Abstract

The Greater Tortue/Ahmeyim gas field discovered by Kosmos Energy in 2015 in the deep-water Mauritania–Senegal Basin opened a new giant gas province in Northwest Africa. The discovery well, Tortue-1, was drilled in Mauritania by Kosmos and the Mauritania National Oil Company (SMHPM) and discovered dry gas in Cenomanian reservoirs within a stratigraphic–structural trap of approximately 90 km² (34.7 miles²). Two additional wells, Guembeul-1A in Senegal (drilled by Kosmos and PETROSEN) and Ahmeyim-2 in Mauritania (drilled by Kosmos and SMHPM), support a field resource estimate of approximately 25 tcf (0.71 tcm) GIIP. Additional drilling outside the Ahmeyim Field proves this gas province extends from northern Senegal to central Mauritania and may contain up to 50–100 tcf (1.42–2.84 tcm) GIIP.

The Mauritania–Senegal Basin saw several exploration campaigns throughout approximately 60 years. However, the deep-water petroleum system had not been recognized as it was assumed; (1) the Cenomanian-Turonian (C-T) mudstones were the only viable source rocks, and these would be immature in the deep water; (2) the basin outside of the salt province was undeformed—thus lacking trapping geometries; and (3) a sufficient volume of high-quality reservoir was not likely to be found in the lower slope.

The Greater Tortue/Ahmeyim Field discovery dispelled these paradigms while giving new insights into deep-water exploration. The African margin has been described as a passive margin; however, a clear deformation event occurred in the Late Cretaceous to Early Tertiary on faults linked to hyperextended crust. This deformation event likely coincides with the heat event, which is evidenced today by elevated heat flow across the outer basin. This heat event matured an Early Cretaceous source or sources producing high-maturity dry gas. The lower slope to basin floor depositional systems contain very thick, stacked packages of sand, which have high-quality reservoir properties because of rework by multiple current processes.

The Greater Tortue/Ahmeyim Field reached final investment decision (FID) in December 2018 with partners BP Exploration and national oil companies SMHPM (Société Mauritanienne des Hydrocarbures et de Patrimoine minier) and PETROSEN (Senegal). The field will be developed with a floating liquefied natural gas (FLNG) design.