors controlling organic-rich formations deposition in the Babouri-Figuil Basin (Northern Cameroon)

The Babouri-Figuil Basin is an intracratonic basin (half-graben) in northern Cameroon that is genetically connected to the Benue Trough from Nigeria, and is an area of interest in terms of petroleum prospectivity. Recent studies highlighted the presence of organic-rich formations in the basin. However, none of these works have identified factors that governed the accumulation of organic matter (OM) in the sediments. The main objective of this work is the characterization of these formations through palynofacies and organic geochemical techniques (total organic carbon - TOC, total sulfur, insoluble residue and biomarkers), in order to determine the organic facies, their depositional environments and the main drivers for organic enrichment in the basin. The current study reveals that black shale and massive claystone lithologies constitute the main organic-rich formations in the basin, with TOC reaching up to 26.08 wt%, being characterized by a dominance of bacterially-derived amorphous OM. Palynofacies and biomarker data revealed that these formations are positively associated with anoxic conditions and a partly highly saline and stratified lake water column. The deposition of organic-rich formations in the Babouri-Figuil Basin was mainly controlled by restriction conditions which developed in connection with the regional tectonic framework. The Lower Cretaceous rifting episode in the West and Central African Rift System (WCARS) basins led to the formation of accommodation space, a reduction in water levels, and the development of anoxic conditions within the basin, facilitating the deposition of organic-rich formations. Therefore, the organic enrichment of the Babouri-Figuil Basin has been predominantly controlled by its tectonic evolution, particularly during the syn-rift phase. This phase created favorable conditions for the deposition and preservation of OM, including the establishment of anoxic conditions. Additionally, the paleoclimate (arid conditions), the development of bacterial biomass, and the basin's paleogeography all played a significant role in this process. The organic-rich formations of the Babouri-Figuil Basin show characteristics of prospective petroleum source rocks (high organic content, high proportion of oil-prone kerogen, significant thickness and lateral extension). The combination of organic-rich formations with sandstone deposits above and extensive claystone/shale deposits on top can indicate the presence of an oil play in the basin. A detailed study with broader sampling is needed to investigate thoroughly the variation of organic facies and the influence of paleoenvironmental factors that control the deposition of thick source rock intervals.

Integrated Geophysical‐Petrological 3D‐Modeling of the West and Central African Rift System and Its Adjoining Areas

Integrated Geophysical‐Petrological 3D‐Modeling of the West and Central African Rift System and Its Adjoining Areas

Tectonic and sedimentary evolution of the Northern Cameroon Cretaceous rift basins: A study of the Babouri-Figuil and Mayo Oulo-Lere basins

The Babouri-Figuil and Mayo Oulo-Lere basins located in Northern Cameroon are small elongated sedimentary asymmetric half-grabens associated with the development of the West and Central African Rift System (WCARS). The evolution of these basins began during the Precambrian pre-rift period and continued through the Late Jurassic - Early Cretaceous periods, although details of their development remain unclear. The aim of this study is to understand the evolution and development of the border faults and their control on syn-rift geometry, accommodation space, and the resulting stratigraphic architecture. Using a combination of multiples surface and subsurface well logs data, we demonstrate the relationship and evolution of various stages of rifting and the associated sedimentary architecture. The basin is composed of an alluvial to lacustrine succession that developed during its tectonic evolution. A 3-stage tectonic model is proposed for the evolution of the basin that uses fault propagation analysis to describe the sedimentary architecture and evolution of basin geometry. The three stages of the model are the early rift, rift climax, and late rift; here, the early rifting phase is marked by the reactivation of faults in the Lower Cretaceous. The rift climax phase resulted in the interconnection of small segmented border faults leading to the high rate of activation of major border faults and to the development of the half graben structure of the basins. The late rift stage is marked by a decrease in border fault activity and accommodation. Sediment distribution and the facies architecture are controlled by tectonic subsidence created throughout the rifting phases. The stratigraphic signature of the Babouri-Figuil and Mayo Oulo-Lere basins are punctuated by the lacustrine water transgression-regression cycles, and controlled by the balance between the rate of sediment supply and tectonic accommodation. The progressive change between overfilled to starved sedimentary architecture is illustrated by this stratigraphic succession. Additional age assessment and sub-surface data in future work could help confirm the structures identified on the surface.

