SEDIMENTARY BASINS IN NIGERIA: BENUE TROUGH

A WINDOW INTO THE BENUE TROUGH

GROUP V (2014/2015)

OCTOBER 2014

GROUP MEMBERS

UMA JOSHUA AJIKE - 12CN014027

ONOKWAI ELIZABETH -12CN014017

AGBATOR SHEDRACK -12CN013977

OKONEDO TOSAN RACHAEL -12CN014011

ETAFO WENDY -12CN013995

OGAR SCHOLARSTICA -12CN014008

EMMANUEL EBODAGHE -12CN013988

ADETOKUNBO KAYODE - 12CN013974

ASUQUO VICTOR -07CF05823

ABSTRACT The Nigerian Benue Trough is an intracratonic rift structure which evolution is related to the Early Cretaceous opening of the South Atlantic Ocean and the Gulf of Guinea. The Benue Trough in Nigeria consists of a series of rift basins which form a part of the Central West African Rift System of the Niger, Chad, Cameroon and Sudan. Basement fragmentation, block faulting, subsidence and rifting accompanying the opening of the South Atlantic Ocean led to the deposition and accumulation of sediments ranging between 4000 to 6000m in the greater Benue Trough along the 800km axis over a width of ca.120km from the northern parts of the Niger Delta Basin in the south west to the fringes of the Chad Basin on the north east .The trough, is divisible geographically into the Lower, Middle and Upper Benue basins with the Lower Benue consisting of the Abakaliki and Anambra Basins and the Middle Benue Basin occupying the region

While exploration efforts for oil and gas have been concentrated on the adjacent Tertiary Niger Delta Basin, very little attention have been paid to the Cretaceous inland basins especially the southern segments of the trough with obvious stratigraphic continuity despite the commercial discoveries of the structural related rift basins of the Niger, Chad, Sudan and Cameroon within the same trend.

TABLE OF CONTENTS- INTRODUCTION- LOCATION OF THE BASIN- GEOLOGY OF THE BASIN- TECTONIC HISTORY- PETROLEUM POTENTIAL OF THE BASIN- CONCLUSION

A basin is a depression on the earth’s surface relative to surroundings. Basins are usually caused by erosion. Examples are deep ocean basin, in tramontane basin etc. A sedimentary basin is a low area on the earth’s surface relative to its surroundings e.g. deep ocean basin (5-10 km deep), intramontane basin (2-3 km a.s.l.). It is a large area in which thick layers of sedimentary deposits accumulate. Most sedimentary basins form under oceans & are called Marine Basins. Some sedimentary basins develop on land & are called Continental Basins.

A basin may be of tectonic or Erosional origin. Sedimentation may be interrupted with unconformities. Basins may be simple or composite (sub-basins). They are regions of long-term subsidence creating accommodation space for infilling by sediments.

They are also regions where considerable thicknesses of sediments have accumulated (in places up to 20 km) and have been preserved for long geological periods. Sedimentary basins are widespread both onshore and offshore. The way in which they form was a matter of considerable debate until the last 20 years. The advance in our understanding during this very short period is mainly due to the efforts of the oil industry.

There are several sedimentary basins in Nigeria namely:-Benue Trough Complex-Bida basin-Borno/Chad Basin-Eastern Dahomey Basin-Niger Delta basin-Sokoto Basin This report is for the case study of the Benue Trough Basin of Nigeria

INTRODUCTION The Benue trough is a sedimentological feature in the evolution of the Cretaceous and Tertiary geology of Nigeria. It is also a marine basin. The Benue Trough is a major geological formation underlying a large part of Nigeria and extending about 1,000 km northeast from the Bight of Benin to Lake Chad. It is part of the broader CENTRAL AFRICAN RIFT SYSTEM. It may be assumed that prior to its inception (before the break-up of the Gondwana super continent), no part of the Nigerian land-mass had any sedimentary cover. The various regions of the basement present today were part of one large mass of land known as the CENTRAL GONDWANA SHIELD.

Figure 2: CENTRAL AND WEST AFRICAN RIFT SYSTEM (BENUE TROUGH TO THE WEST IN NIGERIA)

This is the largest basin in Nigeria volumetrically. It is 500 miles long and 140 miles wide in the south and tapers to about 70 miles wide at the north end. It has sub-basins like - Anambra basin, -Abakaliki trough,-Gongola trough

LOCATION OF THE BASIN As noted during the introduction, it is a geological formation underlying a large part of Nigeria and extending about 1,000 km northeast from the Bight of Benin to Lake Chad. This means that it also includes the Borno Basin which was seen as a minor basin, only to be declared a major basin in Nigeria in recent years. The trough has its southern limit at the northern boundary of the Niger Delta, where it dips down and is overlaid with Tertiary and more recent sediments. It extends in a northeasterly direction to the Chad Basin, and is about 150 km wide. The trough is arbitrarily divided into lower, middle and upper regions, and the upper region is further divided into the Gongola (Gongola Sub Basin for this presentation) and Yola arms. The Anambra basin in the west of the lower region is more recent than the rest of the trough, being formed during a later period of compression, but is considered part of the formation

It is a very long basin by length stretching from the area of confluence, over the middle belt zone and down to the savannah region and the Borno basin.

