Pre-salt of West Africa, twin or distant relative of Brazil sub-surface?

Pre-salt of West Africa, twin or distant relative of Brazil sub-surface?

19th July 2016

Billions of barrels of hydrocarbons contained within world class carbonate reservoirs lie beneath circa two kilometres of salt in the Santos and Campos Basins of offshore Brazil. At the time that the pre‑salt petroleum system was developing (Aptian), Brazil was immediately adjacent to the Salt Basins of West Africa extending from the Niger Delta in the north to the Walvis Ridge in the south (Figure 1). With the success of the Brazil pre-salt, it is no surprise that interest in the pre‑salt plays of West Africa has developed, but to what extent is Brazil an analogue?

Figure 1: Location of the Aptian Salt Basins in West Africa

145 million years ago, Africa and South America formed part of the Gondwana supercontinent. Large scale rifting led to the formation of these two continents throughout the Cretaceous (Figure 2). The initial rift phase produced a series of asymmetrical basins, within which restricted depositional environments were established. Deposition within these basins often occurred under anoxic conditions leading to the formation of petroleum source rocks in addition to localised clastic reservoir facies. As active rifting ceased, a “sag phase” developed, and resulted in deposition of a series of continental, fluvial and transgressive lagoonal sediments on the developing passive margins, which included microbial carbonate and siliceous facies draped over basement highs. Successive marine incursions across the Walvis Ridge into the partially restricted basins to the north during the Aptian led to development of a thick evaporite sequence through repeated marine incursion‑evaporation cycles. The evaporite sequence is predominantly comprised of salt and acts as a semi‑regional seal for the pre‑salt petroleum system. Fully marine conditions were then established following deposition of the salt on both sides of the southern Atlantic Ocean, resulting in deposition of the highly productive post‑salt marine petroleum systems.

Figure 2: Plate reconstruction of the development of the South Atlantic from onset of rifting in the Berriasian (left) to open marine conditions in the Albian (right)

Pre-salt basins are exciting but technically challenging plays to explore and develop. In addition to the technical difficulties associated with the presence of the salt, the pre‑salt plays are often deeper, resulting in a bias toward the shallower post‑salt reservoirs in the early exploration phase. As a result it has largely been the post-salt plays of worldwide basins which have been the focus of the majority of exploration efforts throughout the history of oil and gas exploration, with several obvious exceptions including the Southern North Sea Anglo‑Dutch Basin and the Dnieper‑Donets Basin of Ukraine.

In this context West Africa is different, as pre‑salt exploration has been relatively contemporaneous with post‑salt exploration and has been actively explored since the 1950’s. Early exploration was exclusively onshore or in shallow water but there has been a progressive trend towards deeper water exploration targets since that time (Figure 3). Whilst the renewed vigour in exploration efforts in the pre‑salt domain of West Africa is almost certainly attributed to the success in the pre‑salt carbonate reservoirs of the Santos basin in Brazil, it is important to remember that pre‑salt clastic plays have long been exploration targets, particularly in the northern segment of the Aptian salt basins. This play is far from exhausted, with several very significant discoveries made in recent years, notably two discoveries in the Republic of the Congo namely Nene Marine (2012) and Minsala Marine (2014) and two discoveries in Gabon: Diaman (2013) and Nyonie Deep (2014).

Figure 3: Proportions of E&A wells targeting Aptian Salt Basins of West Africa from 1956 to present by water depth

It stands to reason that the palaeo‑proximity of the two margins yields significant potential for a successful analogue for pre‑salt petroleum plays. However, we need to be cautious in the application of this analogue, as the margins are often extremely diverse along-strike. This is particularly noticeable within the Aptian salt basins of West Africa where the thickness and geometry of the evaporite sequence, basement geometries and stratigraphies are highly variable along‑strike (Figure 4). Clearly one analogue cannot be uniformly applied to this margin, especially when we consider that the southern Atlantic Ocean opened from south to north (Figure 2), and therefore different depositional environments and rift phases were contemporaneous along the margin.

Figure 4: Structural and stratigraphic variation of the Aptian Salt Basins along-strike

A possible use of this analogue would therefore be comparison of the areas directly adjacent to each other on the opposing margin. The Campos and Santos basins of Brazil, which have had tremendous recent success in pre‑salt carbonate plays were adjacent to the Kwanza and Namibe basins respectively. When these basins are compared however (Figure 5), it is clear that whilst they share similarities, there are also major differences. Notably, the ubiquitous salt is much thicker (~2 km) in the Santos basin, when compared to the distinctly rafted salt geometry of much of the Kwanza basin, which has several implications to the petroleum system. Additionally it is becoming increasingly apparent, from recent well data from West Africa, that whilst the stratigraphic sequence is comparable in both basins, the relatively thick, homogenous microbial carbonate reservoirs of the Santos basin are more complex and spatially limited in the Kwanza basin.

Figure 5: Cartoon cross‑sections of the Santos basin, Brazil and Kwanza basin, Angola.

Recent exploration activity has seen an increase in the proportion of E&A wells targeting pre‑salt reservoirs in comparison to post-salt reservoirs of the Aptian salt basins of West Africa (Figure 6). The impact of the success in Brazil is evident, with a distinct increase after the Tupi‑1 discovery was made in Brazil in 2006. To a certain extent, the analogue has delivered perfectly, as the success in Brazil has encouraged further exploration efforts in West African pre-salt carbonate, resulting in several discoveries and opened up an entirely new play in Angola. However, pre‑salt exploration drilling in Angola has revealed that Brazil does not provide the perfect analogue and considerable additional work is required to understand the full potential of the extensive Kwanza basin. 

Figure 6: Proportion of E&A wells targeting pre vs post‑salt reservoirs in the Aptian salt basins of West Africa from 1954 to present.

Perhaps in terms of the relationship between the two margins, rather than twins or distant relations, we should consider the margins to be cousins, who lost contact ~110 million years ago.

REFERENCES

· Brownfield, M. E & Charpentier, R. R (2006). Geology and Total Petroleum Systems of the West-Central Coastal Province (7203), West Africa. U.S. Geological Survey Bulletin 2207-B, 52p.

· Feijo, F. J. (2013). Santos Basin: 40 years from Shallow to Deep to Ultra-Deep Water. AAPG Search and Discovery, Article 10553.

· Heine, C., Zoethout, J. & Dietmar Müller, R. (2013). Kinematics of the South Atlantic rift. Solid Earth 4, 215-253.

Authors

Pre-salt of West Africa, twin or distant relative of Brazil sub-surface?
Pre-salt of West Africa, twin or distant relative of Brazil sub-surface?

Dr. Stephen Wright

Technical Director: Geoscience - stephen.wright@gaffney-cline.com

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