AAPG ACE 2018

Datapages, Inc.Print this page

Characterisation of the Pre-Salt Microbialite in the Kwanza Basin, Angola

Abstract

The recently discovered and prolific “Pre-Salt” play has invigorated exploration on both sides of the South Atlantic. Many discoveries are hosted within Aptian “Sag 2” phase lacustrine carbonate reservoirs. Despite many similarities with the equivalent Brazilian carbonates, it appears that the Angolan “Sag 2” reservoir facies or “Microbialite” has some characteristics that are unique to the Kwanza Basin. The variability resulted from interplay between lake chemistry, climate, tectonics, hydraulics and depth of the water in which they formed. Diagenesis was also complex and the core presented in this study exhibits a number of features that distinguish it from published work in Brazil and elsewhere in the Kwanza Basin. Better understanding of these processes will aid in reducing uncertainty in further “Pre-Salt” exploration.

The core is characterised by calcite shrubs, spherulites and a range of intermediate facies. Re-worked facies have also been identified. Magnesian clays are rare, possibly due to diagenetic removal. All calcite phases are recrystallized to varying degrees and this is reflected in geochemical data; however shrubs and spherulites often preserve an original radiaxial-fibrous crystal structure.

Chert layers often preserve stromatolitic or shrubby/spherulitic textures. Chert is believed to be an early diagenetic phase precipitated via a drop in pH, possibly triggered by pluvial events, marine incursions or locally, by the degradation of organic matter as may be the case with stromatolitic textures. Fractured, massive chert often displays a pseudo-chalcedonic microfabric with organic inclusions that may reflect microbial activity and/or be associated with an opal-CT phase transition. Scanning electron microscopy has also revealed the presence of microbial filaments within the shrubby calcite fabrics, but it is not certain if the microbes provided merely a passive substrate for mineral precipitation.

Diagenesis was multi-phase, with early dolomite, quartz and calcite phases present alongside later quartz, dolomite, fluorite and sphalerite. No saddle dolomite was identified; but the presence of sphalerite implies circulation of high temperature fluids during later diagenesis. Flushing by mantle derived CO2 has affected reservoirs in the Angolan Microbialite and may have triggered de-dolomitisation locally within the core.