Pre-Salt Lacustrine Carbonates, Diagenetic Silicification and Hydrothermal Overprinting in Kwanza Basin (Offshore Angola): A Tale of Two Silicas

Abstract

The Kwanza Basin in Angola presents a remarkable correlation between tectonic domains, outlined by the thickness of the lithospheric crust, and the resulting pre-salt carbonates diagenetic history. The studied well is representative of several other wells located within the ultra-stretched domain. A thorough petrographic and geochemical study on pre-salt side wall cores was conducted to decipher its depositional and diagenetic history within this tectonic scenario. The original alkaline lacustrine carbonate rocks, similar to Santos Basin (Brazil) pre-salt carbonates, underwent very different diagenesis where silica metasomatism plays a key role. Dolomitization occurred prior to the early-diagenetic main pervasive silicification event. Silica appears as fine-grained opal-microquartz mass replacing carbonate components, and opal-chalcedony-(mega)quartz cements precipitated within corrosion-enhanced, primary porosity (framework, interparticle,…). Silica diagenesis caused opal-A to transform to microcrystalline quartz, with accompanying microtexture destruction, porosity changes and geochemical modifications resulting in a texturally complex rock.

A second, high-temperature hydrothermal event overprinted the already silicified rock, triggering the almost complete replacement of the scattered residual microbial carbonate resisters by a second silica generation. The latter contains fluid inclusions (brines and hydrocarbons), and solid corroded calcite and pyrobitumen fragments with clear evidences of coeval trapping. Oil cracking temperature is well constrained by hydrothermal Tpeak (219-227°C) calculation after pyrobitumen reflectivity (Ro) data. Paleofluid reconstructions (brine PTX and oil PVTx) indicate that a liquid oil-filled reservoir was telescoped by a hot (197-221°C), high salinity (≈25wt% eq NaCl) hydrothermal plume, thermally cracking the pre-existing oil to residual pyrobitumen and wet gas (51-53 API) generating local, very high fluid overpressures (11400-12400psi/780-850bars). Further in situ cracking has left the reservoir filled with mostly dry gas and CO₂. High He contents coupled with noble gases isotopic compositions suggest a possible mixing of deep (mantelic) and crustal (carbonate corrosion, oil cracking) sources for the CO₂.

Similar processes, in different degrees, have been recognized in other reservoirs on trend within the same tectonic domain, suggesting that this diagenetic evolution is typical of Kwanza’s ultra-stretched domain.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018