Old basins still attract new exploration investment, with great success in the case of the Tano Basin in the transform margin of West Africa. Spanning the eastern Cote D’Ivoire and western Ghana offshore, the Tano Basin represents a large Mesozoic margin wedge, with a thick succession of Cretaceous clastics and localised carbonates, caught between the abrupt margin of the coast and the major transform faults sweeping west to east in the deep water.
The resulting series of thick marine clastics and prolific source rocks have been clearly proven across the basin, from Gye Nyame in the east to Baleine in the west. A great number of opportunities still exist amongst these large discoveries, with farm-in opportunities and open acreage available.
Graben fault blocks
The original focus in this area was on the shelf through the 1970’s and 1980’s, with companies testing the large broad pre-rift Albian graben fault blocks trapped against the syn-rift basin of the transform margin. This resulted in finds such as Espoir in Cote D’Ivoire and the shallow Tano fields in Ghana. Post-rift discoveries were made in these shallow and shelf environments, and operators only now are returning to smaller, mixed-phase fields such as at Ibex, Eland and Kudu in Cote D’Ivoire and North and South Tano and Ebony in Ghana. These represent excellent opportunities to monetise smaller fields in jack-up territory.
Moving into deep-water
By the 2000s, deep-water exploration had taken off, and 2007 saw the discovery of Jubilee, a giant oil field with turbidite style reservoirs in the Turonian and world-class source rocks in the Cenomanian and Turonian. This so-called CT play has now been extended across both sides of the border. Whilst there was a noticeable gap in any major discoveries immediately on trend with Jubilee, with Tullow, Anadarko and Lukoil amongst others drilling several dry and sub-commercial wells in that play, recent exploration has produced better results at Sankofa and Afina in the east and most recently Baleine in the west.
These latest discoveries have pushed the exploration model to include Cenomanian and Albian targets, leaving the Jubilee and TEN cluster of fields to sit in the sweet spot of the Upper Cretaceous fairway. Jubilee itself has recently been extended with drilling at the Jubilee Southeast extension. In the last 18 months Eni in particular has had great success extending these slightly deeper trends, through Aprokuma. Eban and Akoma, on trend with Gye Nyame and Sankofa, and at Baleine and Baleine East, setting the trend in Cote D’Ivoire.
Whilst there was a noticeable gap in any major discoveries immediately on trend with Jubilee, with Tullow, Anadarko and Lukoil amongst others drilling several dry and sub-commercial wells in that play, recent exploration has produced better results at Sankofa and Afina in the east and most recently Baleine in the west.
Busy data rooms
Whilst these successes have been notched up, the industry is catching up and moving quickly to take advantage of experience and best practice learnt in this mature basin. The Petroleum Commission in Ghana has been around the world with a 6 Block roadshow, encouraging investment in shallow water marginal fields and deepwater exploration. Petroci likewise is heavily promoting their acreage, with new online data rooms available. Press reports recently suggest that Murphy, Petroci and Tullow have all been awarded new blocks in and around the Baleine discovery at CI 709 for example. DNO have made a significant entry to the west, joining Foxtrot and Heling in the CI 27 developments. Data rooms are busy, and further awards and applications are expected to be announced soon.
Farm-in opportunities are also grabbing serious attention, with firms looking to enter the basin with established and experienced players, laden with good data and mature prospects. Heritage Oil have a number of low-risk prospects in Offshore Southwest Tano (OSWT), adjacent to Jubilee and TEN. Amni, Eco, Springfield and Base Energy have opportunities in the area, while Aker and partners seek to develop the Pecan cluster of fields with new partners and investors.
There will always be mileage in the aphorism “the best place to find oil is to look where it has already been found”, and this is fast becoming a call to arms across most of the exploration industry, as greenfield exploration struggles to find funding, and major basins like the Tano Basin deliver excellent lessons in perseverance and value creation.