Petroleum geological characteristics and exploration targets of the oil-rich sags in the Central and West African Rift System

Based on seismic, drilling, and source rock analysis data, the petroleum geological characteristics and future exploration direction of the oil-rich sags in the Central and West African Rift System (CWARS) are discussed. The study shows that the Central African Rift System mainly develops high-quality lacustrine source rocks in the Lower Cretaceous, and the West African Rift System mainly develops high-quality terrigenous organic matter-rich marine source rocks in the Upper Cretaceous, and the two types of source rocks provide a material basis for the enrichment of oil and gas in the CWARS. Multiple sets of reservoir rocks including fractured basement and three sets of regional cap rocks in the Lower Cretaceous, the Upper Cretaceous, and the Paleogene are developed in the CWARS. Since the Late Mesozoic, due to the geodynamic factors including the dextral strike-slip movement of the Central African Shear Zone, the basins in different directions of the CWARS differ in terms of rifting stages, intervals of regional cap rocks, trap types and accumulation models. The NE–SW trending basins have mainly preserved one stage of rifting in the Early Cretaceous, with regional cap rocks developed in the Lower Cretaceous strata, forming traps of reverse anticlines, flower-shaped structures and basement buried hill, and two types of hydrocarbon accumulation models of “source and reservoir in the same formation, and accumulation inside source rocks” and “up-source and down-reservoir, and accumulation below source rocks”. The NW–SE basins are characterized by multiple rifting stages superimposition, with the development of regional cap rocks in the Upper Cretaceous and Paleogene, forming traps of draping anticlines, faulted anticlines, antithetic fault blocks and the accumulation model of “down-source and up-reservoir, and accumulation above source rocks”. The combination of reservoir and cap rocks inside source rocks of basins with multiple superimposed rifting stages, as well as the lithologic reservoirs and the shale oil inside source rocks of strong inversion basins are important fields for future exploration in basins of the CWARS.

Maastrichtian – Paleocene environmental and climatic conditions in Koum Basin (northern benue trough, Cameroon) inferred using a multiproxy approach including sedimentology, mineralogy, and geochemistry

The Koum Basin is one of major intracontinental basins in northern Cameroon. It is part of the Upper Benue Trough and has genetic ties to the West and Central African Rift System. The main purpose of studying the Koum Basin sedimentary deposits was to re-examine the time of the deposition and constrain the palaeoenvironmental and paleoclimatic conditions during deposition. To accomplish this, we used a combination of sedimentologic, palynologic, mineralogic, and geochemical data. Claystone units interbedded with calcareous siltstones dominate sedimentary sequence, including few clay beds, fine to medium-grained sandstone, and rare marl layers. The palynomorph assemblage suggests a previously unknown Maastrichtian – Paleocene age in this basin. The sediments originated from intermediate igneous parent rocks with a relative contribution of felsic rocks. These rocks underwent low detritism (Si/Al: 2.51–4.43; K/Al: 0.11–0.36; Ti/Al: 0.05–0.10) and chemical weathering (CIA: 55–64; PIA: 56–68) in the source region. Consequently, deposits are immature (ICV values > 1) and weakly recycled (Si2O3/Al2O3 ratios ≤5). The organic matter analysis and the palynoflora assemblage, reveal Type III kerogen with significant contributions of Type II-III and type II kerogens, as well as a continental paleo-depositional environment with open rainforest to savannah vegetation. The clay minerals distribution suggests moderate illitization and chloritization processes and thus moderate burial diagenesis at temperatures ranging from 70 to 90 °C, also attested by immature to mature Tmax values (≤436 °C) marginally. The low CIA values and very low kaolinite contents, indicate dry climatic conditions. In addition, two major-element-based discrimination diagrams show that the parent rocks of clastic sediments formed primarily in a continental arc system, with a moderate contribution from the rift system and a low contribution from the oceanic island arc, implying compressional conditions, which is consistent with previous studies on the tectonic background of the region during the late Maastrichtian to Paleocene period.

