Although oil and gas were first found offshore Ghana in 1970, petroleum companies showed very little interest in the West African country until exploration moved into deeper water. Since 2007, however, major discoveries in the deepwater Tano basin in West Ghana have changed the opinion of many companies, and look set to revolutionise the country as well. The Jubilee, Odum and Tweneboa fields have combined estimated recoverable reserves of possibly three billion barrels. This is a significant volume and of great value to a country where the majority of people live on an average income of US $1,500.
Long history – little success
The existence of seeps had been known for many years in western Ghana, and they were first investigated with shallow drilling as early as 1896. A number of wells found oil, and drilling continued intermittently over the next 70 years, but commercial quantities of hydrocarbons were not discovered.
In 1968 the Government of Ghana announced the first licensing round, offering 22 blocks offshore. By 1980, a further 31 wells were drilled, resulting in merely three discoveries: Cape Three Points and Tano in the Tano Basin, straddling the Ghanaian/Cote d’Ivoire border, and Saltpond in the central Saltpond Basin. Only this last was put into production, in 1978, producing 3.5 MMbo before it was shut-in seven years later.
With little further interest from the industry, the country decided to make itself more attractive, establishing the Ghana National Petroleum Corporation in 1983 and introducing new laws and incentives for exploration. This resulted in the acquisition of large quantities of seismic and the drilling of a number of wells, but still little of significance was found. Not until drilling technology advanced into deep waters was the true potential of Ghana discovered.
Success in deep water
Companies began to move further offshore, led by Hunt’s 1999 Cape West Three Points minor discovery in 1,000m water. They started looking at the western part of the Tano Basin, where 3D seismic identified interesting prospects in the deeper water. In all, four deep water wells drilled between 1999 and 2004 found hydrocarbons, proving the existence of an active petroleum system. Finally, the oil industry in Ghana had began to take off – but still nothing in commercial quantities.
Not, that is, until June 2007, when a consortium of Tullow, Kosmos, Anadarko and E.O Group announcement a ‘significant’ discovery with their Mahogany 1 well in Block West Cape Three Points. This was the Jubilee field, now estimated to contain between 600 and 1,800 MMbo recoverable, confirmed a few months later by Hyedua-1, 5km to the south-west. The two wells intersected a large continuous accumulation of light sweet 37° API oil in excellent quality stacked reservoir sandstones. Further appraisal wells confirmed the size of the discovery, and also identified extensive underlying sands which could hold further reserves. Well tests showed that production may be as high as 20,000 bopd.
The Jubilee field was followed in 2008 by Odum, about 20 km to the east, also in block West Cape Three Points, in water depth of 955m. This found heavier oil, with 22m of net pay. The following year Tullow and partners moved further west, and near the border with Cote d’Ivoire found the Tweneboa Field, thought to hold 1.4 Bboe. The second well on this field also found a deeper oil and gas condensate section.
The most recent discovery was Dzata, a Cenomanian/Albian faulted anticlinal trap found in March this year, which lies about 100km south-east of Jubilee and opens a new trend in the eastern part of the Tano Basin.
Thick Cenomanian source horizon
Much of Ghana comprises metamorphosed sedimentary and volcanic rocks over 2,000 million years old, the source of the gold which originally attracted European colonisers. The offshore, however, is dominated by Cretaceous and younger sediments deposited during and after Africa and South America rifted apart. As the continents separated, a major fracture zone system formed, trending roughly south-west to north-east, along with associated faults and folds, and these fractures and structures play a significant part in the hydrocarbon geology of the region.
Offshore Ghana comprises three main basins. The western Tano Basin and the eastern Keta Basin are primarily Mesozoic, with the Palaeozoic Saltpond Basin sandwiched between them. For many years, exploration efforts were centred on Tertiary plays in shallow water. Despite this, the only producing field, Saltpond, has a Palaeozoic reservoir, the Middle Devonian – Carboniferous Takoradi Formation, which was deposited in a fluvial-marginal marine environment when Africa and South America were still one land mass.
Pre-rift continental rocks were overlain by Lower Cretaceous (Aptian-Albian; 125–100 MA) syn-rift fluviatile to shallow marine sediments. A number of lakes formed in the rift, and the resultant lacustrine sediments provide a good source rock. The South Tano High, a major structural feature influencing the prospectivity of the area, developed at this time.
During the Cenomanian (Late Cretaceous), the whole West African margin was rapidly inundated, and thick organic-rich shales were deposited over a wide area, forming another important source horizon for hydrocarbons. Their maturity was a matter of debate, until the recent discoveries were shown to be sourced from this level.
Major fan systems
By the Late Cretaceous a deltaic complex had developed in the west of Ghana, depositing thick layers of basin floor fans and turbidity flows into the deepening Tano Basin. These are the large fan systems which form the main reservoirs in the Jubilee, Tano and Odum fields. Continued extension and subsidence resulted in deposition of thick shales, which seal Jubilee and the other prospects in the margin.
Deposition was slower in the late Cretaceous in the Saltpond Basin, with marked erosion surfaces and a thinner sedimentary package. The Keta Basin also appears to have suffered more erosion that Tano, although the main potential of the basin is still perceived to be in the Upper Cretaceous. African specialist Afren, which holds a large block in this basin, believes that this section has thick fan sandstones similar to the Jubilee play, and has identified a number of prospects. Its first well, Cuda-1, spudded in 2008, targeted a Campanian fan complex, but encountered high pressure and had to be abandoned.
A thick transitional marine sedimentary package was deposited during Cenozoic times as the whole area of offshore Ghana underwent thermal subsidence. The Tertiary has not proved productive, probably because the traps are not of sufficient size to warrant exploitation.
Legislation to avoid problems
There is much discussion in Ghana over the best way to exploit this oil bonanza, with a determination not to copy the errors of its neighbour, Nigeria. Ghanaians are keen that oil riches will be used to benefit the whole country, and they have been trying to develop an oil and gas policy that is acceptable to the general public, yet sophisticated enough to ensure oil revenues are spent responsibly. The Norwegian government has been assisting them to construct the appropriate legislation, and they have held open consultation forums around the country.
There have already been problems, as Ghana blocked the sale of Kosmos Energy’s stake in the Jubilee field to Exxon, preferring the state-run Ghana National Petroleum Corp to acquire it. Ghana does not have production-sharing contracts, and the carried GNPC participation is 10%, so the main return to the government will be in taxes and royalties. Ghana’s neighbour, Cote d’Ivoire, has threatened action, claiming that some of the fields may actually lie over the border in their territory.
Ghana has begun to build a deep sea port and improve infrastructure in the area, ready for the first oil, due ashore late in 2010. Meanwhile, it is needs to ensure transparent institutions and regulations in order to avoid the violence and corruption brought by oil booms elsewhere in Africa.
