Wamba Field, offshore Cabinda, People’s Republic of Angola, produces significant quantities of oil from sandstones of the Upper Cretaceous Vermelha Formation. This sequence of mixed carbonate-siliciclastic sediments was deposited as a transgressive-regressive barrier island system. Approximately 320 ft (97.5 m) of conventional whole core was examined in order to define depositional facies and understand their relationships. Three principal facies were recognized: (1) inner shelf to shoreface sediments characterized by bioturbated, argillaceous, dolomitic siltstones, very fine-grained sandstones, and silty/sandy dolowackestones; (2) beach/barrier bar complex ranging from well-sorted, very fine- to fine-grained to poorly sorted, very fine- to very coarse-grained subarkoses and arkoses commonly containing planar and low-angle planar cross-laminations; and (3) lagoon and tidal-flat sediments composed predominantly of argillaceous dolomudstones and wackestones punctuated by dolomitic sandstones.

In this reservoir complex facies relationships and diagenesis cause extremely variable reservoir properties. Generally the sandstones of the beach/barrier bar complex exhibit the best overall reservoir quality resulting from sparse cementation, preserving intergranular pores and some secondary porosity development. The shoreface and lagoon facies exhibit the poorest reservoir quality and may act as local or regional permeability barriers. The mixing of siliciclastic and carbonate rocks occurs primarily by lateral facies mixing (spatial variability) along facies boundaries and transition zones, and by storm-punctuating processes. The larger-scale mixing variations in the stratigraphic succession (temporal variability) are caused by longer-term eustatic sea-level changes.