Lower Congo Basin Coupled System—Linked Up-Dip Extension and Down-Dip Contraction: Does it Add Up?

Kendall, Jerome, Richard R. Gottschalk, Ilsa Schiefelbein, Arlene Anderson, ExxonMobil Exploration Co, Houston, TX

The Lower Congo Basin of offshore Angola contains a coupled system of linked up-dip extension and down-dip contraction. A series of sequentially restored cross sections encompassing the complete system from the onshore basement outcrops to the oceanic crust tracks the evolution of the basin. The Lower Congo Basin is well imaged, relatively simple and small compared to other linked systems such as the Gulf of Mexico and Niger Delta. An investigation of the distribution and magnitude of the up-dip extension and more poorly defined down-dip contraction helped constrain the entire system. The three main pulses of deformation occurred in the Albian, Latest Cretaceous to Paleocene, and Late Oligocene to Early Pliocene. A systematic tracking of the strain illustrates a rough balance between the extension and contraction for each deformational pulse.

For the Late Oligocene to Early Pliocene phase of deformation, integration of isochore maps and the regional cross sections demonstrate the spatial and temporal distribution of deformation. The gradient of strain increases in proportion to the depositional load. Lateral variations in the system are related to sediment load, base salt topography, tectonic tilt, and relative thickness of salt and sediments. The resulting Neogene sediment load is inboard of the older sediment loads. This retrograding depositional load is different from the normal passive margin prograding loads. The Lower Congo Basin coupled system controlled the distribution of load, reservoir, and traps, creating an exceptional petroleum system