Thermomechanical Modelling of the Tertiary Basin Formation and Subsidence History of the Anambra Basin

This study develops an original thermomechanical model for the formation and subsidence history of the Anambra Basin, South East Nigeria, integrating geophysical constraints to produce the first quantitative tectonic analysis of this important sedimentary basin. The Anambra Basin occupies a structural sag between the southern Benue Trough and the Niger Delta, but its formation mechanism, the transition between Benue Trough rifting and Niger Delta passive margin thermal subsidence, and the implications for petroleum maturation remain poorly understood. Thermomechanical modelling provides the framework for quantitatively testing alternative formation hypotheses using subsidence, heat flow, and seismic velocity constraints. This study develops a finite element thermomechanical code incorporating rheological stratification of the lithosphere, syn-rift crustal extension, thermal subsidence, flexural isostasy, and sediment loading effects. The model is calibrated against stratigraphic constraints from reinterpreted 2D seismic data (260 kilometres of newly acquired profiles) and geothermal gradient data from eight deep exploration wells. Backstripping analysis of five composite stratigraphic sections provides the subsidence history curves used for model constraint. Findings reveal that the Anambra Basin formed by two-phase subsidence: a mid-Cretaceous rifting phase (beta extension factor of 1.35) followed by Maastrichtian-Eocene thermal subsidence and loading. The absence of significant volcanic activity distinguishes it from syn-rift processes in the main Benue Trough. Thermal maturation modelling shows the Campanian Nkporo shale is currently at peak oil generation phase in the deepest basin sections, providing an important constraint on petroleum system timing. The study establishes an original basin evolution model with direct implications for exploration timing and fairway delineation.

Keywords: thermomechanical modelling, basin formation, Anambra Basin, subsidence history, petroleum maturation.