Pre-Salt Stratigraphy and Depositional Systems in the Kwanza Basin, Offshore Angola

The Lower Cretaceous pre-salt section in the Kwanza Basin contains an excellent petroleum system that includes “syn-rift” strata (Barremian) overlain by a “sag” interval (Aptian) and capped by the Loeme Salt (seal). The upper syn-rift is generally limestone with widespread mollusk packstones and grainstones (coquinas) deposited in a fresh to moderately saline (alkaline) lake. Carbonate platforms and silica-rich isolated buildups formed in highly-evaporated, highly-alkaline lakes during deposition of the sag interval. Shrubby (dendritic), probably microbial-influenced, boundstones and intraclast-spherulite grainstone accumulated in shallow water on platform tops. Microbial cherts were deposited as organic buildups on large, isolated structural highs basinward (west) of platforms. Chert microbialites formed at low temperatures in very alkaline lake water. Shrubby boundstones and microbial cherts have vuggy pores that are generally primary and commonly result in high permeability. Wackestones and packstones with calcitic grains (commonly spherulites) surrounded by dolomite or argillaceous dolomite were deposited in slightly deeper, low-energy sag environments. Clays (especially stevensite) precipitated out of the silica-rich, highly alkaline lake waters during sag deposition. During sag deposition, calcitic material precipitated on the shallow lake floors with morphologies that varied from spherulites to shrubs including a continuum of intermediate forms. Spherulites probably formed just below the sediment-water interface. Spherulites and shrubby calcites are commonly recrystallized. Spherulites floating in stevensite apparently formed in deeper lacustrine environments. Organic-rich mudstones were deposited in deep-water lacustrine environments during syn-rift and sag time. These interpretations are supported by seismic, core, petrographic, stable carbon and oxygen isotope data.