|PLAY FAIRWAYS ANALYSIS AND HYDROCARBON POTENTIAL OF THE KETA BASIN, DEEP-WATER GHANA|
|Exploration Hot Spots|
The Offshore Keta basin, a part of the Gulf of Guinea province which includes the Ivory Coast, Tano, Saltpond, Central Benin basins and the Dahomey embayment has recently been an area of strong interest by the oil industry. Exploration offshore Ghana received a tremendous boost with a number of large discoveries recently made in Cape Three points West and Deep-Water Tano. This basin developed as a transform margin along the Romanche Fracture Zone (RFZ) as Africa separated from South America beginning in the Early Cretaceous. Water depths range from 0m – 2800m. Figure 1 shows the location of the Keta basin study area, wells and proximity to other fields in the Gulf of Guinea Province.
WesternGeco, in a joint venture with the Ghanaian National Petroleum Company acquired a GSI 1983 survey comprising 7226km of 2D seismic survey. The data was reprocessed in 2003. The project dataset consists of 17 widely spaced 2D lines covering a total area of 7226km. This includes 5 Strike lines (NE– SW) with a spacing ranging from 10 to 15 km and 12 Dip lines (NW – SE) with a line spacing of 10km. Data quality is fair.
Previous exploration activity in the area was focused on the shallow water and onshore area, mainly targeting tertiary plays and was unsuccessful. There has been some shift from the shallow water areas to the deepwater areas of the offshore basins. Four deepwater wells drilled in Ghana between 1999 and 2003 have proven the existence of an active petroleum system in the deepwater. Significant analogues are the Jubilee and Odum discoveries in the west of the country. Only two exploratory wells were drilled in offshore Keta in deep water in 2003 (Tarpon 1X) and 2008 (Cuda 1X).
This study assesses the prospectivity of this basin by interpreting key stratigraphic sequences, an understanding of the basin evolution, an identification of key elements and processes for a working petroleum system and an identification of structural and stratigraphic prospects. The evaluation of the hydrocarbon potential of the Keta basin, the Jubilee and Odum discoveries in deepwater Ghana, similar proven petroleum systems in offshore Cote d’Ivoire and growing interest by the industry suggests that there is significant exploration potential in this relatively frontier deepwater basin.
The Ghanaian continental margin formed at the culmination of Late Jurassic to Early Cretaceous tectonism. This was characterized by both block and transform faulting superimposed across an extensive Paleozoic basin during breakup of the African, North American and South American paleocontinents. It is highly influenced by transform tectonics and evaporates and halokinesis is absent in the region. A prominent marginal ridge separates the Ivorian basin from this area of study. This province is structurally divided by three major transform fault zones, the St. Paul fracture zone along the northwestern boundary, the Romanche fracture zone that separates Ivory Coast and the Central and Saltpond Basins from the Keta Basin and the Chain fracture zone along the eastern boundary. The keta basin is regionally located within wrench modified basins initiated during the Late Jurassic rifting of the Atlantic ocean basin. The area has undergone a complex history which can be subdivided into three main phases:
|REGIONAL STRATIGRAPHY AND SEISMIC INTERPRETATION|
Data analysis mainly involved the identification of seismic events using Western Geco’s 2003 reprocessed 2D seismic survey. Published information on the Dzita-1 stratigraphy was used to tie information in the onshore part of the basin and the top of the middle Miocene and the top of Cretaceous were interpreted from the Tarpon 1X deepwater well. A generalized geo-seismic section shows the structural character and features of the Ghana transform margin and its offshore extension in figure 2.
The Keta basin is located in a passive continental margin. Seismic shows that it has a narrow shelf and steep continent ocean transition. The Ghana margin formed along the paleo Romanche transform fault and coincides with the most prominent fracture zone in the Equatorial Atlantic. The Romanche Fracture zone divides the Ghana shelf and separates Ivory Coast from the study area. The RFZ (Romanche Fracture Zone) is a principal bathymetric feature of the Ghana margin and is represented by a prominent submarine escarpment where water depths drops from 500m on the shelf to >6000m across a wide zone and transform displacement is interpreted to have occurred in quite a narrow zone (12 – 25km).
Three principal stratigraphic sequences representing Pre – Rift, Syn – Rift and Post Rift strata were identified on seismic sections. Lack of access to well data made this correlation very challenging but the most reliable interpretation was made from sparse information from articles on the Dzita 1 onshore well and the Tarpon – 1X deepwater well. The Pre-rift sequence consists largely of Paleozoic strata ranging in age from Pre - Devonian to Carboniferous. The overlying Synrift sequence ranges in age from Aptian to Albian and consists of siliciclastic strata.
This study confirms the presence of a working petroleum system. The most important petroleum system in this area is the Cretaceous Total petroleum system consisting of Albian, Cenomanian and Turonian marine shales. Basin modeling suggests the presence of several source rocks, importantly Early Cretaceous Neocomian lacustrine source rocks and Albian mature source rocks.
Reservoirs include syn-rift Albian sandstones, Coniacian to Maastrichtian sandstones and tertiary reservoirs.
Migration pathways - The first suggestion here is that hydrocarbons would migrate up vertical faults and lead to charging of synrift Albian reservoirs, late Cretaceous reservoirs and Tertiary reservoirs. Also in the case of stratigraphic traps, migration would most likely have occurred by direct charging of sandstones directly overlying Cretaceous shales and Miocene shales. Secondary migration is thought to have occurred both laterally and then vertically up the major faults into the unconformity related closures.
Traps are primarily stratigraphic and some structural traps exists.
Extensive seals occur throughout the Cretaceous and include marine shales
The petroleum potential of this basin shows exploration plays both in the Tertiary and in the Upper cretaceous. These include an Albian growth fault play, Cretaceous basin floor fans, ponded turbidites, Tertiary basin floor fan and drape anticlines. Uncertainties include migration pathways, seals and potential reservoir quality.
This study recognizes the effects of rifting, block faulting, uplift and erosion in the Keta basin. It’s geological history and petroleum systems is common to the other basins of the Gulf of Guinea. Prospectivity of this area is based on the:
Significant leads have been identified in this basin and may develop into drillable prospects with further analysis. Most of the leads will receive and entrap oil expelled in late Cretaceous and early Tertiary. The quality of reservoirs remains an uncertainty in the area and has been assessed mainly by analogy with nearby oil and gas fields.
In summary, seismic interpretation of this area suggests that the area is highly prospective.
We would like to thank WesternGeco for providing the seismic data utilized in this study and for giving permission to publish this paper.
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