Modelling of high overpressure in an off-shore Nigerian Basin: slant fault models with non-horizontal beds

The main feature of this young (9.5 Ma) off-shore Nigerian basin is rapid sedimentation of shales leading to very high overpressure development. The ratio of total fluid pressure to sedimentary load is estimated to be between about 0.9 to 0.95 at the bottom of well A, while the main tectonic activity appears to be gravitational, characterized by the development of growth faults. To model these conditions, where slant faults could have played an important role in the development of overpressure, a 2-D fluid-flow/ compaction model must reproduce the geometry of the basin according to the development of faults and in relation to their possible effects as pathways for fluid migration.

For this reason, the 2-D model is designed to respect, as much as possible, the geometry of the system. A significant improvement over previous uses of the 2-D fluid-flow/compaction code is the ability to account for inclined layers around faults, instead of only horizontal layers. As a consequence, the basin geometry is much better reproduced. This improvement requires a fine grid of points and a fault throw less than the thickness of the smallest layer offset by the fault, so that no layer is “missing” at any position along the fault. The evolution of excess pressure or temperature with time, especially when fault occur, is consistent with results obtained synthetic tests and using flat layers around faults. Finally, the model reproduces correctly the measured fluid data. To obtain such overpressure, the fracturing must be higher than 0.9 to 0.95, as estimated by previous studies, and the faults cannot be always open.