Evaluating the Impact of Genetic Turbidite Facies and Rock Physics Properties on Acoustic Impedance of Deepwater Clastics – Offshore West Africa

Abstract

Variations in reservoir acoustic impedance (AI) relative to overburden shale create challenges for seismic interpretation, particularly when mapping turbidite boundary reflectors. This study integrates data from 30 wells with available broadband seismic to evaluate Miocene deepwater (DW) slope channel complexes. In this study, most shallow reservoir sands exhibit lower AI than the overburden shales, but this relationship reverses with increasing burial depth, where sands become higher impedance than shale.

The objective is to assess how genetic DW facies, fluid type, burial depth, and rock properties control these relative AI changes. Workflows included well-to-seismic ties and 1D synthetic seismograms for selected wells to identify reflectors interpreted as the bases of individual channel complexes. For younger, low-impedance reservoirs (SC1-SC8), channel bases commonly correspond to trough-to-peak zero-crossings. In contrast, older, high-impedance reservoirs (DC1-DC6) mainly show peak-to-trough zero-crossings on quadrature-phase data.

Interpreted horizons were propagated using Vsand and Phit seismic inversion volumes, improving mapping reliability. Sedimentological classification identified seven genetic turbidite facies (R, S, Ta, Tb, Tc, Td, Te/Sls/Slm). Comparing facies distributions within selected low- and high-impedance intervals helped constrain the geological context. Crossplots of density, AI, compressional velocity (VP), and VP/VS versus depth were also generated to define rock-property trends in sands and shales. AVO analysis, supported by fluid-substitution modeling, was used to evaluate potential fluid effects on AI.

Results indicate that AI is controlled primarily by VP rather than density. Shallow reservoirs and gas sands generally show low AI, whereas deeper reservoirs and oil-saturated or wet sands show higher AI. High-impedance sands are dominated by traction/bedload turbidites (R, S, Ta), while shalier intervals are enriched in suspension or lower-concentration turbidites (Td, Te/Sls/Slm). Overall, fluid fill and genetic DW lithofacies exert minor influence compared with burial depth and compaction, which are the main drivers of AI variation in this field.