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Jubilee Field

Jubilee Field
Jubilee Field, Ashantiland Peninsula
Country	Ghana
Region	Ashantiland Peninsula
Location	Tano Basin
Coordinates	4.49278,-2.916667
Operator	Tullow Oil - 35.48% Kosmos Energy - 24.08% Anadarko - 24.08% Ghana National Petroleum Corporation - 13.64% PetroSA Ghana - 2.73%
Field history
Discovery	2007
Start of production	2010
Production
Current production of oil	74,000 barrels per day (~3.7×10^6 t/a)

The Jubilee field is an oil field located in the deep waters offshore Ghana in the Ashantiland Peninsula. The oil field, which spans roughly 27,000 acres, was globally one of the largest finds of 2007, and the largest find of the decade in Africa. The geology of the Jubilee field shows reserves are located in the upper Cretaceous structural-stratigraphic play off the coast of West Africa.

Discovery

Evidence of hydrocarbons off the coast of Ghana has been evident since the early 1890's with recordings of onshore oil seeps; however it was after several months of exploration in the South Atlantic Ocean, that Kosmos Energy identified upper Cretaceous structural-stratigraphy along the transform margin of West Africa. Two exploration wells; Mahogany-1 (M-1) and Hyedua-1 (H-1) were drilled 5km apart in this region, and intersected intersected large continuous accumulations of crude oil. On Saturday, December 22, 2007 it was announced that oil reserves nearing 3 billion barrels had been discovered off the countries coast.

Geologic History

The Jubilee field is located in one of Ghana's four sedimentary basins: the Tano-Cape Three Points or simply the Tano Basin. This basin spans from Ghana's eastern border to the eastern border of Cote d'Ivoire and was formed in the Cretaceous period. Tectonic activity throughout this period shaped the basin.

Africa and South America plate boundary during the Cretaceous Period

Early Aptian Stage

The Early Aptian Stage of the Cretateous period saw the beginning of the separation of the South America and Africa plates. The separation was initiated by strain localisation and rapid lithospheric weakening in the southern Atlantic segment. The result of this crustal weakening was progressively increasing extensional velocities in oceanic crust, in addition to a significant increase in rotation of the extension direction to North East–South West.

Early Albian Stage

An increase in oceanic crust separation gave way for subsequent weakening and extension of the central Atlantic segment during the Early Albian Stage. As extension began to move northward to the equatorial Atlantic segment, South America and Africa fully separated with the last point of contact being the western coast of Cote d’Ivoire. At this time, the Tano Basin received substantial clastic sediment input from the African continent.

Middle Albian Stage

During the Middle Albian Stage, the hotter, softer plates between the central and equatorial Atlantic segments allowed for thermal uplift and subsequent erosion, which added significant sedimentary deposition to the Tano Basin. During this stage, continued separation of the equatorial Atlantic segment was evident. Unlike the opening of the southern and central Atlantic segment, the opening of this segment was not only perpendicular to the West African transform margin, but plate motion was coupled with oblique movement and tearing along faults.

Cenomanian-Turonian Stage

With a reduction in plate motion during the Cenomanian-Turonian Stage, further deposition to the Tano Basin was due to sediment gravity flow. This sedimentation allowed for the development of turbidite fans from West Africa's coastal region to the deep ocean.

Stratigraphy

The stratigraphy of the Tano Basin can be categorized by age of the lithostratigraphy and the corresponding depositional environment. Sequences in these categories are outlined below;

Lower Cretaceous

Initial deposition to the Tano Basin began in the Aptian age of the lower Cretaceous with the deposition of clastic sediments in the form of sandstone with a mix of shale. This deposition formed the basement rock for the basin at a depth of 7000 to 9000ft. The next series of deposition was during the early Albian age with the deposition of another shale called the B-shale series. The B-shale is considered the most important system in the Tano basin as it is the most abundant series in the Tano Basin and also assumed to be the source rock of this basin. Sandstone was deposited after the B-shale deposition was followed by breakup in sedimentation. During the upper Albian age, there was an uplift event followed by erosion which signified a major unconformity in the basin.

Upper Cretaceous

At the start of the upper Cretaceous period during the Cenomanian stage, shale was deposited in the basin, followed by sandstone mixed with huge deposits of limestone. The Turonian to upper Santonian section was defined by the deposition of medium brownish-grey shales and claystone, often non-fossiliferous, with traces of limestone and dolomite. The thickness of this depositional sequence was about 920 feet. During the Campanian age, there was a further deposition of a shale mixed with stringers of limestone and dolomite. The Maastrichtian age saw relatively thin deposition of claystone with occasional sandstone and dolomite.

Tertiary

The early Tertiary period, during the Middle and Lower Eocene ages consists of finely laminated, dark grey to brown claystone with thin beds of fossiliferous dolomite and fine sandstone. Large portions of the Paleocene, upper Eocene and Oligocene consist of thin bed of laminated claystone. These ages are attributed with extensive uplift and erosion.

