Multi-proxy reconstruction of the burial history and porosity evolution of the TOGA carbonate formation in the Lower Congo basin (South West Africa)

The timing of porosity destruction in reservoir rock is key for hydrocarbon exploration. This study constrains the porosity destruction of the syn-rift Barremian (130-125 Ma) TOCA Fm. from one drill core located in the Lower Congo basin (West African passive margin). We apply an unprecedented multi-proxy approach to reconstruct the absolute timing of mechanical compaction, pressure-solution and cementation as well as their relative contribution to porosity destruction. LA-ICPMS U-Pb geochronology combined with petrological observations suggest that the main interstitial diagenetic cement Cl filling the porosity of the packstones and dated 127.4 +/- 2.2 to 119.4 +/- 6.4 Ma predates pressure-solution. The quantification of the cement distribution by image analysis indicates that Cl clogged similar to 30% of the initial porosity before the strata were buried at 400 m. We assume that the early cementation limited the effect of mechanical compaction on porosity reduction which is estimated to similar to 2% in the first 100 m of burial. In contrast, a first order estimate of the chemical compaction-reprecipitation can account for up to similar to 6% of the initial porosity destruction, as estimated by the inversion of sedimentary stylolite roughness. This technique also documents that the burial-related pressure-solution in the TOCA Fm. occurred in the 550-1750 m burial depth range corresponding to a time spanning from 117 to 95 Ma considering the burial-time model of the TOCA Fm. Our study reveals that the initial porosity of carbonates was reduced down to its current value of 4-8% within the first 35 Ma of its burial history, reaching similar to 10% after only 10 Ma, i.e. in the first 400-500 m of burial. This case study indicates that the fate of reservoir properties in bioclastic carbonate formations such as the TOCA Fm. may be largely controlled by early, very shallow diagenetic processes rather than subsequent deep-burial mesogenetic reactions.

Automated summary

The timing of porosity destruction in reservoir rock is crucial for hydrocarbon exploration. This study uses a multi-proxy approach to determine the absolute timing of mechanical compaction, pressure-solution, and cementation in the syn-rift Barremian TOCA Fm. The results show that early cementation played a significant role in limiting the effect of mechanical compaction, while chemical compaction-reprecipitation contributed to a larger percentage of porosity destruction. The study also reveals that burial-related pressure-solution occurred within a specific depth range and timeframe. This case study suggests that early diagenetic processes have a major impact on reservoir properties in bioclastic carbonate formations like the TOCA Fm.