INTRODUCTION

GAMBIA RIVER BASIN

Gambia is a West African country bounded by the

Atlantic ocean by the west and Senegal by the north, south and east.

-A river basin is an area of land drained into by a river and its tributaries.-Gambia river basin was initially called the GAMBIE and it originates from the FOUTA DJALLON mountains in the north of central guinea.

The climatic condition of gambia river basin consists of a long dry season

(november –may) and a short wet season from june to october .

The average annual rainfall ranges from 1700mm in the southern part of the

mountains to 800mm in the northern part.

Gambia river basin offers interesting potential in several aspect including hydro power ,irrigation and fishery.

The location of Banjul the capital of Gambia and its surroundingCountries.

A pictorial view showing the flow of Gambia basin.

Basin evolution of the Gambia River

The Gambia Basin is formed at the final point of the Permian(299 million to 250 million years ago) to Triassic(250 million to 200 million years ago) rift system during the breakup of North America, Africa, and South America(Paleozoic period).

Basin historypre-rift (Upper Proterozoic to Paleozoic),;sync-rift (Permian to Triassic);post-rift (Middle Jurassic to Holocene).

Evidences of the presence of folds and powerful land movement have been revealed by the study of the geographical formation.

Tectonic period after the Cambrian led to the formation of a strip of land violently folded and slightly metamorphosed from the NNE-SSW direction.

The following layers are observed when moving along the Gambia from west to east:

The Bedrock. A synclinorium worm running east. An area of peak corresponding to the Bassari

hills and showing metamorphic facets. It is a vault anticline limited to the West by a large loop.

A synclinal basin (Yukon basin). Another area of metamorphic peaks with

folds.

Coastal and marine sediments of the Eocene period are covered everywhere by sandstone deposits of continental origin, which form the "Continental terminal".

The most common faces of the Continental terminal are white stoneware clay, with large patches of reddish brown iron oxides.

Eocene layers appear in one place, at the confluence of the Gambia with the backwater of Bam baya in the region Badi; the outcrop is located in the bend of a very pronounced narrow meandering river. At this point, there is slightly calcareous sandstone, yellow, very fossiliferous, based on clay laminated greenish-grey.

Sandstones appears locally, in the bottom edge of the plateau overlooking the valley of the Gambia after Simenti or on the sides of some hills. The sandstone deposits contain levels of kaolin clay white or pink, more frequent towards the West.

This colorful sandstone grain in variable size (hétérometrique) crumbles easily. That is why it is usually covered with sand or clay loam.

The formations of the Continental terminal are all azoic. They thicken rapidly toward the northwest. They are often covered with hard iron deposits, pitted with grains of quartz.

A survey conducted in Tambacounda has crossed the 134m deep mark. It generally gives an age miopliocene to these detrital deposits.

Tectonic sedimentary layers of the Secondary and Tertiary period remained static, but it can be assumed that during the geological periods there occurred a very slow lifting of the Birrimian base and Paleozoic formations resulted, coinciding with the subsidence of the sedimentary basin of Gambia.

EXPLORATION HISTORY

The Exploration History Of The Gambia Basin.- Gambia has an upstream industry taking place

offshore Banjul. In 1998 west oil held an offshore block under a technical operation agreement, offshore block A and onshore block B.

- In October 1999 fusion oil and gas NL signed a petroleum production license for the deepwater offshore block of Banjul previously held by west oil. In December 1999 a 1000 km 2-D seismic survey was shot over the block. The license was issued for 6 years. The survey showed that Gambia had real offshore hydrocarbon potential but the production its neighbouring

countries were on disrupted vast production.

- Banjul yet again signed a license with Britain's planet oil over the offshore acreage. The largest prospect, G-8 is estimated to have oil in place reserves of up to 1.4 billion barrels.

- Still on 1st OCT 2006 the Gambia government issued two licenses to Buried Hill Energy for the exploration and production of oil and gas in the off shore areas. The Alhamdulillah A1 and A4 blocks were evaluated by the company with a high resolution 3D seismic imaging. They identified five distinct plates with 26 prospects of oil production, the result showed more oil potential than expected. The drilling of the first Gambia wells were set to start 2009.

- Along the line, African petroleum corporation announced on the 24th of August 2010 that its subsidy African petroleum Gambia entered into an agreement with the Buried Hill Gambia to acquire license for Alhamdulillah block A1 and A4 licenses covering 2,668 km sq off shore Gambia. The license extended for another two years until 31st December 2013.

the location of the offshore Gambia.

