Beyond turbidites: Core insights from the Rovuma Basin

The Rovuma Basin evolved from Early Jurassic rifting into a passive margin by the Mid-Cretaceous, followed by significant clastic infill through the Cretaceous and Tertiary. Uplift associated with the early Paleogene East African Rift System drove substan­tial sediment supply, feeding deepwater channel systems and fan complexes.

Progressive sediment loading and gravity-driven deltaic tectonics led to collapse of the sediment wedge, forming a linked system of deforma­tion characterised by numerous listric growth faults linked by a regional de­tachment layer downdip to compres­sional fold-and-thrust belts in deep­water area. Many major hydrocarbon discoveries are located near the toe of this thrust belt.

Core-controlled insights

The insights presented here are derived from a regional core-based study un­dertaken in 2023 by Core Laborato­ries in collaboration with the Instituto Nacional de Petróleo (INP) in Mozam­bique. Core material from across the basin captures the full spectrum of the turbidite system – from proximal chan­nel deposits through to channelised or amalgamated lobes and lobe-fringe facies. Several intervals display thick-bedded sandstones with very good to excellent reservoir quality, reinforcing the effectiveness of these systems as pri­mary reservoir targets.

One important aspect in this area is the influence of bottom currents. Rather than acting independently, these currents interact with turbidi­ty flows – either during deposition or through subsequent reworking, creat­ing hybrid facies.

Turbidite – contourite interaction

The evidence for contourites in the Ro­vuma Basin is mainly seen in the pres­ence of unusually thick, stacked sandy turbidite beds, as the contourite cur­rents strip out the finer-grained mate­rial and deposit them on the overbank margins of the channels as asymmetric drifts. In core, however, it is possible to observe some evidence of bottom current reworking within thin, fin­er-grained units locally preserved be­tween the sandy, high-density turbidite (HDT) flows like the ones illustrated in the core photo (left).

Here we see that the HDT depos­its, interpreted to form part of a chan­nelled fanlobe, are separated by a thin finer-grained interval (A). The lower HDT (B) comprises coarse-grained, massive to fluidised sandstones with abundant dewatering structures (dish and pillar) and large deformed mud­clasts (C), while overlying finer-grained intervals (A) display climbing ripples, clay drapes, and localized bioturbation. Deposition resumes upward with addi­tional fluidized sands (D) and thicker granule-rich units (E), reflecting re­newed high-density turbidity flow. In contrast, the thin finer bed (A) repre­sents a low-density turbidity flow, pos­sibly reworked by bottom-currents.

Reservoir implications

Both HDTs exhibit high porosity, with permeability exceeding 1,000 mD in granule-rich intervals. However, the presence of finer-grained layers intro­duces subtle heterogeneities that can significantly influence reservoir con­nectivity and flow behaviour.

Although typically below seismic resolution, these features are identifia­ble in core and are critical for refining reservoir models in structurally com­plex deepwater settings.