CHLORITE DIAGENESIS IN THE ALBIAN PINDA FORMATION

Detailed studies of clay minerals, particularly chlorite, from the CABGOC 67-4ST and 79-1 wells from offshore Cabinda, Angola, reveal a complex diagenetic history for the Albian Pinda Formation. Petrographic, XRD, SEM, and geochemical analyses have been integrated to determine the relative timing of cements and the origin of coeval pore fluids. Chlorite was formed during nearly all stages of diagenesis, either as grain rimming or pore-filling phases.

The sedimentary rocks of the Pinda Formation were deposited in the Lower Congo basin, in environments ranging from tidal flat to shallow marine shelf. Detrital clay grains were derived from the metamorphic basement rocks of the Congo Craton, while smectite-rich soil was transported into the Lower Congo basin. Authigenic clay minerals formed either by alteration of detrital clay grains, or direct precipitation from pore solutions (neoformation). Increasing temperature and pressure associated with burial diagenesis transformed detrital kaolinite and smectite into mixed layer illite/smectite, illite, or chlorite. Changes in pore water chemistry caused by dissolution of mafic minerals and/or carbonate rocks and subsequent flushing through clastic units during diagenesis also may have been important in controlling clay mineral formation and chemistry. Successive transgressive/regressive events altered pore fluid conditions and led to the development of halite, calcite, dolomite, albite and clay mineral cements.

Clay mineral distribution in the 67-4ST and 79-1 wells appears to be controlled to some extent by original depositional facies. Chlorite, while present in all facies, is most prevalent in the more distal environment (shoreface, marine shelf), whereas illite is the dominant clay mineral in nearshore environments (tidal flat, lagoon).