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Origin of Silica in Pre-Salt Carbonates, Kwanza Basin, Angola

Abstract

Various forms of silica (e.g., chert, chalcedony, mega-quartz) have been identified in pre-salt lacustrine carbonate reservoirs in the conjugate South Atlantic margins, offshore Santos and Kwanza basins. However, their mode of precipitation with respect to biogenic or burial diagenetic processes remains debated. The preponderance of microbial boundstone textures commonly associated with chert may suggest syndepositional chert formation through microbial mediation (Saller et al., 2016). Such relationships are commonplace in modern hot spring environments and alkaline lakes, where high dissolved silica concentrations promote rapid silicification of microbial mats (Renaut et al., 1998). Here we present a case study from offshore Kwanza, where chert is not a primary biogenic precipitate, but rather replacing a carbonate precursor of boundstone texture during burial diagenesis. The replacement process is partially fabric-preserving, hence the microbial texture of the chert. Petrographic observations suggest the following relative timing of events: 1) formation of microbial boundstone carbonate, 2) complete dolomitization and burial dolomite cementation, 3) silicification (i.e., partial replacement of microbial boundstone by chert), silica cementation (i.e., fibrous chalcedony followed by drusy and mega-quartz cements) and coeval corrosion. Silica cementation and corrosion may occur as multiple, repeating events. Fluid inclusion microthermometry, Raman spectroscopy, and petrography indicate that silica cements precipitated from high-T fluids (Th=98-123°C) associated with HC gases in the burial diagenetic realm. These observations imply that the high quality of these silicified carbonate reservoirs (k up to 100 mD, Φ up to 15%) is not only due to the presence of primary microbial framework porosity but also the enhancement of secondary vuggy pores generated by high-T fluid flow and corrosion. Thus, the recognition of potential migration pathways for high-T/hydrothermal fluids (e.g., faults), in association with build-up geometries on seismic profiles, may help to optimize the identification and characterization of these types of reservoirs. Renaut, Jones, Tiercelin 1998, Rapid in situ silicification of microbes at Loburu hot springs, Lake Bogoria, Kenya Rift Valley: Sedimentology, 45, 1083-1103. Saller, Rushton, Buambua, Inman, McNeil, Dickson 2016, Presalt stratigraphy and depositional systems in the Kwanza Basin, offshore Angola: AAPG Bulletin, 100/7, 1135–1164.