Abstract: Salt Tectonics and Sedimentation: an Integrated Interpretation Ultra Deep Water Area, Lower Congo Basin, Offshore Angola

Dominey, Jerry R. - Shell E&P Angola & Susanne Wiffe - Shell International E&P

Recent exploration within the Lower Congo Basin outboard of the Tertiary deltaic depocenters has yielded major oil discoveries (Girassol, Dalia, etc.) in time equivalent turbidite reservoirs. Adjacent to this recent trend, but in deeper water, is the Ultra Deep Water (UDW) area, which is some 150 km from the Angolan coast in water depths averaging 2 km. The interpretation of this unexplored area was derived from an integration of regional well data, 2D seismic, gravity and magnefic data in preparation for a bid round scheduled for early 1998.

Within the Lower Congo basin, the Middle Cretaceous to Tertiary sedimentary section has undergone extensive gravity sliding on an Aptian salt layer. "Rafted" blocks commonly cored with Albian - Aptian carbonates rest on the detachment surface. The size, distribution, and amount of movement of these carbonate cored blocks, particularly up dip of the study area, have influenced turbidite deposition and salt deformation within the down dip UDW area. The key influences include:

- inter-raft lows down dip of shelf margins can develop as long lived "feeder" channels for funneling turbidites into the UDW area (Fig. 2),

- thrusting within the UDW area is influenced by compressional forces generated from continued down dip raft block movement due to Tertiary deltaic and turbidite depositional loading of up dip inter-raft zones.

Within the UDW area, variable amplitude discontinuous seismic facies are common and are interpreted as amalgamated turbidite channel / levees. The primary topographic controls on the channel flow directions are:

- salt cored highs (channel flows are deflected from the crest to the flanks),

- thrust induced highs (axial transport along the thrust front is common),

- subtle depositional relief generated by a succession of channel / levees and inter-channel areas.

Several "backstop" basins were observed in the southern portion of the study area. High turbidite depositional rates and differential loading of the underlying salt are characteristic of these mini-basins. Continuous seismic events, which are interpreted as turbidite sheet sands, frequently baselap onto salt.

Mobilization of the salt within the study area has resulted in a complex of salt diapirs, canopies, and thrust faults. The three main structural areas are (N to S):

- a small northernmost area of relatively thin salt (mainly deep) with little thrusting and NE / SW oriented pre-salt rifts,

- an extensive central area of complex thrusting and common salt canopies (gravity modeling indicates thin allochthonous salt),

- a large southern area of salt turtles to the E (underlain by a thick pre-salt section) with increasing thrusting and allochthonous salt to the W (Fig. 1). Mapping of the salt mini-basins, intrusives, and carbonate cored raft blocks were enhanced by the analysis of the marine and satellite Bouguer gravity data, 2D modeling, and regional / residual separation.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil