Taking advantage of recent deepwater plays and new global investment

Following Innoson Oil & Gas’ success offshore Sierra Leone, between May and September 2022 the country embarked on its Fifth Licensing Round, enabling investors to share in the recent renaissance in deepwater exploration, providing an opportunity to license large areas of deepwater acreage, containing proven light oil and condensate accumulations identified on a newly-available, high quality well and 3D seismic database (PSTM & PSDM).

Following on from the recent successes in the deepwater plays of South America, including the world-class Liza discovery in Guyana, could the Sierra Leone margin be the next in line for continued deepwater exploration success? Although an underexplored basin in terms of hydrocarbon offshore exploration with 8 exploration wells drilled to date, the Sierra Leone Basin can be considered to have great potential as shown by recent finds in neighbouring countries in recent years.

The Fifth Licensing Round was launched against a back-drop of new global investment in deepwater; with exploration activity and commercial success in Guyana, Suriname and Senegal providing excellent analogues for future exploration in Sierra Leone and the oblique margin of North West Africa as a whole. In addition, a major reduction in drilling costs since 2014 and a more stable long-run oil price have significantly enhanced the deepwater investment environment.

Sierra Leone’s geological composition and technical structural analysis

According to Getech, one of PDSL’s technical partners, most of Sierra Leone is underlain by the Sierra Leone Basin, which is the southernmost of the MSGBC basins. The Monrovia Transform Fault (MTF) divides the broader attenuated crustal zone to the north (Sierra Leone) from the much narrower West Africa transform margin offshore Liberia, showing a rapid transition from continental crust to oceanic crust (Figures 1 and 2). Getech’s integrated structural analysis utilises the company’s comprehensive gravity and magnetic data holdings, together with key seismic and well data from PDSL.

Figures 1 and 2

In its structural assessment Getech incorporates advanced data processing, integrated modeling, characterisation of crustal architecture, sediment and crustal thickness inversions to form the basis of the integrated structural analysis, which is then used to unravel some of the structural complexities associated with the Sierra Leone continental margin. One of Getech’s studies presents an understanding of the mechanisms involved in the development of play concepts at transform margins and their controlling factors. These ideas can be used predictively to inform exploration evaluations.

According to Getech, “one of the key findings is that the offshore Sierra Leone margin to the north of the MTF is influenced by classic extensional tectonics akin to the MSGBC Basin zone (albeit with some transtensional influence), whereas the area to the south of the MFZ is influenced by transform margin tectonics”. Furthermore, the undrilled northern Sierra Leone Basin has a similar structural style to the offshore Guinea region, therefore giving potential for extensional trapping styles akin to discoveries in these areas.

Challenging past quality risk conclusions

In Sierra Leone, the presence of kaolinite cementation and quartz overgrowths have in the past been cited as risks to reservoir quality. Getech’s new palaeo drainage analysis and source-to-sink studies challenge this conclusion.

Of the 8 wells drilled in the offshore area, 6 have targeted Upper Cretaceous turbidite systems with varying degrees of success. All 6 of the recent wells found slope channel and fan sandstones with gross thickness from 10 to 200m and porosities ranging from 5% to 28%. What lithologies the palaeo-drainage systems were eroding will have had a significant effect on the reservoir quality of the shelfal and slope sandstones.

Recent source-to-sink studies by Getech highlight the overall proximal nature of the drilled reservoirs in Sierra Leone and the effect of hinterland geology on reservoir quality; in particular the difference between the Sierra Leonean hinterland and the margin further south (Figure 3).

Figure 3

As stated by Getech, “typically, the Sierra Leone-Liberia Basin is considered as a single system, but palaeosurface geological reconstructions strongly suggest that while the Cretaceous hinterlands were broadly similar, the southern portion of the margin comprised a much greater proportion of metamorphic assemblages. Conversely, the Sierra Leone (northern) section accessed a greater proportion of clastic-derived sediments (for example of the Rokel River Group) which in turn increased the overall compositional maturity of sediments entering the Sierra Leone portion of the basin”.

