Basin

There are four basins in Ghana and they are as follows;

Tano-Cape Three Point Basin/ Western Basin

The Tano-Cape Three Points Basin is a Cretaceous wrench modified pull-apart basin. It is bounded by the Saltpond Basin in the East and the St. Paul Fracture Zone in the West. The basin is the eastern extension of the Cote D’Ivoire-Ghana Basin and formed as a result of trans-tensional movement during the separation of Africa and South America, and opening of the Atlantic in the Albian. Active rifting and subsidence during this period resulted in the formation of a deep basin. Prevailing conditions at the time were ideal for the deposition of shales, thus thick organic rich shale was deposited in the Cenomanian and Turonian. Several river systems contributed significant clastics into the deep basin and led to deposition of large turbidite and channel complexes.

The basin is made up of a rift section which comprises shallow marine to continental deposits and a thick Upper Cretaceous drift section which is dominated by basin floor fans, stratigraphic traps and channel systems. The working play type is the Cretaceous Play, which consists of Cenomanian-Turonian and Albian shales as source rocks with Turonian slope fan turbidite sandstones and Albian sandstones in tilted fault blocks as reservoirs. Trapping is both stratigraphic and structural.

The hydrocarbon potential of Ghana’s portion of the basin has been known since the 1890’s based on onshore oil seeps but the first major discovery was made in 2007 by Tullow Oil Plc (Jubilee Field) with oil production commencing in 2010. The hydrocarbon prospectivity of the basin is also well known in Cote D’Ivoire where there are producing fields including Baobab, Belier, Espoir, Foxtrot, Lion and Panthere.

The Saltpond Basin/Central Basin

The Saltpond Basin is a Paleozoic wrench modified pull-apart basin centrally located between the Tano-Cape Three Points and Accra-Keta Basins. It covers an area of approximately 12,294 sq km. Sediments in the basin were deposited in non-marine to coastal marine environments, and ranges in age from Ordovician to Cretaceous. The basin has been stratigraphically divided into formations based on lithofacies and depositional environments.

These are from oldest to youngest: Elmina Sandstone (Late Ordovician-Early Silurian), Tarkoradi Sandstone (Devonian), Takoradi Shales (Middle Devonian-Early Carboniferous), Efia Nkwanta Beds (Late Carboniferous-Permian), Sekondi Sandstone (Triassic-Early Jurassic) and the Lower Cretaceous sediments. The structure of the basin is characterized by multiple faulting, which has resulted in a complex set of horsts and grabens. Unlike the Tano-Cape Three Points and Accra-Keta Basins, the Romanche Fracture Zone has limited the shallow water area available in the basin for exploration and development.

The only known and proven petroleum system in the Saltpond Basin is the Lower Paleozoic Petroleum System. This system has Devonian source rocks and Devonian to Carboniferous reservoirs. The two main source rocks are the Lower and Upper Takoradi Shales. These are dark grey shales with predominantly Type II kerogen and moderate to good TOC and HI values. The reservoirs are sandstones of the Takoradi Sandstone Formation. Trapping is both structural (fault bounded blocks) and stratigraphic (sandstones interfingering into shales) with sealing provided by the Tarkoradi Shale formation.

The Accra-Keta Basin/ Eastern Basin

The Accra–Keta Basin is a Cretaceous wrench modified pull-apart basin structurally bounded by Chain Fracture Zone in East and Romanche Fracture Zone in the West. It covers an area of approximately 33,900 sq. km of which 1900 sq. km is onshore. This basin is the western extension of the Dahomeyan embayment that stretches east covering Togo, Benin and Western Nigeria and ending just before the Niger Delta in Nigeria.

The basin’s formation history has been divided into:

• Pre- Rift Stage comprising Precambrian to Late Jurassic rocks
• Syn-Rift Stage comprising Lower Cretaceous rocks and,
• Post Rift Stage represented by Upper Cretaceous to Tertiary sequences

Studies by Abu et al. (2010) suggest the presence of a working Cretaceous Petroleum System, with at least two key mature source rocks: Early Cretaceous lacustrine shales with Types II and III kerogen and Turonian-Coniacian organic rich shales. Numerous Syn-rift Albian, Late Cretaceous and Tertiary reservoirs exit. Both stratigraphic and structural trapping mechanisms are present, with the former predominating. Several horizons mapped and interpreted on seismic sections as Cretaceous-Tertiary shales could provide sealing. Potential exploration plays such as basin floor fans and ponded turbidites exist in the Upper Cretaceous and Tertiary.

The Voltaian Basin

The Voltaian Basin is an asymmetrical inland basin covering about 40% of Ghana’s continental landmass. It stretches into Togo, Benin and Burkina Faso. The basin formed during the Pan African Orogeny about six hundred million years ago (600 Ma) as a result of the collision between the stable West African Craton and the Pan African Mobile Belt, which are represented in Ghana by the Birimian Supergroup and the Dahomeyan System respectively. Sediments are of shallow marine to continental origin and comprise sub-horizontal beds of sandstones, shales, mudstones and conglomerates, which range in age from Precambrian to Paleozoic.

The thickness of sediments exceeds 6000m in the deepest portion of the basin. The Voltaian Basin has great potential to hold significant petroleum resources. Soviet Geologists observed viscous black oily bitumen in core samples of sandstones, shales and siltstones of the Oti Group in Middle Voltaian, suggesting the presence of a working petroleum system. The basin has been stratigraphically divided into:

• Upper Voltaian, which comprises massive and thinly bedded sandstones with some shales.
• Middle Voltaian, which comprises sandstones with limestone intercalations (Upper Greenish-Grey Series), shales with limestone intercalations, siltstones, silty sandstones (Variegated Series) and conglomerates.
• Lower Voltaian, which comprises sandstones, shales and siltstones (Lower Greenish-Grey Series) and quartz sandstone (Basal Sandstone Series)