Quatenary

The Quatenary period saw little to no deposition in the Tano Basin.

For the sake of being restricted from posting a cartoon image of the stratigraphy of the Tano Basin, a simple table outlining this stratigraphy is provided below;

Age		Deposits	Depositional Environment					Tectonic Events
Quaternary								Drift Sequence (Post Rift)
Tertiary	Miocene	Thin beds of laminated claystone
	Oligocene	Thin beds of laminated claystone
	Eocene	Laminated claystone with thin beds of fossilliferous dolomite and fine sandstone	Submarine Channels
	Palaeocene	Thin beds of laminated claystone	Marine shelf deposits
Upper Cretaceous	Maastrichtian	Thin deposition of claystone	Marine shales (Calcerous in parts)
	Campanian	Shale with stringers of limestone and dolomite
			Dark organic rich marine shales
	Santonian	Brownish-grey shale with claystone with occassional limestone and dolomite
	Coniacian
	Turonian		Organic rich marine shales
	Cenomanian	Sandstone mixed with limestone
		Shale	Marine shelf limestones
Lower Cretaceous	Albian	Sandstone	Shallow marine nearshore sands					Rift Sequence (Onset of rifting in Southern Atlantic)
		B-Shale (source rock)	Mixed marine and terrestrial sands and shales
			Continental deposits
	Aptian	Sandstone with a mix of Shale

Hydrocarbon Potential

Source Rocks

There are three source rocks in the Tano Basin; the upper Albian source rock, the Cenomanian source rock and the Turonian source rock, all made up of shales. These source rocks are interbedded with the reservoir rocks. The kerogen types contained in these shales are primarily the type II and type III with evidence of type I and type IV. Most of the source rock samples proved to be in the range of immature to marginally mature source rock. The Total Organic Carbon (TOC) potential of the source rock samples ranged from 1-4% wt, indicative of good potential for oil generation.

Reservoir Rocks

There are three main reservoir plays in the Tano Basin; the Albian reservoir play, the Cenomanian series and the Turonian series, all of sandstone lithology. The Turonian series is the most successful and important of all the three plays in terms of accumulation of hydrocarbons in commercial quantities. This reservoir drives majority of the Jubilee Fields' hydrocarbons. Porosity values in this play ranges from 17-23% with permeability of 100- 3000mD. The sandstone in the Albian reservoir is the second most productive with porosity between 17% to 22%, maximum permeability of 2000mD and maximum oil pay of 58m. The Cenomanian sandstone reservoir has not been proven to be productive in this scope.

Seals and Traps

In the Tano Basin, Marine shale forms most of the seals in the basin with both stratigraphic and structural traps being the trapping mechanism. Unconformity surfaces and local fault patterns also contribute to seal integrity. In addition, faults that run down to the basin act as hydrocarbon leakage barriers in the reservoir. There are three seal plays in the Tano Basin; the Albian shale series, the Cenomanian Shale series and the Turonian Shale series. The Albian series consists of structural traps with domes, fault blocks, and anticlines. The Cenomanian play has stratigraphic traps formed by sand deposits, while active rifting and subsidence form normal faults and anticlines creating structural traps. The Turonian play also consists of structural and stratigraphic traps including pinch outs, normal faults and anticline.

Migration

Majority of the seals and reservoir rock in the Tano Basin are predicted to be in close proximity to the source rocks therefore a minimal hydrocarbon migration pathway is assumed. Turbidite fan systems create migration routes in productive reservoirs

Production

Kwame Nkrumah Floating Production, Storage and Offloading (FPSO) vessel above Jubilee Field

With the discovery of accumulations of light crude oil in the Ashantiland Peninsula, operators sought to produce the field; including Tullow Oil, Kosmos Energy, Anadarko, Ghana National Petroleum Corporation (GNPC) and PetroSA. In late 2010, Tullow Oil company began the production phase with 9 producing wells, 5 water injectors and 3 gas injectors. Production from the wells were directed back to Kwame Nkrumah, a Floating Production, Storage and Offlaoding (FPSO) vessel above the field. Production from these wells averaged 103,000 barrels of oil per day.

In February 2016 an issue with the turret bearing of the Jubilee FPSO vessel forced the vessel to be shut down in order to implement new operating procedures. It wasn't until 9 May 2016 that production resumed. Tullow Oil is predicted to end 2016 with an average gross production of 74,000 barrels of oil per a day.

Further Reading

• Pletsch; Erbacher; Holbourn; Kuhnt; Moullade; Oboh-Ikuenobede; Soding; Wagner (Pergamon). "Cretaceous separation of Africa and South America: the view from the West African margin". Journal of South American Earth Sciences. 14