Diagenesis, paleoenvironments, and petroleum geology of sediments lining the West and Central African Rift System, Koum Basin, North Cameroon

Diagenesis, paleoenvironments, and petroleum geology of sediments lining the West and Central African Rift System, Koum Basin, North Cameroon

A new Moho map of the African continent from seismic, topographic, and tectonic data

We use seismic surveys (853 stations and 79 seismic profiles) to compile a new continental Moho model of the African continent of which 90% is formed by the Precambrian crust. The result reveals considerable Moho depth variations, with values ranging from 14 to 20 km for the Afar Triple Junction and Turkana Lake to 46 km beneath the Rif Mountains, parts of the Ethiopian Plateaus, and the Namaqua-Natal Belt. A localized Moho deepening (∼44 km) is found beneath the western and eastern blocks of the Congo Craton, the central part of Victoria Lake, the Rehoboth Province, the eastern part of the Kaapvaal Craton, and the Irumide Belt. A shallow Moho depth is detected beneath wide areas in North, Central, and East Africa, particularly along the North Africa coast (24–30 km), the Mauritanian Belt (26–30 km), the West and Central African Rift System (30–32 km), the Benue Trough (24–28 km), and beneath the Rwenzori Mountains (26–30 km) and the southern part of Somali Plate (22–26 km). At many locations, a Moho relief is characterized by significant changes across different tectonic provinces; from the Afar Triple Junction, Turkana Depression, and the Somali Plate to the Main Ethiopian Rift, between the Rwenzori region, from the Gabon-Cameroon Shield to the Benue Trough, and from the western border of the West African Craton to the Mauritanian Belt. According to our estimates, an average Moho depth beneath the African continent is 36.1 km. Comparison of our result with selected global, continental, and regional Moho models shows large differences that locally even exceed ±15 km.

Role of two-stage strike slip faulting in the tectonic evolution of the Doseo depression in the central Africa rift system

Doseo depression of Central African Rift System is an important onshore frontier area for oil and gas exploration in Africa, with rift development characterized by the dual control of fault-depression development and strike slip faulting. According to the latest drilling and seismic data in recent years, through the fine seismic interpretation, the calculation of paleo-fault throw and fault growth index and the method of balanced profile cross-section restorations, the fault characteristics and tectonic setting of Doseo depression are analyzed in detail, especially the fault activity, tectonic evolution and genetic mechanism of two large-scale strike slip movements in Doseo depression. The results show that the Doseo depression experienced five main tectonic phases, including two stages of strike slip tectonic movements with different mechanisms. The first stage is characterized by strike slip tectonic movements under tilting extension, which controls the overall structure of the depression. The second stage is characterized by strike slip tectonic movements under transform compression, which play an important role in the formation of some inversion structures and SW-NE trending anticlines and faulted anticlines. The superposition of an early faulted-depression and two later stages of strike slip tectonics led to current pattern of Doseo depression.

A new lithostratigraphic profile for Cretaceous to Paleogene successions in the West and Central African Rift System, Koum Basin, northern Cameroon

Six new formations (Gaba, Roumi, Poula, Mbissiri, Lende andKali) are proposed for Cretaceous to Paleogene sediments of the Koum Basin, northern Cameroon. The Koum Basin is one of numerous sedimentary basins lining the West and Central African Rift System and presents a complex evolutionary history involving extensional shearing and compressional events spanning the Cretaceous to Paleogene. Recent discoveries of hydrocarbons in associated basin such as Doba in Chad,motivated exploration of the Koum Basin. However, existing studies of the Koum Basin lack a viable lithostratigraphic framework, which is crucial for ongoing exploration efforts in this basin. The present study integrates lithologic, petrographic and palaeontologic data to map, stack and correlate several outcrop sections across the basin. Facies analysis suggests that deposition occurred in a fluvio-lacustrine palaeodeposional environment. The new units were described following the guidelines of the International Stratigraphic Guide. An updated geologicmap depicts the spatial distribution of the lithostratigraphic units that make up the Koum Basin. Palynomorphs, bivalves and dinosaur footprints are used to date the studied sections.

