Cretaceous deposits of the northern part of the southern offshore Tanzania: depositional system, seismic manifestation, well data expressions and their implications on petroleum system elements

The northern part of the southern offshore Tanzania basin contains an operational gas field. This field is produced from the lower Cretaceous reservoirs within a faulted system. The Cretaceous depositional system of the study area has been poorly studied. Therefore, paleo sediment flows, depositional processes, sediment distribution, deposit types, and distribution of post-depositional features are not well understood. This work utilized core logging, well logs analysis, and interpretation of 2D reflection seismic data using seismic facies and attribute analysis to establish an understanding of the depositional system of the study area. Results show that the Cretaceous interval contains a sand-rich submarine fan dominated by shaly and sandy successions in the northern and southern parts of the area, respectively. This submarine fan is characterized by alternating sequences of sheet sands, channel sands, and channel levee successions containing turbidites, slumps, and debris flow deposits. These deposits are linked to sediment gravity flows triggered by episodic fault movements and rapid sea-level fall. The gravity flows supplied sediment from the Tanzania mainland through the NE traversing submarine channels. Syn-depositional interaction between these gravity flows and northward-flowing bottom currents caused the deposition of hybrid turbidite-contourite and drift deposits, forming key reservoirs and cap rocks, respectively. The studied stratigraphy has been cut by several deep-rooted faults with flowering patterns. These faults, together with erosional cuts associated with channel deposition, formed potential migration pathways for petroleum generated by the deep-buried source rocks. The flowering faults are also interpreted to be forming major petroleum traps in the study area.