The Importance of Diagenetic Processes in Sandstones Facies of the Hamakoussou Sedimentary Basin in North Cameroon: Influence on Reservoir Quality.

Published studies in the Hamakoussou reservoir sandstones are very few and the characterization of the reservoir quality including diagenesis is unknown. In this paper, after lithological reports, classical petrographic techniques have been used to study the diagenesis and reservoir quality of the Hamakoussou sandstones: Diagenetic processes within and around detrital grains show that early cementation by calcite come from volcanic veins and late cementation originating from silicification. Diagenetic phenomena (early cementation, compaction, fracturation and late cementation) show that these sandstones have a low porosity due to the blockage of intergranular pore spaces by cement. Intense volcanic activity associated with the circulation of fluids (silica and calcite) as well as the dissolution along the contacts of quartz grains are the principal sources of early and late cements which are responsible for the decrease in porosity observed in these sandstones. The immediate consequence is the sudden drying up of boreholes drilled for water supply

Magnetostratigraphy and sedimentology in the cretaceaous formations of the Hamakoussou and Mayo Oulo-Léré basins, Northern Cameroon (Benue Trough)

Les bassins sédimentaires d'Hamakoussou et du Mayo Oulo-Léré sont des bassins à sédiments continentaux. Ils se situent en territoire camerounais, dans le prolongement de la branche de Yola de direction Est-Ouest du Fossé de la Bénoué dont la mise en place est liée à l'ouverture de l'Atlantique Sud à partir de la dislocation du Gondwana. L'âge de ces bassins est basé sur des études biochronologiques les situant dans le Crétacé inférieur. La série sédimentaire est sensiblement la même dans les deux bassins, elle est constituée de dépôts fluviatiles à fluviolacustres montrant une alternance de grés, de siltite et d'argilite.Des prélèvements paléomagnétiques ont été effectués sur des niveaux fins situés dans la partie supérieure de la série sédimentaire de chaque section. Dans le bassin sédimentaire d'Hamakoussou, 50 échantillons sur 11 sites d'épaisseur égale à 69 m, ont été prélevés dans la localité de Djallou et 78 échantillons sur 20 sites d'une épaisseur de 511 m dans la localité d'Ourokessoum. Dans le bassin du Mayo Oulo-Léré, 116 échantillons sur 45 sites sur une épaisseur de 478 m ont été prélevés dans la localité de Tchontchi.La désaimantation progressive au champ alternatif et à la température de ces échantillons montre que les séries sédimentaires de ces bassins portent une aimantation primaire. Les directions d'aimantation ont permis de déceler une tectonique régionale marquée par un mouvement de rotation autour de l'axe vertical et un mouvement de translation des blocs. Les minéraux magnétiques porteurs de cette aimantation sont de faible, moyenne et de forte coercivité. Une séquence de trois polarités a été déterminée le long de chaque section du bassin d'Hamakoussou : une polarité inverse et deux polarités normales. Dans le bassin du Mayo Oulo-Léré, la section Mayo a livré deux polarités dont une normale et une, inverse. La séquence de polarités obtenue pour chaque coupe a été corrélée avec l'échelle de temps de polarités magnétiques. Les trois polarités des coupes du bassin d'Hamakoussou ont été corrélées avec les chrones M1 et M3 avec un âge compris entre 125 Ma et 128,11 Ma. La séquence d'inversions de la coupe du Mayo Oulo-Léré a été corrélée avec le chrone M1 avec un âge compris entre 125 Ma et 127,61 Ma. Le taux de sédimentation des dépôts du bassin d'Hamakoussou varie entre 5,5 cm/ma et 40,5 cm/ma et est de 38 cm/ma dans la section du Mayo Oulo-Léré.Hamakoussou and Mayo Oulo-Léré sedimentary basins are extension of the executive branch of East-West Yola Benue Trough, whose formation is related to the opening of the South Atlantic from the dislocation of Gondwana, in the Cameroonian territory. An age range of lower Cretaceous to Barremian-Hauterivian limit has been assigned based on biochronological studies. The sedimentary sequence in both basins is composed upward finnings fluivial to fluviolacustrine deposits. The deposits present an alternation of fine grained sandstone, siltstone and mudstone overlying coarse sandstone which is underlain by micro-conglomerate to conglomerate facies at the base.A magnetostratigraphic study has been carried out on a fine sediments in the upper part of each sections. 50 samples from about 69.03 m thickness, were collected from 11 sites at Djallou and 78 samples, about 511.03 m thickness, were collected from 20 sites at Ourokessoum localities in the Hamakoussou basin ; and 116 samples, about 478.19 m thickness, collected from 45 sites at Tchontchi locality in the Mayo Oulo-Lere basin.Specimens subjected to progressive alternating field and thermal demagnetization show that the sedimentary sequences have a primary magnetization. The directions of magnetization indicate a regional tectonic marked by a rotation and translation block. Rock magnetic investigations reveal the presence of both high and low coercivity minerals. A sequence of three polarities was determined along each section of the Hamakoussou basin: one reversal polarity and two normal polarities, whereas two polarities: (normal and a reversal) were determined along Mayo section in the Mayo Oulo-Lere basin The three polarities sequences from the two sections from Hamakoussou basin are correlated with M1 and M3 Chrons and suggest an age between 125 and 128, 11 My. While the two polarities sequences from the Mayo Oulo-Lere basin are correlated with M1 Chron, and suggest an age between 125 and 127.61 My. The sedimentation rates of Hamakoussou basin deposits vary between 5.5 cm/kyr and 40.88 cm/kyr and 38.26 cm/kyr in the section of Mayo Oulo-Lere basin

Tectono-stratigraphic evolution and architecture of the Miocene Rio del Rey basin (Cameroon margin, Gulf of Guinea)

International audiencehe Rio Del Rey basin located in the Gulf of Guinea is one of the two basins lining the Cameroonian coast. It evolved since the Cenozoic in a tectono-magmatic and seismic complex setting. During the Miocene, this evolution was controlled by tectonics, relative sea level changes, basin filling and mantle dynamics. Two deposit megasequences of second order, MS1 and MS2, were pointed out. MS1 (lower Miocene), aggradational and corresponding to the Isongo turbidites deposited during a period of relative sea level drop registered two sequences of third order differentiated by the degree of channel incision: S1a (Aquitanian-Burdigalian), hardly incised and S1b (Serravallian), moderately incised. A double extension along N-S (Burdigalian) and NE-SW (Serravallian) directions responsible for a set of horsts and grabens, high subsidence and depocentres trending NW-SE affected these sequences. MS2 (middle to upper Miocene), progradational and corresponding to deltaic alternations deposited during a period of creation of available space were controlled by the tilting, wrenching, a second stretching phase and low sedimentary filling. MS2 recorded three cycles of 3rd order: S2a (lower Tortonian), S2b (upper Tortonian) and S2c (Messinian) differentiated by their morphology in ramp for S2a and S2b and slope for S2c. The formation of grabens and horsts and the tilting of the margin coincide with the high activity of the Cameroon volcanic line interpreted as a result of deep mantle dynamics affecting the inner Congo craton lithosphere. This architecture, which differs from those of the other basins of the Gulf of Guinea, partly explains the low oil production of this basin