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PESGB London Evening Lecture

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September 10, 2013

 

Tuesday 10th September 2013 

Wembley Stadium, Wembley, London (please note: different venue this month)

Getting to Wembley Stadium with ease: http://www.wembleystadium.com/TheStadium/GettingHere.aspx

This lecture proceeds The 12th PESGB/HGS Conference on African E&P 11th - 12th September 2013

For more information on the conference click here 

West Africa Pre-Salt Rush: new ideas on proven and emerging trends.

Abstract

In West Africa the pre-salt is an established play since the 50’s. Even so, only recently the expectations for a potential extension of the Brazilian Santos/Campos Basins proven pre-salt carbonate play boosted spectacular interest on the Kwanza Basin (South Angola). The “Sag carbonate Play” gained most of the attention and promise to be the real “sweet spot” of future pre-salt exploration, at least in Angola. Elsewhere on the West African margin, like in Gabon, in the unexplored Deep water trend established clastic pre-salt plays gained new attention. Moreover, new technologies and ideas, allow to re-approach the great complexity of the pre-salt play on the historically explored trend (onshore and in conventional waters) along the whole West African margin.

Introduction

The deep water pre-salt trend in the entire South Atlantic rift basin is in the exploration spotlight since the giant discoveries in the Santos Basin (Brazil) in 2006. The opening of the pre-salt play in the Santos Basin has added over 30 billions barrels of oil between 2006 and 2011, with just 32 wells drilled, thus resulting in an exceptional gradient of the creaming curve for the play (Wood Mackenzie, 2011). Syn-rift and Sag sequences pre-dating the widespread deposition of South Atlantic evaporite (110-120 m.y.) are subject to receive increasing attention by industry and academia. A number of recent papers have been focused on similarities and differences between the two sides of the rifted margins in the Berriasian to late Aptian time interval (e.g. Karner & Gamboa, 2007; Torvisk et al., 2009; Unternehr et al., 2010).  Rift basins located south of the present day Congo River (Lower Congo and Kwanza Basin) have been reported (Lentini et al., 2010) to share with their conjugate proven pre-salt hydrocarbon provinces (Santos and Campos) geometrical and depositional patterns. The results of the recent Angola pre-salt bid in Kwanza Basin, with 3.5 billions $ bonus paid and 20 exploration wells committed testify the huge interest of most Major oil companies for the potential of the pre-salt play. eni (formerly Agip) has been a major actor in the region since the beginning of the upstream activities and is today in the front row to face the exploration challenges of the pre-salt both on established and frontier areas and themes . In this paper we briefly discuss the pre-salt structural and sedimentary setting along the Gabon, Congo and Angola Aptian Salt Basin, with a focus on the sector outboard of the onshore hinge, with the aim to highlight analogies and differences between proven and emerging trends and their exploration potential. 

West Africa pre-salt exploration background

In West Africa the pre-salt is an established play and has been actively explored in-board of the Atlantic Hinge Line (on-shore and in conventional waters) since the 50’s.  In 60 years of exploration 109 oil fields have been discovered (Fig. 1A) with cumulative reserves close to 4 billion of barrels of oil equivalent (65% on-shore). Apart from a single giant field, few discoveries exceed the 100 MMbbl reserves threshold. A wide spectrum of clastic and carbonatic reservoir (Fig. 1B) distributed along the whole pre-salt stratigraphic section has been proven effective in the past activity. World class lacustrine source levels have been proven both in syn-rift and Sag sequences (Fig. 1C). The play has not yet reached a mature status, mainly because of geologic complexity and technological challenges involved. Moreover, the deep-water “golden age” of the post-salt Tertiary turbidite play, started mid 1990’s and not yet over in Angola, acted as a catalyst for Majors’ exploration investments. As a result, the pre-salt play out-board of the Atlantic Hinge was virtually un-explored in West Africa, since 2011. 

The carbonate Sag Play. Proven and emerging provinces.

