Abstract: Angola - The Lower Congo Basin Tertiary Petroleum Systems Hydrocarbon Distribution in Relation with the Structural and Sedimentary Evolution

Amaral, J.; Biteau, J. J.; and Zaroslinska, P. - Elf Exploration Angola; and DeCosta, L. - Sonangol

The Lower Congo Basin with up to 7 billion barrels of initial recoverable reserves discovered is the most prolific petroleum basin of Angola. The giant oil fields discovered the last two years on deep water blocks focused on Malembo section (Upper Tertiary) confirm the high interest of these plays.

Related to the opening of the Atlantic Ocean, the Tertiary West tilting of the margin and the uplift of the African craton started since Oligocene time linked to a global retrogression cycle. During this period, no sedimentation on Angolan margin and erosion of clastics led to an important influx of sandy turbidites to the West. This induces the development of a large and wide detritical deposition system related with the Congo river alluvial fan that reaches a maximum thickness of about 8500m.

The main depocenter or Tertiary trough in the Lower Congo Basin located in offshore at South of Congo river mouth is dominated by a structural extension domain and detritical deposition with deltaic and turbiditic unconsolidated sands. Structural traps locally with stratigraphic components are the exploration targets in this area.

Two different pressure domains are present in this Tertiary deep offshore basin:

- A geopressure domain restricted to the central basin section at burial depth less than -3000 m /SL., consequence of the high rate of sedimentation and thickness which induced a compaction disequilibrium in this vertical-stress dominated sand-shale depocenter;

- A hydrostatic domain, characterized by a less Tertiary sediments thickness in deep water (more than -1000 m/SL) and at low burial depth (500-1500 m).

Two types of marine and mature source-rocks contributed to the work of the petroleum systems in this Lower Congo Tertiary basin:

- The labe marine upper Cretaceous mainly amorphous type II oil prone with a high to very high initial petroleum potential,

- The deltaic and basinal Malembo series, Upper Tertiary in age, mainly amorphous type II - III with relatively low to medium initial petroleum potential.

Oil to source rock correlations demonstrate that hydrocarbons types and compositions are functions of these two source-rocks with a major contribution of the Upper Cretaceous one to the charge of the Tertiary plays.

Thermodynamic conditions encountered within the different reservoirs are closely related to the hydrocarbon characteristics, the burial of the traps and the location within the basin. On the western edge of the Tertiary extensional domain, these conditions range between 500 and 1500m of burial of traps and the location within the basin. On the western edge of the Tertiary extensional domain, these conditions range between 500 and 1500m of burial with a hydrostatic pressure regime and geothermal gradient between 4.0 and 5° C/Km. Gravity, viscosity of oils as well as biodegradation decrease with burial depths for recent oil discoveries.

The industrial perception of this implications is becoming more and more strategic for the deep offshore exploration issues.

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