Stratigraphic control on facies and diagenesis of dolomitized oolitic siliciclastic ramp sequences (Pinda group, Albian, offshore Angola)

摘要

Pervasively dolomitized, oolitic to siliciclastic ramp sequences are prolific oil reservoirs in the Albian Pinda Group of northern Angola. During the Late Cretaceous, this reservoir series was segmented by salt tectonic gravitational gliding into halokinetic "turtle-back" and raft structures that provide hydrocarbon traps today. The Albian ramp sequences contain a large variety of reservoir facies, reaching from littoral sandstones with interparticle porosity to various types of dolomitized marine shelf to shoreline carbonates with moldic, vuggy, and intercrystalline porosity. Reservoir prediction is substantially improved by an integrated approach combining high-resolution sequence stratigraphy with carbonate geochemistry. As high-resolution stratigraphy documents, most of the important sandstone reservoirs formed at the base of stratigraphic sequences during shoreface transgression. Pisolitic dolostones with moldic porosity are the best carbonate reservoirs. They developed as transgressive shoreface sand sheets above the basal sandstone reservoirs. Regressive facies tracts contain oolitic and pisolitic dolomites with intercrystalline and moldic porosity that originated in progradational barrier and sand-belt settings. The combined stratigraphic and geochemical approach allowed us to determine the Pinda Group dolomites as early diagenetic in origin, whereas fluid inclusion and oxygen isotope analysis alone suggested a late burial dolomite formation. Dolomite neomorphism at higher burial temperatures modified most of the original geochemical signatures of early, near-surface dolomitization. Cathodoluminescence data and trace element analysis suggest that dolomitization was associated with gradually decreasing pore water salinity. Accordingly, barrier oolites were completely dolomitized and developed intercrystalline porosity during the earliest stages of diagenesis. At a later phase of early diagenesis, as waters became less saline and undersaturated with respect to calcium carbonate, fabric-selective dissolution of the remaining calcareous components occurred. The decrease in pore water salinity and associated porosity generation are genetically linked with high-frequency events of relative sea level lowering. This possible relationship implies that high-resolution sequence stratigraphy can be a valid reservoir prediction tool even in pervasively dolomitized series. [References: 44]