Although differences in large scale geometries at crustal level and pre-salt sequences subsidence patterns are evident when comparing West Africa and Brazilian margins, evidences supporting the frontier “Tupi” carbonate play on the deep water trend of West Africa are similarly significant (Fig. 2). Development of a thick Sag Basin (onlap fill sequence of Lentini et al. 2010) and existence of an Outer Trend of basement high at the external edge of the Sag Basin are imaged on recently acquired long–offset seismic. Outer Basin shape and infill as well as the development of a complete syn-rift sequence on the outer trend is instead still a matter of debate due to the lack of quality seismic data.  

Lacustrine carbonates are well known reservoirs in the pre-salt sequence of West Africa margin. Molluscan rudstones (Coquina) and high energy ooidal-oncoidal packstones (Toca Formation) are recognized in Congo, Cabinda (Fig. 3A) and Kwanza basins. Reservoir effectiveness seems strictly related to the hydrotermal diagenetic overprint that locally solved and enlarged fractures and pores. The origin of the carbonatic reservoir of the Lula Field (formerly Tupi, Santos Basin, Brazil) is still controversial (Fig. 3B). A subaerial origin has been envisaged as alternative to the more conventional lacustrine to brackish stromatolites (microbialites) interpretation. Subaerial carbonates (Travertine, Fig. 3C) and their relationship with Volcanic activity gained recently the attention of pre-salt explorationists in West Africa, being the focus of geological reconstruction of potential pre-salt target in Kwanza Basin ( Sharp, I. et al., 2012).

Potential source facies within the large sag basin developed down-dip of the Atlantic hinge is un-proven so far. Scattered well data tent to indicate the presence, at the internal edge of the Sag Basin, of organic rich levels, time equivalent to the well known Organic Boucomazi / Marne of Pointe Noire source. Thermal regime and source reactivity remain a matter of concern in the fetch areas of potential Outer Trend leads. Deep seated syn-rift source, which effectiveness is largely proven in Northern Angola and Congo could still play a basic role in Kwanza Basin emerging Inner Trends. 

West African “Tupi Myth” perspectives. 

Industry have high expectations on the potential of the “Tupi Play” in the pre-salt un-explored deep water trend of West Africa. There are a number of structural and geological evidences confirming that the play elements could be present and effective similarly to the reference proven basins of the Brazilian margin. In Kwanza Basin reconstructed Barremian/Aptian South Atlantic margin configuration have driven attention on the Sag Play (Fig. 4). Preliminary outcomes of the new exploration season started in Kwanza Basin (Cameia-1 well, Cobalt 2011) tent to confirm that this could really be the “true emerging trend” of the next decade in Angola. Supergiant size structures of the Santos Basin are likely not foreseen for the sector of the basin where the current activity is concentrating but still wide geologic uncertainties cover the external domains, volcanic rich, and still poorly imaged on seismic.

Frontier Sag exploration do not means only Carbonate in West Africa. If Angola sag Basin is likely to remain out of the exploration interest for a while (target depths), in DW Gabon a new decade for the clastic Dentale exploration is likely to start after key offshore tests currently ongoing.

Along with the carbonate Sag emerging Play in Kwanza Basin (Angola), an overall re-increase of exploration activity on West African pre-salt is likely in the next few years. Improved seismic imaging could support re-evaluation of under-explored targets (eg. Basal sandstones offshore Congo) and de-risk challenging exploration , as in subtle structural closures of terminal clastics (Gamba, Chela Fm.) in Gabon, or in diagenetic and stratigraphic closures of Toca carbonates in Angola and Congo. The proven Toca trend in North Angola/Congo could receive new attention. Difficulties in predicting reservoir distribution can be now overcome by appropriate depth imaging on seismic as well testified by the recent exploration success in Block 0 (Cabinda, Angola).

For this reason is our opinion that the West African “Tupi” will probably be found looking in more than one direction.

 

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