Furthemore, the company stated that “as for kaolinite, the change in cementation from diagenetic quartz to kaolinite is due to periodic unroofing of the hinterland through time. Kaolinite is derived from K-rich feldspars which decrease in abundance with depth; these results in kaolinite cementation being more prevalent in younger reservoirs (above the K85/88.5 unconformity).

The presence of kaolinite cements, however, do not seem to impact reservoir quality, with kaolinite-rich sands from the Mercury-2 well exhibiting average porosities of 16% and, more importantly, permeabilities over 200mD (Figure 3). In fact, according to studies from Nadeau, Hutcheon and Ehrenberg, small amounts of pore-lining cements have been shown to enhance reservoir quality by preventing compaction.

The observed difference in provenance lithotypes, shown in Figure 3, is therefore very encouraging news for the potential reservoirs offshore Sierra Leone – Liberia being dominated by metamorphic, quartzite, metasandstone, whereas Sierra Leone’s clastics, sandstone and quartz-rich material points to significantly better reservoir quality.

Moreover, Cenomanian–Coniacian sections consist predominantly of fine-grained clastic sediments with sands restricted to the channelised areas. This sequence contains increased amounts of plagioclase deposited by short-headed rivers and derived from metamorphic lithologies in the foothills of the Guinea Highlands. Well sorted sandstone reservoirs are expected in distal locations for the Cenomanian–Coniacian. However, diagenetic quartz-overgrowth cementation at higher temperatures (>120˚C) is common and facilitated by the overall high amount of detrital quartz resulting in reduced poro-perms.

As announced by Getech, “the overlaying Santonian–Maastrichtian sequences display an increase in sand content. The rivers draining into the Sierra Leone depositional basin increased their catchment areas, incising into granitoids and thus increasing K-feldspar abundances in the offshore sediments. Diagenetic quartz overgrowths and kaolinite cement are less pronounced (Figure 4); Kaolinite-rich sands from the Mercury-2 well exhibit average porosities of 16% and permeabilities over 200 mD”. According to studies, the current understanding is that younger reservoirs (those above the Coniacian-Santonian 85Ma unconformity) have much improved reservoir quality with well-preserved intergranular porosity, making slope fan complexes of that age a potential target for future exploration.

Figure 4

Cuttings and Sidewall core data demonstrate high variability in the grain size, sorting and clay content, resulting from short transport distance and debrite deposition on the slope; these are observed in both sequences.

Finally, “in the undrilled deep to ultra-deep southern area thick Cenomanian–Turonian shelfal sands intersected up-dip by wells could well be eroded from the shelf and deposited further offshore in channel-fan complexes with potential quality reservoirs. In the unexplored deep to ultra-deep water off the northern and central shelf Cenomanian turbidite, fan and channel sandstone could form potential reservoirs as seen to the north in the Guinea Basin. Sands reaching the slope and basin floor across the wide shelf from questionable feeder rivers remains to be answered by drilling”, stated Getech.

Excellent opportunities in an underexplored Sierra Leone

The offshore Sierra Leone acreage remains highly prospective and underexplored, though wells have already encountered light oil and condensate in significant quantities within the basin. There is a proven petroleum system; prospective open acreage with water depths ranging from <100m to >4000m; a stable legal and fiscal regime; a wealth of high-quality well and 2D & 3D seismic data, and 4 undeveloped discoveries (2009–2013). Whilst the reservoir quality in Sierra Leone is a complex interplay between provenance sedimentology and diagenetic effects, integration of well results with Getech’s source-to-sink studies indicate that a good quality reservoir is present in the Upper Cretaceous reservoir.

With ongoing successes on the conjugate margins of Senegal and Guyana, Sierra Leone’s Fifth Licensing Round offered global investors an excellent opportunity to enter this exciting area of the Equatorial Atlantic Margin.

About the Author: Felipe Gaitán Michelsen