Presalt stratigraphy and depositional systems in the Kwanza Basin, offshore Angola

Abstract

The Lower Cretaceous presalt section in the Kwanza Basin contains an excellent petroleum system that includes “synrift” strata (Barremian) overlain by a “sag” interval (Aptian) and capped by the Loeme Salt. The upper synrift is generally limestone with widespread mollusk packstones and grainstones (coquinas) deposited in a fresh-to–moderately saline (alkaline) lake. The sag interval is characterized by carbonate platforms and silica-rich isolated buildups formed in highly evaporated, highly alkaline lakes. Shrubby (dendritic), microbially influenced boundstones and intraclast–spherulite grainstones accumulated in shallow water on platform tops. Microbial cherts were deposited as organic buildups on large, isolated structural highs basinward (west) of platforms, and they apparently formed at low temperatures in very alkaline lake water. Shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability. Wackestones and packstones with calcitic grains (mainly spherulites) in dolomite or argillaceous dolomite were deposited in slightly deeper, low-energy sag environments. In addition, clays (especially stevensite) precipitated out of the silica-rich, highly alkaline lake waters. During sag deposition, calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms. Spherulites probably precipitated just below the sediment–water interface. Spherulites and shrubby calcites are commonly recrystallized. Spherulites floating in stevensite probably formed in deeper lacustrine environments. Organic-rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals, and they are likely the source of most hydrocarbons in this system. These interpretations are supported by seismic, core, petrographic, and stable isotope data.