ABSTRACT: The Role of Unconformities in Hydrocarbon Accumulation: Middle and Lower Pinda Formations, Block 2, Offshore Angola

KISUCKY, MICHAEL J., Texaco Portugal, Cascais, Portugal

Of the several unconformities in the Lower Congo basin postrift sequence, two in the Middle and Lower Pinda formations control hydrocarbon accumulation in Texaco-operated Block 2.

The Top Lower Pinda Unconformity (TLPU) is a seismically mappable event recognized as an angular unconformity by seismic signature, log response, and paleontology. In several accumulations the TLPU forms the topmost trap limit, shown by its mappable closure and appearance of hydrocarbons below it, but does not always correspond to top economic reservoir.

Elsewhere the interplay between the TLPU and the steeply dipping subunconformity reservoir carbonates accounts for hydrocarbon column heights in excess of the mapped closure.

A regional unconformity, the Middle Pinda Seismic Marker (MPSM), separates a generally siliciclastic sequence from the carbonate-dominated section below. In many traps the unconformity is angular, recognized by dipmeter and truncation of older seismic reflections.

The angular discordance across the TLPU and MPSM documents separate erosional episodes of tilted structures resulting from the interaction of eustatic falls and coeval deformation.

In addition to creating subunconformity traps and enhancing structural closures, these unconformities possibly supplied paths for dolomitizing and porosity-enhancing fluids, thereby controlling reservoir quality in the Lower and Middle Pinda.

These unconformities are chronologically constrained such that they relate to third-order eustatic falls of the Zuni ZC-1 Supercycle. The TLPU is correlated with a drop between cycles 1.1 and 1.2 (106 Ma), whereas the MPSM relates to the fall between cycles 1.2 and 1.3 (103 Ma).

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)