Abstract: Vermelha Formation (Cenomanian), Area A, Cabinda, Angola: Palynomorph Biozonation and Facies Controls on Populations

Denison, Christopher N. - Chevron Overseas Petroleum Co.

In Area A aquatory, Cabinda, Angola, the Vermelha Formation (Cenomanian) is a siliciclastic dominated succession with subordinate carbonates. Nearshore facies are reservoir-forming shallow marine delta front sandstones, interbedded with marine flooding event shales and restricted bay mudstones. Offshore, these facies pass into marine mudstones with few reservoir quality sandstones. NW SE listric faulting controlled sand distribution, and bay orientation and duration. Carbonate, dolomite and anhydrite cement can degrade reservoir quality; bedded anhydrite is locally present.

Wireline log correlation is complicated by the facies changes, faulting and diagenetic overprint. Age control to aid correlation is hampered by facies controls on microfossil distribution. Planktonic foraminifera and calcareous nannofossils are often rare or absent. Palynomorphs are more consistently present, but environmental factors greatly influence their distribution. Four newly defined biozones, based on population data from 12 wells, provide age control (Fig. 1). These biozones currently offer the best potential for chronostratigraphic correlation in Cabinda and to coeval successions on the Brazilian continental margin

At the labe/Vermelha boundary there is a change from sparse but diverse marine shelfal microplankton assemblages in upper Subzone 40A to more abundant but frequently low diversity, Subtilisphaera dominated, assemblages in lower Subzone 40A and Subzone 40B. Subtilisphaera continues to be a characteristic element of many assemblages throughout the Vermelha. In Zone 45, the P. polymorphum group includes Cyclonephelium compactum, C. membranophorum, Circulodinium distinctum subsp. longispinatum, and species of Pseudoceratium, e.g. P. eisenackii and P. polymorphum. Limbacysta is an informal name for a plexus of undescribed species with an acme in Zone 45B. The pollen grain Afropollis jardinus became extinct within the earliest Cenomanian, but it is currently used as a pragmatic Albian top in Cabinda.

Dinocysts from offshore mudstones are of low diversity compared with higher latitude shelfal assemblages. Endemic species are sometimes numerous but their value for age control has yet to be established. Bay-fill mudstones are often characterized by abundant to super-abundant, sometimes virtually monospecific, assemblages of Subtilisphaera. These algal blooms are probably a response to high-stress. restricted circulation, possibly hypersaline conditions. Leiospheres could tolerate the most extreme conditions. Bay-margin tidal flats are characterized by common algal tubules or abundant microforaminiferal linings. Terrestrial palynofloras are often of low diversity and abundance. A general dominance of Ephedripites suggest an arid hinterland. Classopollis influxes may represent riparian floras that flourished during more humid climatic periods.

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