The Deep Senegal Offshore is part of the Mauritania – Senegal – The Gambia – Guinea Bissau – Conakry (MSGBC) Basin, classified as an Atlantic passive margin basin of Middle Jurassic to Holocene age overlying a Palaeozoic basin. The Basin has experienced three broad tectonic phases: a Palaeozoic pre-rift phase, a Triassic-Early Jurassic rift phase and a Middle Jurassic to Present Day drift phase.
The Deep Senegal Offshore is one of the most prolific hydrocarbon provinces of the world. Intensive exploration efforts over the last 5 years has led to a succession of significant discoveries, notably the FAN, Sangomar (ex-SNE), Tortue, Teranga and Yakaar. However, the full potential of the basin and the continental slope is still to be realised.
Modern seismic data and improved geological models indicate a prospective zone in the MSGBC, located in the deep offshore. Extensive regional 2D and 3D multi-client seismic data provides a high-quality dataset that enables an unprecedented understanding of the evolution of the MSGBC Basin and especially the deep offshore.
The post-rift section is composed of a carbonate platform of Middle Jurassic to Neocomian, overlain by transgressive and regressive sequences from the Albian to the Turonian. A globally regressive Upper Cretaceous clastic sequence is capped by Tertiary sediments.
Within the MSGBC Basin, there are three major source rock intervals that contribute the majority of the hydrocarbons found in the region: the Neocomian, the Aptian-Albian and the Cenomanian–Turonian intervals. However, the quality of these source rocks is variable. One controlling factor is their relative position in the area to the shelf and the related influence of detrital influx. Generally, source rock character improves towards the west with the decrease of the sand/shale ratio and the establishment of more anoxic conditions favourable for the preservation of the organic matter.
Recent discoveries prove extensive reservoirs from Campanian-Maastrichtian deltaic sandstones and the outer basin sands of Cenomanian and Aptian-Albian age. For the deep offshore area, quality reservoirs still lie within the Santonian-Campanian section deposited in turbiditic environments.
The recent Guembeul-1 well, drilled north of the Saint Louis offshore deep block, penetrated sandstone gas-bearing levels within the Cenomanian, Albian and Aptian, sourced from Neocomian shales.
During periods of low sea-level, deltas migrated seaward to the shelf edge and large amounts of sediments were transported to the slope and deep-sea by turbidity currents and related mass flows via submarine canyons. Turbidity flows were confined within deep leveed channels on the upper and middle fan, but spread laterally outwards as sheet flows on the lower fan, dispersing large amounts of coarse sediments across broad areas.
The principal trapping system is often a combination of structural and stratigraphic traps. In the North, low relief structures associated with massive sand flows from the Senegal River, create the extensive gas fields of Greater Tortue/Ahmeyin and Yakaar. This model of traps extends further west in the deep and ultra-deep offshore.
The FAN-1 discovery is a basin floor fan turbidite reservoir, which develops across the area, south of the Dakar Peninsula. SNE (now Sangomar), the world’s largest discovery in 2014, is a shelf edge Albian turbidite play associated to migration pathways of mature sources located deep in the basin.
The salt diapir zone, in the south, shows a highly prospective area, along the shelf edge and deep into the basin.