Abstract

Cretaceous sediments from the Bima (Albian-Cenomanian), Gongila (Lower Turonian), and Fika Shale (Turonian-Santonian) Formations were investigated from three exploratory wells (Kanadi, Albarka and Mbeji) situated along a SW-NE transect in the Bornu (Chad) basin, northeastern Nigeria. These sediments were analysed by organic geochemical (Rock-Eval pyrolysis) and organic petrological (vitrinite reflectance) methods. The average total organic carbon content (TOC) of all studied formations exceeds 0.5%. The highest TOC values are recorded in the dark laminated shale within the upper part of the Fika Shale Formation (3.87%), where the TOC content increases from the southwestern edge to the centre of the basin (lake Chad area). Organic matter occurring in the Cenomanian-Santonian formations belongs to type III and especially type IV, except in the lake Chad area where mixed organic matter of types II and III is encountered in the Fika Shale Formation. The investigated sediments are mainly gas prone and their hydrocarbon potential is low. Only the dark laminated shale of the Fika Shale Formation has a higher gas and especially oil potential. Tmax and vitrinite reflectance values indicate that the sedimentary organic matter is immature to early mature within the Fika Shale Formation, while only early mature in the Gongila and the Bima Formations. The present depth of the "oil window" threshold ranges from ca. 1,400 m in the Kanadi well to ca. 2,100 m in the Albarka and Mbeji wells. The latter value, which characterizes the lake Chad area, agrees with the one found in the neighbouring basins of Kanem (Chad) and Agadem (Niger). The lesser depth of the threshold of the "oil window" in the Kanadi well could be related to magmatic activity in the Senonian. The maturation gradients from vitrinite reflectance in the Kanadi (0.52 Ln %Rr/km), Albarka (0.41 Ln %Rr/km) and Mbeji (0.35 Ln %Rr/km) wells decrease from the Maiduguri depression (SW) to the lake Chad area (NE). Based on vitrinite reflectance profiles, the estimated amount of erosion during the late Cretaceous has shown various results. The erosion section in the Kanadi well is 1,000 m, while 900 m in the Albarka well, and up to 1,400 m in the Mbeji well. The first two values stand in reasonable agreement with the seismic data of the investigated area. The higher degree of sediment removal in the Mbeji well could be explained by a thicker sediment accumulation in the Maastrichtian and by a stronger uplift during the late Cretaceous, resulting in greater erosion. If these results are valid, they should be confirmed by vitrinite data drawn from above the unconformity in the Chad Formation. For the Kanadi well, the maximum burial of the Cenomanian-Santonian sediments was reached after the deposition of the Gombe Sandstone at the end of the Cretaceous and after the deposition of the Chad Formation during the late Tertiary and Pleistocene for the Albarka and Mbeji wells. The maximum burial temperatures calculated from vitrinite reflectance data range from 135 degrees C (3,290 m) in the Albarka Well to 151 degrees C (2,660 m) in the Kanadi well with an intermediate temperature of 144 degrees C (3,595 m) in the Mbeji well. The temperature palaeogradient in the Cenomanian-Santonian sediments decreases from the Maiduguri depression (42 degrees C/km) to the lake Chad area (33 degrees C/km to 28 degrees C/km). The Cenomanian-Santonian sediments reached their maximum temperature (151 degrees C) during the late Cretaceous at the northern edge of the Maiduguri depression, whereas in the lake Chad area, the maximum temperature (135 degrees C) was attained perhaps only during the Pleistocene. Hydrocarbon generation possibly began earlier (Turonian-Santonian) in the Maiduguri depression than in the lake Chad area (late Maastrichtian), where it could have continued through the Pleistocene.

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