Controls on Reservoir Development in the Toca Formation of Block 0, Offshore Cabinda, Angola

Wasson, Matthew S.; Saller, Arthur; Self, Dan

Lacustrine carbonates are important reservoirs in Block 0, offshore Cabinda, Angola, with total production from Toca carbonate reservoirs being greater than 450 MMBO mainly from 3 fields. A recent discovery in the Toca in western Block 0 has led to renewed interest in Toca reservoirs. Toca carbonates are interbedded with Bucomazi shales and were deposited during the early sag stage of the break-up of Gondwana during the early Cretaceous. The Toca contains fossiliferous grainstones to wackestones with variable amounts of microbially-mediated grains such as oncoids, ooids, and other facies including boundstones similar to those deposited in today's East African Rift Lakes (Cohen and Thouin, 1987). The main reservoirs in the Toca are hydrothermally-dolomitized fossiliferous grainstones and packstones (mollusk coquinas). Grainstones to wackestones and boundstones with microbially-mediated grains like oncoids are common, but are generally not productive reservoir facies within Block 0.

The diagenetic history of Toca deposits was critical to the development of reservoirs. Early diagenesis of the Toca includes recrystallization and dolomitization. In places, later burial diagenesis was influenced by faulting and hydrothermal fluids. Stable carbon and oxygen isotopic analyses of cores and cuttings show a distinction between early, syndepositional dolomite with heavy oxygen values and later, hydrothermal dolomite with light oxygen values. Isotopic variations in the Toca have also helped interpret lake chemistry and how that influences depositional patterns and diagenesis.

Stratigraphic correlation of the Toca between western and eastern Block 0 has been difficult due to the rifting across the block. Biostratigraphy was used for correlations within the Toca and for determining the age of the Toca. Core description and facies stacking patterns helped refine correlations along strike.

Results of this study indicate that the optimal conditions for Toca reservoir formation are (1) proximity to rift-related faults and fracture systems that acted as conduits for dolomitizing hydrothermal fluids and (2) depositional facies with good primary permeability and porosity intersected by faults and fractures, causing the best reservoirs to be hydrothermally-altered grainstones and packstones.Dolomitization is also critical for preserving reservoir porosity during deep burial with dolomites commonly having porosity while adjacent limestones have lost porosity due to compaction.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013