Sedimentology, T-R sequence stratigraphy and geochemical characterization of sedimentary successions of the West and Central African Rift System, Koum Basin

The Koum Basin (KB) is among the numerous relatively unexplored sedimentary basins lining theWCARS(West and Central African Rift System), hence limiting the understanding of how sedimentation evolved along the full extent of the WCARS. The intensification of petroleum exploration within WCARS has further emphasized the need to study these greenfields. This study contributes in solving the above issue by establishing a sequence stratigraphic framework and expanding the sedimentological databank of the KB. Systematic fieldmapping and lithofacies analyses of theKB along three NNW-SSE traverse lines cross-cutting the basin exposed alternations of conglomerates, sandstones and shales, intercalated by siltstones and carbonate mudstones. Dinosaur, bivalve and plant fossils are common. X-Ray Fluorescence data of studied samples plot as non-marine, immature, low to moderately weathered shales and greywackes of intermediate to felsic igneous and quartzose sedimentary sources. Sedimentation occurred under oxic, through anoxic to dysoxic depositional conditions, in a semi-arid active continental margin. These deposits inferred to be fluvio-lacustrine, have been subjected to outcrop-based sequence stratigraphic analyses through section matching and correlation of its material-based sequence stratigraphic surfaces, following the Transgressive-Regressive sequence stratigraphic model. The deposits have been subdivided into five repetitive T-R sequence stratigraphic units, bounded by four first order unconformities (SB-1, SB-4, SB-5 and SB-6) and two correlative conformities (SB-2 and SB-3), representing deposition under repetitive base level rise and fall regimes. These results demonstrate that any potentialmarine incursion in the WCARS probably did not reach the Koum Basin.

The Mesozoic West and Central Africa Rift System (WCARS) and the older Kandi Shear Zone (KSZ): Rifting and tectonics of North Africa and South America and fragmentation of Gondwana based on geophysical investigations

Intra- and trans-continental tectonic lineaments and tectonic structures cutting Africa and Gondwana have long been identified by many authors. These structures were initially identified from topography, coast lines and geological data but more recently from remote sensing and geophysical data. Here gravity and aeromagnetic data for Africa and South America have been used to identify, track and model these structures at depth within the crust/upper mantle. The two structures investigated are the West and Central African Rift System (WCARS) and the Kandi Shear Zone (KSZ). WCARSthe nature and spatial extent of the WCARS have been identified using gravity data. In its simplest form, the WCARS consists of a set of strike-slip shear zones linked to extensional rift basins that can be traced as continuous structures from the Atlantic coast of Nigeria to southern Algeria and from the coast of Cameroon to Kenya. The rift system developed as a set of discrete tectonic pulses throughout the Mesozoic at the same time as the Central, South and Equatorial Atlantic Oceans developed into a single unified spreading centre. Each tectonic pulse had a different basin response due to the changing orientation of the plate tectonic controlled displacement field. The African rift basins formed by crustal extension involving brittle failure and ductile flow of the upper and lower crust respectively, resulting in the isostatic subsidence of the surface and rapid infilling of the basins with mainly continental derived sediments. Gravity and seismological studies support this crustal basin model originally proposed by McKenzie. This contribution shows that the evolution of the WCARS is clearly linked to the oceanic plate tectonic processes. New analysis of the fracture zone geometry in the Central Atlantic shows that these rift pulses involving crustal extension, strike-slip faulting, stratigraphic unconformities and short episodes of crustal contraction e.g. Santonian and Maastrichtian events, which can be identified and correlated with changes in the azimuth of the fracture zone geometry. KSZThe second, shorter part of the paper uses predominantly aeromagnetic data to track the KSZ from the Atlantic coast of Benin north into central Algeria before splaying out beneath the Saharan Platform. To the south the KSZ is shown, prior to the formation of the Equatorial Atlantic, to link with the Trans-Brazilian Lineament but its continuation south of Brazil still remain highly speculative.Finally, this review raises new insights and questions that are ideal for future research topics, both in developing the analytic methodologies and their application to other continental scale datasets.

Pyrite Dissolution in the Cretaceous Yogou Formation of the Niger (Chad) Basin: Implications for Basin Evolution under a Rift Tectonic Setting.

