Lessons learnt from recent dry wells in northwest Africa
Industry interest in the Mauritania-Senegal-Guinea Bissau-Conakry (MSGBC) Basin has heightened in recent years with the SNE and FAN discoveries, offshore Senegal in 2014, as well as a successful drilling campaign in 2015-16. Recently, however, many high-profile dry wells suggest that the geological risks in these basins are not fully understood at present. The first exploration-drilling programme during 2015-16 (led by Kosmos Energy in partnership with BP) boasted a 100% success rate across five wells. It included major dry gas discoveries at the Greater Tortue Complex (Tortue, Ahmeyim, and Guembeul), Marsouin, and Teranga. During this campaign, more than 50 Tcf of gas was discovered. The wells targeted gas reservoirs in Cenomanian channel sandstones in combination traps with a strong structural component. Although industry excitement was high regarding the large gas volumes discovered, many exploration and production (E&P) companies hoped for the potential of significant volumes of economically recoverable oil in the basin. With the objective of discovering a new oil province, the 2017-18 drilling campaign shifted from slope-channel sandstones to focus on an outboard basin floor fan fairway. A liquids-rich pay zone was believed to be more likely here because of expected lower maturities of potential source rocks basin-ward. Four independent prospects were high graded from a large inventory to provide the best chance of discovering liquids in volume. The wells were drilled between May 2017 and January 2018 and resulted in a 25% success rate (with the only discovery of dry gas). The results of wildcats provide hard evidence for where the petroleum system works and where it does not. Commonly, many of these failures constitute a prospect-specific risk. However, understanding the likely chance of this risk reoccurring on a new prospect is key to future exploration campaigns. Therefore, placing these wells in the wider context of a basin review, it is possible to better understand and differentiate basin-scale risk and prospect-scale risk.