Wamba Field, People’s Republic of Angola, a Cenomanian Mixed Carbonate-Siliciclastic Reservoir
Anthony J. Lomando, Tracy L. Walker, 1991. "Wamba Field, People’s Republic of Angola, a Cenomanian Mixed Carbonate-Siliciclastic Reservoir", Mixed Carbonate-Siliciclastic Sequences, Anthony J. Lomando, Paul M. Harris
Download citation file:
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.
Figures & Tables
Mixed Carbonate-Siliciclastic Sequences
The study of carbonate-siliciclastic mixed sequences has seen an increase in the number of investigations that focus on mixed settings as part of the continuum between the carbonate and clastic end members. Cyclic deposition in mixed basins by reciprocal sedimentation has become one of the foundation blocks for sequence stratigraphy. In addition, these mixed sequences have a variety of distinctive petroleum reservoir characteristics, important for both exploration and development programs. The emphasis now is on reevaluating ancient sequences in the light of a more dynamic understanding of spatial and temporal variations and controls on these sequences. Examples in this volume are subdivided under the following headings: Shelf Wide, Coastal and Inner Shelf, Middle to Outer Shelf, Slope to Basin and Paleokarst. Many mixed sequences have been described in the literature, but understanding the controls of these sequences from a process approach in now in an adolescent stage.