This is the first study of pyrite minerals in the entire West and Central African Rift System (WCARS). Several polished organic-rich core samples from the Cretaceous Yogou Formation of the Niger (Chad) Basin located in the WCARS were investigated for their pyrite content using FE-SEM and SEM-EDS imaging techniques. An attempt was made to classify the types and provenance of the pyrites and to highlight the control of rift fractures on the oxidation and dissolution of pyrites in the region. Three major types of pyrites are present in the studied formation, including euhedral pyrite (EPy) crystals, pyrite framboids (FPy), and sunflower pyrites (SPy). A statistical analysis of 307 FPy shows that the framboids are diagenetically formed with an average diameter of 6.61 μm. SPy is present in a relatively low amount compared to framboids. The pyrites underwent a variety of diagenetic modifications, from mechanical compaction to oxidation, dissolution, and recrystallization. Unoxidized pyrites primarily contain Fe, S, and C, but oxidized pyrites also contain O, Al, and Si. There is a strong correlation between the fractures and the spatial distribution of the physicochemical alteration of the pyrite in the study. Dissolution in relatively deep-buried samples occurs mainly along fracture planes. The fractures provide a pathway for oxidants and other metal elements to reach the pyrites. The pattern of pyrite dissolution reflects the timing of fracture formation and fracture activities as a purveyor or drainage for fluids in the organic-rich samples investigated. The pyrites are associated intimately with organic matter (OM); thus, the relationship between the fracture and the pyrites' transformation is significant in the assessment of organic matter preservation at deep-burial depth.

Optical and geochemical assessment of petroleum source rock potential and palaeodepositional environments of Cretaceous mudstones, Koum Basin, North Cameroon

Limited organic matter (OM) characterisation studies on Cretaceous mudstone successions in the Koum Basin have resulted in a significant knowledge gap in the source rock potential; a petroleum system prerequisite. Field mapping and lithofacies analysis of outcrop sections point to fluvial channel and marginal to profundal lake deposits. Representative mudstone samples were evaluated using multiproxy organo-petrographic and bulk geochemical analyses to assess the kerogen type, thermal maturity, hydrocarbon generation potential, and deduce the paleodepositional environment. Results reveal that ∼90% of the samples are organic-lean and ∼10% organic-rich with TOC content of 2.81–6.91 wt%. Rock-Eval pyrolysis S2 yields of 14.50–54.18 mg HC/g rock and hydrogen index (HI) 784–846 mgHC/gTOC, suggest excellent potential for oil generation. High S1 values of 1.18–1.45 mg HC/g rock and low production indices (PI) of 0.02–0.03 indicate in situ generated free hydrocarbons. PI and Tmax of 438°C–443 °C, consistent with measured vitrinite reflectance (Ro% = 0.28–0.53%), Spore Colour Index (SCI = yellow to golden brown) and Thermal Alteration Index (TAI = 1.5 to 2.6) on the Chevron scale, portraying an immature to marginally mature OM. Oil-prone kerogen Type I-II dominates and to a lesser extent gas-prone Type III, derived from algal-terrestrial organic facies, typified by the predominance of amorphous OM 51.2%, liptinites 24.6%, vitrinite 16.4% and inertinite 7.8%. Based on lithofacies, and palynological evidence, a fluvio-lacustrine depositional setting with limited marine influence, under fluctuating warm humid to hot semi-arid paleoclimates, is inferred. A comparison with surrounding petroleum-producing basins further enhances the understanding of the Koum Basin's hydrocarbon potential within the West Central African rift system context.

