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Hydrothermal Diagenesis and Reservoir Quality in the Pre-Salt Carbonate-Clastic Series of the Kwanza Basin, Offshore Angola


The Pre-salt reservoirs in offshore Angola are predominantly composed of synrift/sag continental deposits of Aptian age and represent important targets for hydrocarbon exploration. They often exhibit very variable sedimentary facies and complex diagenetic transformations, with a significant impact on reservoir properties. A detailed integrated study was conducted on a reference well located in the southwestern part of the Kwanza basin, near the Benguela Transfer Fault Zone, in order to reconstruct the diagenetic history of the Pre-salt reservoirs presently occurring at ~5-6 km depth and ~100°C temperature. Petrography, fluid inclusions, isotope geochemistry and U/Pb age data were gathered for the continental carbonates of the Chela Formation and the underlying silty-sandstones of the Cuvo Formation.

The studied Pre-salt reservoirs exhibit multiple generations of cements occurring as matrix-replacive, pore-fill or fracture/vein fill, and predominantly composed of calcite, dolomite, chalcedony and quartz. Several dissolution events are also documented. Fluid inclusions record paleo-temperatures much hotter than present-day, exceeding 150°C, and the ubiquitous involvement of highly saline brines (>200g/l). The common presence of CO2 gas inclusions in several cements suggests multiple pulses of CO2 mixed with variable CH4. Stable isotopes indicate that cement-forming waters were 18O-rich brines. U/Pb ages determined for a few diagenetic cements and combined with fluid inclusion temperatures document the existence of an early heat pulse, and constrain the timing of hydrocarbon emplacement. Overall, the results demonstrate that Pre-salt reservoirs were heated to unusually high temperatures shortly (within 15-20 Ma) after deposition and affected by a profound hydrothermal diagenesis driven by CO2-rich highly saline brines through most of their post-depositional history.

This intense diagenesis had positive and negative effects on reservoir properties. An appreciable fraction of the present-day porosity is of secondary (dissolution) origin in both formations. However, the Chela reservoir also preserved a high proportion of well-connected primary porosity. This is postulated to be largely related to early HC charge, in relation to early high thermal regimes. The Cuvo sandstones tend to show poorer reservoir quality, due to lower permeability values. This is related to the presence of clay/carbonate matrix and the predominance of poorly connected moldic porosity.