Abstract Research in Cretaceous shales from West Africa has demonstrated that significant permeability can develop within shales at shallow depths (<100 m), equivalent to a permeability of >1 m day −1 . Much of the variation in permeability is related to the degree of burial metamorphism, with shales that have been altered and that approach the anchizone having the highest permeability and those that are largely unaltered (early diagenetic zone) having the lowest permeability. However, further research targeting largely unaltered shales dominated by smectite clay has shown that the presence of small igneous intrusions can radically alter the hydrogeology. Twenty-four exploratory boreholes were drilled into smectite-dominated shale and nine of these boreholes were targeted to small dolerite intrusions within the shale. The dolerite was intensely fractured at the intrusion edge, with significant zeolite growth along the fracture surfaces. The permeability in the fractured dolerite was the highest measured in any shale borehole, with transmissivities of up to 60 m 2 day −1 measured from pumping tests. Fracturing was less where dolerite was intruded into sandstones, however, and the measured transmissivity was lower (<0.5 m 2 day −1 ). We postulate that the low permeability and high water content of the shale enabled high pressures to develop during intrusion, facilitating the development of fractures along the intrusion contact zone.

Abstract Nigeria’s under explored and poorly understood frontier basins include the Nigeria, Chad, Bida, Dahomey, Sokoto, and Benue basins. They have their origins in the multi-phased rift systems that were formed during the breakup of Gondwanaland in Early Cretaceous time. The rifting is widely attributed to the stretching and subsidence of the African crustal blocks accompanied by reactivated plate movements in the early Tertiary. These basins are part of the West African Rift Subsystem (WARS) of the West and Central African Rift Systems (CARS). The Chad basin, the largest, is an intracratonic rift basin having an area of 2,335,000 km 2 that covers Chad, Niger, Cameroun Republics, and the northeastern part of Nigeria. Only about 10% of the Chad basin lies within Nigeria. It is a two-stage rift basin comparable with the petroliferous south Chad basins (Doba, Doseo) by having (A) a Lower Bima early rift stage generated by east–west gravity faults of Albo-Aptian age, followed by an Upper Bima Sag phase (Albian); and (b) an Upper Cretaceous rift phase with deposition of lower Fika source rocks followed by a mild Tertiary sag phase corresponding to the sedimentation of Lower Kerri Kerri and Chad formations. Of the 23 dry wells drilled in the Chad basin, only the Wadi-1 and Kinasar-1 wells recorded non-commercial gas accumulations. Three possible petroleum systems have been mooted. The petroleum systems of this and other Nigeria frontier basins are discussed. Suggestions are made for the successful search for hydrocarbons in these basins.

Alkaline magmatism along the Cameroon Line has been active for at least 67 m.y. and is currently defined by an almost SW-NE geological lineament (mean value: N30°E). Available petrological, geochemical, and structural data obtained over the last 20 yr lead us to reappraise its mechanism of emplacement. Known as the second most important geological curiosity in Africa, after the East African Rift system, it displays a continental part and an oceanic part, a unique feature in Africa and even in the world. The continental part contains both plutonic and volcanic massifs, and the oceanic part consists only of volcanic massifs. Plutonic rocks as a whole define a complete series of gabbro-diorite-monzonite-syenite-granite type, whereas volcanic rocks display abundant basic (basalt-hawaiite) and felsic (trachyte-phonolite-rhyolite) lavas with very few intermediate ones (mugearite-benmoreite). The formerly entire alkaline nature of these rocks is here ruled out by the discovery of volcanoes with geochemically transitional affinities in some areas of the continental sector. On the other hand, new K-Ar and 40 Ar/ 39 Ar dates confirm the absence of any age migration associated with the SW-NE linear trend. This lack of steady time-space migration and the SW-NE trend have also been observed in the magmatic provinces of Nigeria and Benue Trough, which share similar geochemical features with the Cameroon Line, and along the NE-SW major igneous lineaments in South Africa. The mechanism of such episodic emplacement of alkaline magmatism can be better explained in terms of complex interactions between hotspots and lithospheric fractures during African plate motion.

Abstract Course Note 41 represents a unique documentation of the wide variety of structural styles in the offshore Niger Delta region. Twenty-five seismic profiles are presented for their structural and sequential stratigraphic interpretation. Each is accompanied by instructions and discussion and keyed to a line drawing of a regional profile. These materials, together with background information and a reprint of a key article, not only provide a realistic context for geologic education but they are also highly relevant to global deepwater exploration.

The Cretaceous coal-bearing facies of North and West Africa are directly related to the breakup of Africa and North and South America, global sea-level fluctuations, and the resulting marine transgressive and regressive cycles. In the Cretaceous, the coals of North Africa formed near the equator in a warm and humid climate, in contrast to the North American temperate coals. Lower Cretaceous coal-bearing facies have been reported in North Africa in Algeria, Egypt, Ethiopia, Libya, Mauritania, and Senegal. Upper Cretaceous coal has been reported in Benin, Egypt, Mali, Niger, Nigeria, Senegal, and Sudan. A better understanding of Cretaceous facies relationships and shoreline trends should lead to the discovery of more coal deposits within North Africa. Major marine transgressions and regressions associated with rifting, the breakup of Gondwana and Laurasia, and eustatic sea level cycles developed during Cretaceous time in North and West Africa. During the Mesozoic Era, most of Africa was above sea level and marine deposition took place only in the marginal basins associated with the rifting of the continents. The opening of the South Atlantic and the breakup of Africa and south America occurred along a north-south rift system that began in the south during the latest Jurassic and migrated northward in the Early Cretaceous, reaching Nigeria by mid-Cretaceous time. The first marine connection between the North and South Atlantic occurred during late Albian time. Final separation of the continents occurred during the Santonian, and the permanent seaway developed in early Turanian time. The Tethys sea advanced across the Saharan Platform during the late Cenomanian-Turonian, Coniacian, late Campanian-Maastrichtian, and the Paleocene. Concurrently, the South Atlantic transgressed through the Benue Trough and Nupe Basin and linked with the Tethys sea in the Chad and lullemmeden Basins, forming the four trans-Saharan epeiric seaways.