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Quantitative analysis of vitrinite reflectance data from the Anambra basin, Nigeria, has revealed for the first time tectonism and unconformities corresponding to the Campanian-Maastrichtian and Paleocene-Eocene boundaries. This analysis also afforded definite subsurface evidence for the Cenomanian as well as the Santonian deformations. The estimated thickness of each rock section that was removed by erosion to give the Cenomanian, Santonian, and upper Campanian unconformities is approximately 373 m, 512 m, and 217 m, respectively. The estimates for the Cenomanian and Santonian erosion also represent the height of the corresponding uplifts in the lower Benue trough. The thicknesses of the rock sections eroded upon cessation of subsidence in the Anambra basin in the early ertiary to produce the present topography vary systematically from 73 to 3063 m. Therefore, the reconstructed depth corresponding to the top of the oil window in the early Tertiary ranges from 1521 to 5127 m (a temperature range of 81 to 191°C).
Because the top of the oil window was located at that time within the Nkporo Shale or younger units, the Eze-Aku Shale, Awgu Shale (Nkalagu Formation), and, in many cases, the Nkporo Shale were originally placed at temperatures (>150°C) that favored mainly generation of gaseous hydrocarbons in many parts of the basin. This could have played, at least, a supportive role in producing the predominance of gas plays in the Anambra basin. Hence, reconstruction of the maximum burial depth (and paleotemperature) of viable source beds is recommended as necessary input into a successful exploration strategy for the basin.
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