Predicting Granitic Buried-Hill Reservoirs Using Seismic Reflection data—A Case Study From the Bongor Basin, Southwestern Chad

Predicting the distribution of fractured and dissolved reservoirs in granitic buried hills is a prerequisite for finding high-quality hydrocarbon reservoirs and improving the success rate of exploration activities. Granitic buried-hill reservoirs in the Bongor Basin are characterized by strong heterogeneity and poor lateral continuity. Through the analysis of seismic, core, logging and dynamic production data, this study predicts the distribution of fractured reservoirs and fractured-cave reservoirs in the granitic buried hills in the BC Block of the Bongor Basin. In this study, we predict fractured reservoirs using coherence cubes and linearly enhanced attributes and identify fractured-cave reservoirs with single-frequency attribute bodies. Integrated predictions of fractured reservoirs and fractured-cave reservoirs are conducted using attribute fusion techniques. With these methods, good results have been achieved in reservoir prediction in BC buried hills. Furthermore, this study summarizes a set of granitic buried-hill reservoir prediction techniques for densely inverted rift basins in the Central African Rift System.

Delineating structural features of North Cameroon using the EIGEN6C4 high‐resolution global gravitational model

The Cretaceous basins of the West and Central African Rift System (WCARS) are characterized by sinistral strike–slip movements observed between the Benue and Arabian–Nubian microplates. These basins share a similar basin‐filling structure, history, and tectonic processes. They have attracted a significant interest in the study of their geodynamics and subsurface structure through geophysical exploration methods. Nevertheless, North Cameroon located in this microplate confluence has not yet been investigated in terms of identifying geological structures, such as lineaments and faults. To contribute to a better understanding of the northern Cameroon structural framework and its tectonic context, we investigate the lineaments in northern Cameroon by using the EIGEN‐6C4 gravitational model and a variety of recent processing techniques (Tilt derivative [TDR], Theta Map [TM], Horizontal tilt angle [TDX], and Edge Detector [ED]). Our results show that the lineaments are trending in ENE–WSW, NW–SE, NE–SW, and WNW–ESE directions. These directions are consistent with the Cretaceous Garoua Rift and granitoids of the Cameroon‐Chad line. The presented result clearly delineates continuous architectonic features of the structural edges of the Saharan Metacraton, the Cretaceous Garoua Rift, the Bibemi‐Léré granitoids, and the Adamawa faults.

Discovery of Early Mesozoic Magmatism in the Northern Muglad Basin (Sudan): Assessment of Its Impacts on Basement Reservoir

The crystalline basement in the Central African Rift System (CARS) has been confirmed as a good unconventional reservoir in the surrounding basins of the Muglad area, Sudan. This study has recognized at least five Early Mesozoic intrusions for the first time at the northern margin of the Muglad Basin. Field and petrographic investigations show that they are mainly granite, syenite, diorite, and monzonite, with dominant alkali feldspar, plagioclase, quartz, and minor hornblende, biotite, and magnetite. Zircon internal structures, Th/U ratios, and rare earth element (REE) patterns indicate their magmatic origin. A total of 210 zircons from five samples were analyzed using LA-ICP-MS and they yielded U-Pb ages from 195.0 ± 6 to 225.0 ± 3 Ma, belonging to the Late Triassic-Early Jurassic. These results suggest that the Muglad Basin basement is not universally Precambrian as previously considered. The older basement was intruded and modified by Early Mesozoic magmas as evidenced by field contacts and our dating results. These magmatic events produced diverse Mesozoic igneous rocks prior to the formation of the Muglad Basin (Early Cretaceous), becoming new members of the crystalline basement. This composite basement then experienced prolonged weathering and fracturing for 65–95 Ma, resulting in porous and permeable weathered layers or fractured zones, making the basement a good unconventional reservoir for oil and gas.

Petrography and geochemical framework of guébaké dolerites dyke swarms (north Cameroon, central Africa)

Dolerites dykes of 4.5 to 25 m wide extend from 200m to 1km, crosscut the Guébaké granitoids basement in northern Cameroon along EW to N100E directions. Petrographic studies reveal the microlitic porphyritic texture to classical doleritic texture of ophitic to sub-ophitic types. Guébaké dolerites are mainly composed in various proportions of skeletal plagioclase and feldspar, clinopyxene, amphibole and oxides crystals. ICP-MS and ICP-AES geochemical analyses have distiguished a lava series composed of trachybasalts, basaltic trachyandesites, trachyandesites, trachytes and rhyolites of continentale tholeiites affinity. Lavas have been differenciated trough fractional crystallization process coupled with crustal contamination and metasomatism. Guébaké dolerites are products of relatively high partial melting rate of E-MORB mantle component. They stand as fingerprints of the post pan African crustal consolidation and precursor of the development of central African rift system at Cretaceous times. Â

