Geochronology and mantle origin of CO2-rich fluid flow in a Cretaceous carbonate reservoir, offshore Angola

Drilling on the volcanic rifted margin of southern Angola has encountered high volumes of CO2 and evidence for hydrothermal fluid flow, but the sources of CO2, the timing of fluid flow and its impact on basin thermal structure, reservoir evolution and resource distribution have so far remained unresolved. This study integrates in-situ U-Pb geochronology, noble gas and clumped isotope geochemistry, and petrography to reconstruct the burial history and fluid-rock interaction in a CO2-rich Aptian carbonate reservoir. The carbonates have not preserved depositional ages, but they were recrystallized at 111.2 ± 3.4 Ma and 87.7 ± 5.5 Ma, coincident with thermal pulses at temperatures of 114-138°C and 145-149°C (1SE), respectively. Evolved basinal fluids that had interacted with siliciclastics, volcanic and/or basement rocks, but also contained a significant mantle volatile component, were responsible for recrystallization. Decompaction analysis suggests that at least the older event can be considered hydrothermal. The occurrence of such a thermal and fluid pulse within c. 6-16 Ma of deposition will have significantly altered the thermal structure of the basin. The Albian age of fluid flow and calcite recrystallization complements age data from the conjugate Brazilian margin and suggests that this widespread but spatially variable event was genetically linked to the early stages of oceanic spreading. The second fluid pulse coincides with the age of the Peri-Atlantic Alkaline igneous Province (PAAP). This study highlights the importance of early diagenetic fluid flow involving mantle-derived volatiles for basins on the South Atlantic conjugate margins.