Lithofacies Analysis of Multistorey Braidplain Sandstone, Lokoja-Bassange Formation, Benin Flank of Anambra Basin, Nigeria

Abstract

A braidplain sandstone body in the Cretaceous Lokoja-Bassange Formation was studied for the purpose of classifying its constituent lithofacies, developing a conceptual sedimentological model that reflects its alluvial architecture, and assessing the implications of this architecture for reservoir development. The NE-SW trending sandstone is approximately 1.2 km long, 55 m wide and, at least, 15 m thick. Four sedimentary logs, produced at selected points across the sandstone body, were combined with its sedimentary attributes to construct a conceptual sedimentological model that captures its geometrical framework. The morphology of the channel sandstone is classified into three groups based on decreasing magnitude: (1) cross-bedded, tabular morphology with sheet geometry, extending to maximum length of 1,200 m and maximum thickness of 15 m, (2) multistorey channel morphology with laterally stacked geometry, extending to 90 m in length and 8 m in thickness, and (3) single channel morphology with lenticular geometry, extending over lateral and vertical distances of 2-5 m and 1.7 m, respectively. Amalgamation of beds is common, enabling thick laterally extensive channel-fill sequence to develop. The sequence consists of pebbly sandstone and medium to very coarse sandstone with claystone clasts that are randomly distributed. The sedimentological model suggests that the Campanian braidplain sandstone has a complex history of bar migration that produced laterally-stacked multistorey channel sandstone. This architecture will facilitate effective pressure communication across producing reservoir intervals. Lack of organised stratification and discontinuous mudstone may turn these reservoirs into a single hydraulic unit that supports effective lateral displacement of fluids to production wells. Such laterally connected channel sandstone, however, requires a trapping mechanism for hydrocarbon to accumulate and impermeable floodplain mudstone to preserve it. Laterally extensive floodplain mudstone, on the other hand, poses a major risk to vertical flow because of its potential for flow segregation across net pay intervals.