NEOCOMIAN – BARREMIAN LACUSTRINE SHALES IN THE MAYO OULO‐LERE BASIN, NORTH CAMEROON: DEPOSITIONAL ENVIRONMENT AND PETROLEUM POTENTIAL

The synrift Mayo Oulo‐Léré Basin in Northern Cameroon is located in the transition zone between the West and Central African Rift Systems. Structural and stratigraphic elements of the basin resemble those of the Yola Basin in NE Nigeria, an extension of the Upper Benue Trough. The Lower Cretaceous lacustrine shales with source rock potential which occur in nearby rift basins are also present in the Mayo Oulo‐Léré Basin. These shales were investigated at two outcrop locations (Badesi and Tchontchi), and samples collected (n = 60) were subjected to palynofacies and bulk geochemical analyses to evaluate their petroleum generation potential and to interpret their depositional environment. At the studied locations, shales were divided into two lithofacies: grey to black laminated shales containing algal‐bacterial OM together with common woody (phytoclast) debris (“facies Fml”); and cm‐bedded shales which had a higher content of algal‐bacterial OM but a lower phytoclast content (“facies Fmlc”).Palynological and bulk geochemical data indicate that the shales contain well‐preserved organic matter (OM) and locally display good to excellent oil generation potential. Average TOC contents are 2.7% and 1.4% for samples of the Fmlc and Fml facies shales respectively. HI values (94‐889 mg HC/g TOC and 131‐638 mg HC/g TOC respectively) suggest that the shales contain Types I to III kerogen. Organic material in the Fmlc facies shales is dominated by amorphous organic matter (AOM: 90% on average) with a low phytoclast content (6% on average); whereas samples of the Fml facies shales contain less AOM (74% on average) and have a higher phytoclast content (23% on average). AOM in the Fmlc shales is highly fluorescent; these shales are interpreted to have been deposited in dysoxic to anoxic conditions. The AOM in the Fml shales is weakly fluorescent and the shales were deposited under more oxic conditions. The kerogen in the shales ranges from immature to early oil window mature. Average values of the pyrolysis S2 yield are 15.5 mg HC/g of rock and 7 mg HC/g of rock for samples from Fmlc and Fml facies shales respectively. The shales increase in thickness northwards towards the Logone Birni Basin where they may have reached the oil window, as in neighbouring basins. The results of this study of lacustrine shales from the Mayo Oulo‐Léré Basin suggests that there may be potential for oil exploration in northern Cameroon.

The crustal configuration of the West and Central African Rift System from gravity and seismic data analysis

SUMMARYThe West and Central African Rift System (WCARS) is the only stable continental geological structure on Earth that is formed by large-scale topographic massifs (swells). However, knowledge of the origin and evolution of the WCARS remains limited mainly due to the scarcity of high-resolution geophysical observations. To better understand the crustal structure beneath the entire WCARS, we use the XGM2016 global gravitational model and the ETOPO1 global topographic-bathymetric model to determine a gravimetric Moho (crust-mantle boundary) model constrained by seismic Moho depth estimates at 41 seismological stations distributed irregularly within the study area. The result reveals a regional Moho deepening to ∼40 km beneath the Hoggar, Aïr and Tibesti Massifs. The largest Moho deepening to ∼46 km is detected beneath the Archean Congo Craton, while the Moho depth under the Adamawa Plateau reaches 42 km. The Moho geometry beneath the Chad Basin, the Chad Lineament and the Termit Basin is relatively even with the Moho depth mostly within 24–26 km. A significant Moho deepening as well as large Moho depth variations within 32–45 km beneath the Saharan Metacraton and the Congo Craton (especially under its northern margin) reflect the metacratonization processes that occurred during the Neoproterozoic. The Niger Delta and the Benue Trough are characterized by a very thin continental crust with the Moho depth varying from ∼20 km in the south along the Atlantic coastline to ∼24 km in the northeastern branch of the Cretaceous Benue Trough around the Garoua-Yola Rift.