Characteristics of marine carbonate reservoirs in the Northern Lower Congo Basin

The Lower Congo Basin of west Africa, a passive continent-marginal and salt-bearing basin, is located in the north of offshore Congo (Brazzaville). Abundant hydrocarbon accumulations have been found in the Cretaceous carbonate reservoirs there. However, critical problems still remained with hydrocarbon exploration and development of the Cretaceous marine carbonate reservoirs, such as the deposition and evolution of the carbonate deposits, and the characteristics and control factors of the origin of reservoirs. Study of the deposition of marine carbonate reservoir characters reveals that relative sea-level changes influenced the deposition and evolution of the carbonate deposits during the period from Cretaceous Sendji Formation to Madingo Formation, suggesting facies changes from the offshore carbonate ramp platform facies to shallow shelf and submarine fan facies. There are three sets of carbonate reservoirs in the basin. The carbonate reservoirs of the study area mainly occur in the Lower Cretaceous Sendji Formation, which belongs to the shallow marine carbonate ramp platform system. Four subfacies are recognized, which includes the mixed shore, back ramp, shallow ramp and deep slope. Reservoir are well developed in the grainstone shoal microfacies of the shallow ramp subfacies, and the shoal and sandy shoal microfacies reservoirs in the back ramp subfacies. The lithologic assemblages of carbonate reservoirs in the study area mainly include four types: grainstone, sandstone, sandy limestone and dolomite, controlled by sedimentary facies and influenced by diagenesis.