Theoretical Development of a Spatially Explicit Model for Hydrocarbon Migration Pathway Prediction in the Deepwater Pre-Salt Carbonate System Offshore South East Nigeria

Hydrocarbon migration pathway prediction in pre-salt carbonate systems offshore South East Nigeria involves modelling fluid buoyancy-driven flow through heterogeneous carbonate fracture networks overlain by evaporite seal sequences, a geological configuration for which existing migration modelling theory, primarily calibrated to siliciclastic Niger Delta conditions, is inadequate. This study develops a theoretical model for hydrocarbon migration pathway prediction specifically applicable to the deepwater pre-salt carbonate system offshore South East Nigeria. The theoretical framework integrates a buoyancy-capillary flow invasion percolation model for primary migration in fractured carbonate host rock with a salt tectonics-derived structural restoration framework that tracks the evolution of migration pathways through geological time as salt movement modifies structural geometry. An original percolation threshold function incorporating fracture aperture, mineralisation, and stress-dependent permeability is derived from calibration to fractured carbonate analogue outcrop data from onshore Nkalagu exposures and subsurface image log data. Three-dimensional migration pathway simulations are constructed for a regional-scale geological model built from available seismic and well data in the study block. Results identify four potential migration fairways where thermally mature pre-salt source rocks charge Cenomanian carbonate reservoirs through relay faults in the evaporite seal. Charge risk assessments are developed for mapped structural closures, reducing exploration uncertainty by providing a ranked migration pathway probability for each prospect. Keywords: hydrocarbon migration, pre-salt carbonate, invasion percolation, migration pathway prediction, deepwater Nigeria.