Coastal Basin Blocks

Structural and Geological Context

The geology of the Coastal Basin is set in the context of the sedimentary basins of the passive margin of West Africa. Its architecture is related to those of neighboring countries including the Republic of Angola (Northern Angola and the province of CABINDA) and the Republic of Congo.

Two stages of structural evolution can be observed:

1. Continental Rift basins located above the metamorphic basement and presenting a horst, graben and half-graben architecture, with confined and discontinuous sedimentation;

2. Marine basins open to the open sea, which have been influenced by “diapiric” movements filled with prograding marine formations.

This evolution took place in two periods:

Late Jurassic to Aptian

During this period, a setting up of horsts, grabens, half-grabens and furrows is observed. This stage precedes the opening of the Atlantic Ocean and is made up of pre-salt sediments.

The Pre-Salt formations rest on a metamorphic basement and are made up of coarse, discordant “Zenze” sediments which are surmounted by fluvio-lacustrine deposits of “Lucula” including arkosic and clayey sandstones in the lower part.

Above these formations come palustrine and lacustrine sediments which are clays of "Middle Bucomazi" and "Upper Bucomazi" which laterally contain coarse deposits of "Toca".

Finally above the sandstone clays, the sandstones of the “Chela” formation are unconformable with the earlier series.

During the Aptian the salt layer was deposited thanks to which the sedimentary series of the Congolese coast is subdivided into two sequences: Pre-salt and Post-salt.

Aptian to Tertiary

This stage is marked by the opening of the Atlantic Ocean, the salt deposits known as the “Loeme” formation and marine sediments. It was during this period that the stabilization of the continental margin took place.

The marine deposits are carbonates and clayey sandstones of “Pinda” with regressive episodes of continental origin of “Vermelha”.

Until the end of the Tertiary, the sea deposited series of marine clays and marls, carbonate rocks and some sediments of continental origin which are lenses of sand, river contributions. These are formations of "Liawenda", "Cretaceous Salt and Landana", "Malembo" and "Cirques".

Petroleum System of the Coastal Basin

Source Rocks

The source rocks are made up of deposits of black clay rich in organic matter called "BUCOMAZI" and whose Total Organic Carbon (TOC) is greater than or equal to 10%.

Reservoirs

In the Pre-Salt section, the reservoirs consist of the formations of:

- ZENZE - LUCULA - LOWER KINKASI (LK 5)

- TOCA - TSHIBOTA - CHELA

In the Post-salt section, the reservoirs are formations of:

- MAVUMA - PINDA - VERMELHA

- KINKASI-LIAWENDA

Petrophysical Parameters

For carbonates which are dolomites and limestones (TOCA, MAVUMA, PINDA, VERMELHA and KINKASI):

- Average porosity (in %): 12.9

- Average permeability (in m): 5.0

For sands (LUKULA, LK5, TSHIBOTA, CHELA, KINKASI):

- Average porosity (in %): 22.0

- Average permeability (in md): 679.0

For the silts (LIAWENDA):

- Average porosity (in %): 13.9

- Average permeability (in m): 10.0

For sandstones (LUKULA, TSHIBOTA, CHELA, VERMELHA, KINKASI)

- Average porosity (in %): 12.2

- Average permeability (md): 93.0

Cap Rocks

In the Pre-salt complex, sedimentation during rifting mainly favored the deposition of clays which constitute a good cover for the Pre-salt reservoirs associated with them.

However, the compact dolomites in the upper part of the Toca can serve as cover for the reservoirs of this same formation.

Loeme salt is also an excellent and ideal cover for all pre-salt reservoirs especially for Chela.

In the Post-salt, the covers are more varied and consist of marine silty clays and marls for the Turonian, and compact dolomites for the Cenomanian, anhydrite in thin beds for the Aptian and Mavuma.

Mechanism of migration and trapping of hydrocarbons

Migration: the hydrocarbons migrated from the Pre-salt section to the Post-salt section via faults linked to the halokinesis phenomenon and laterally via the Chela reservoir.

Trapping: the trapping of hydrocarbons was conditioned by the main stages of the structural evolution of the coastal basin. We note a basement tectonics at the level of the Pre-Saliferous having generated anticlinal traps, traps against faults and mixed. Traps due to beveling or the phenomenon of discrepancy are also observed in the Pre-salt section.

The halokinetic tectonics of Loeme generated the structural traps of the Post-salt section including the anticlines on salt domes, the flanks of domes and mixed traps (Tertiary channels on domes).

Prospectivity of the Coast Basin

The studies undertaken in the Coastal Basin, particularly in the Nganzi, Yema II and Matamba-Makanzi II Blocks selected for the Calls for Tenders, demonstrate the good conditions for the maturation of the source rocks. Very interesting prospects have been identified in the Pre-Salt section of these renderings and constitute real targets in this part of the DRC Coastal Basin.