TRAPPING SYSTEM

Trapping Systems in the Gambia River Basin

Several primary reservoirs and seals in the Mesozoic- Cenozoic section of the Gambian river basin are as follows:

-Jurassic-Lower Cretaceous carbonate section sealed by Cenomanian or other lower Cretaceous shale;

-Upper Cretaceous sandstone units and overlying shale units;

-Lower Tertiary clastics and carbonate-rock units and overlying and intercalated shale units.

-According to Clifford(1986),the Mauritania offshore has rocks like the cretaceous deltaic sandstone which have porosities ranging from 17 to 25 percent. He also talked about the Jurassic-Lower Cretaceous carbonate rock-platform that showed good porosities ranging from 10 to 23 percent but have not yet been fully penetrated by drilling. Also the Upper Cretaceous sandstone sequences in the eastern part of the basin become interbedded with shale to the western offshore part of the basin.-The Mesozoic-Cenozoic section in the Gambia River Basin contains diverse oil and gas trapping configurations. These include salt-related structures, structures related to volcanic intrusion, sandstone pinchouts along the eastern margin of the Gambia river Basin and Jurassic-Lower Cretaceous carbonate bank deposits. Seals consist of Mesozoic and Cenozoic marine shales and faults.

Sandstone reservoirs associated with sync-rift rocks might be present and interbedded with the inferred Permian-Triassic source rocks underlying the Triassic salt. In the sync-rift section in the Gambia River Basin ,the thick Triassic salt is the major seal.- Potential sandstone reservoirs are abundant in lower Paleozoic rocks. The Ordovician sandstones are intensely fractured and could constitute good secondary reservoirs, whereas the Devonian fine- to coarse-grained sandstone beds have porosities ranging from 15 to 20 percent. In the onshore portion of the Paleozoic basin, regional seismic data have shown that the Paleozoic section has been faulted and could form traps in conjunction with the Paleozoic unconformity.

HYDROCARBON POTENTIALS

It is believed that the Gambia Basin has good prospects for hydrocarbon (Deep water PPL Gambia). The area marks the extent of the CASAMANCE-BISSAU sub basin which forms part of the Mauritania- Senegal – Gambia – Guinea Bissau coastal basins.

The most significant hydrocarbon formation in this area is from ;

1. The MESOZOIC section underlying the Cape Verde Peninsula onshore, &

2. The CASAMANCE sub – basin offshore

MATURATION HISTORY

The cretaceous source rocks display a highly variable maturation history.1. ALBIAN source rocks in the Mauritania sub – basin started to generate oil in the late Eocene (55 to 38 million yrs ago)2. TURONIAN and SENONIAN source rocks began to generate oil in the Miocene (24 to 5 million years ago)3. UPPER CRETACEOUS source rocks began to generate oil in the Miocene (55 to 38 million yrs ago)4. PALEOCENE source rocks were found to be immature in the Mauritania offshore.

SENEGAL PROVINCE

Hydrocarbons are gotten from two (2) areas in the Senegal Province.

1. In the offshore Mauritania and Northernmost Northern sub – basins

2. In the CASAMANCE sub – basins and Guinea – Bissau offshore

NORTH OF CAPE VERDE

The amount of generated hydrocarbons increases seawards due to the combined effect of the thickening and deepening of the Cenomanian and Palaeocene source rocks. The quality source rock onshore north of Cape Verde is not favourable for the generation of oil.

The vicinity of the Cape Verde is related to volcanism and ranges in depth from 900m (Dakar marine 2) to 1200m (Cape Verde marine 1).

CASAMANCE AND MAURITANIA SUB – BASINS

The Triassic diapiric salt has brought about the modification of a maturation gradient because of the good thermal conductivity of the salt. Within this basis, at least 2500 tons of hydrocarbon per square km have been generated mainly from the cenomanian and Turonian sources .The Casamance and Mauritania sub – basins are relatively shallow due to elevated geothermal gradients related to Salt Diapirism. The depth of the zone ranges from less than 1000 to more than 3000m depending upon the local geological and thermal parameters in the basin.

RESERVE ESTIMATESIt was measured that the geothermal gradient in those wells were nearly 450c/km. In areas where the average geothermal gradient is about 300c/km, the top of the zone oil generation ranges from 2285 to 2680m. The top of the zone of oil generation is at 2800m, where Cenomanian source rocks are immature.Gas resources may be very significant and accessible in areas where the zone of oil generation is relatively shallow.

MIGRATION OF HYDROCARBONS

Migration of hydrocarbons most likely began in the late Miocene (55 to 38 million yrs ago) and continues to the Holocene (10000 yrs ago) Maturity of the source rocks in the basin increases southward. This may be misleading due to the lack of data north of Cape Verde.A zone of salt spreads off the Mauritania coast more widespread than previously thought and may have increased maturation of the cretaceous source rocks.