Abstract: Regional Evaluation of the Post-Salt System of Cabinda, Angola

Bretthauer, H. - Chevron; L. Casimiro - Sonangol; P. Orsolini - Elf; A. De Carvalho - Agip

The Post-Salt sedimentary section of Eastern Cabinda (Area A) is located in the Lower Congo Basin. The Post-Salt Stratigraphic section consists of marine Albian to Tertiary siliciclastic and carbonate rocks. Over 1,750 MMBO of proven recoverable reserves exist in the Post-Salt objectives with one individual field being as large as 500 MMBO recoverable.

The regional evaluation of Cabinda Post-Salt Petroleum System based on exten sive available well data and recent high-quality 3D seismic surveys has been performed by a four-geoscientist team provided by the J.V. partners in Cabinda. (1. Sonangol, 2. Chevron, 3. Elf Aquitaine, 4. Agip). The group worked for 16 months at Chevron's facility in San Ramon, California, with technical assistance from specialists in each company provided when necessary.

The Petroleum System of Cabinda can be briefly described as follows:

- Reservoirs: Albian to Campanian sandstone, siltstone, and carbonates.

- Seals: shales, silty shales or combination of shale and limestone.

- Traps: anticlines, and rotated fault block closures created with rafting of sediments on salt during extension of the basin.

- Source rocks: Neocomian to Barremian (Pre-Salt) lacustrine shales averaging 7% TOC.

Geochemical analysis shows that the oils produced from Post- and Pre-Salt reservoirs have a Pre-Salt source. The salt windows are therefore critically important to Post-Salt Hydrocarbon charging. The bulk of the oil present in Post-Salt traps is migrated during Upper Cretaceous and Tertiary time. The Post-Salt migration pathways of the oil in the eastern area have primarily been limited to the Turonian-Coniacian (Upper labe-Mesa/Lago) while the oil in the western region is found throughout the section.

The tectonic fabric in the Post-Salt is directly influenced by Pre - Salt morphology and sediment loading on top of the Salt decollement layer. The major production trend is oriented NW-SE and situated above a Pre-Salt hinge line separating two areas with contrasting structural styles:

- Stable platform Area. East of Takula-Limba Production Trend, the Loeme Salt is thick and tabular with limited internal deformation. The structural traps correspond to broad simple four-way closures (Banzala and Malongo Fields) and structurally closed fault blocks.

- Mobile "rafted area." West of the Takula-Limba Production Trend, the amount of horizontal stretching in the Post-Salt increases drastically. The Loeme Salt is generally thin or missing (Salt windows). The structural traps correspond generally to rafted and rotated faults blocks with a maximum amount of growth in the Pinda (Albian) and Vermelha (Cenomanian) section.

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