The Benue Trough covers Bauchi, Benue, Gombe, Kogi, Nasarawa, Plateau and Taraba states, among others

Figure 3: SUB BASINS OF THE BENUE TROUGH

Figure 4: LOCATION OF THE BENUE TROUGH

GEOLOGY OF THE BASIN Benue Trough contain folded and unfolded mainly Cretaceous sediments which were deposited in two late Mesezoic "Failed Arms" or Rift Trough which formed the third arm of the Niger Delta Triple Junction. This Junction may have had its Centre within a triangle formed by a line joining Warri, Port Harcourt and subaerial delta nose. Due to high tectonic activity, the faults in the basin are structural e.g. roll over anticlines and growth faults.

FOSSIL CONTENT: Generally majority of fossils found in the Benue trough are at the lower division around the Anambra basin. Trace fossils from the outcrop section of the Enugu Formation in Anambra Basin, southeastern Nigeria, have been studied based on the varieties, types, and mode of occurrence, orientation of the burrows, wall structures and fills. Five ichnogenera identified belong to the Skolithos and Cruziana ichnofacies and include; Skolithos isp. Thalassinoides isp., Planolites isp., Teichichnus isp, and Chondrites isp.

Figure 5: Aerial view of the Benue trough showing the third arm of the Chum Trilete Junction

The Benue Arm also formed the third arm of the Chum Trilete Junction consisting of the Benue, Yola and Gongola arm. More than 15000ft of the pre-Middle Santonian sediments were deposited in various parts of the Troughs. Parts of these sediments were intruded, mineralized and some were metamorphosed, especially in the Abakaliki Fold Belt. Because of this, sediments in parts of the Troughs are thought to be poor prospects for oil and gas because high geothermal gradients must have prevailed above the zone of modified continental lithosphere and crust which appears to underline the Benue Trough and above the zone of narrow "Oceanic" crust which may occur beneath the Abakaliki Trough. It is the largest basin in Nigeria volumetrically. It is 500 miles long & 140 miles wide in the south & tapers to about 70 miles wide at the north end. It is situated in an area of high porosity loss.

The Benue Trough is thought to have resulted from the crustal stretching of the African plate upon the initiation of proto-equatorial Atlantic. Its peculiarity is its evolution into an intra-continental fold chain through o sequence of tectonic events starting from the late Jurassic. Its present day structure is the result of compressional events leading to widespread folding and faulting in the Cretaceous period.

This produced a magmatic outpouring that progressed linearly southwards from the present day Niger republic. Areas of prospective interest especially for gas, may remain in the Benue Trough marginal basins. In Post-Santonian basins marginal to the Troughs such as the Anambra, Wuse-Gombe and Wukari- Mutam Biya basins, unfolded or gently tilted, mainly Cretaceous Post- Santonian sediments accumulated to great thickness. In the Anambra Basin such sediments exceed 30,000ft and may exceed 15,000ft in other basins marginal to the fold belts. Prospect in Pre-Santonian rocks are limited but good prospect probably exist for gas in Post-Santonian basins, e.g. in the Anambra Basin. The Benue Trough is generally Cretaceous, having Basement complex rocks on the left and right boundary and Cenozoic age rocks (Niger Delta Basin) below it. The Benue Trough is filled with up to 6000 m of Cretaceous sediments associated with some volcanics. It is part of a mega-rift system termed the West and Central Africa Rift System (WCARS). The WCARS includes the Termit Ba- sin of Niger and western Chad, the Bongor, Doba and Doseo Basins of southern Chad, the Salamat Basin of Central African Republic and the Muglad Basin of Sudan.

Figure 6: Outline geological map of the Benue Trough and adjacent areas LBT, Lower Benue Trough;MBT. Middle Benue Trough; UBT Upper Benue Trough. 1. Precambrian. 2. Jurassic “Younger Granites”. 3. Cretaceous.4. Post-Cretaceous sediments. 5. Cenozoic Recent basalts including those of Cameroon Line.

The Benue Trough is generally divided into Three Zones and they are

The Upper Benue Trough: It is made up partially of the Gongola Sub basin. It has a rugged topography which consists of the- Fika formation which is rich in shale containing far to bonded hydrocarbons but highly frackable.-Gongola Basin: This is located on the upper part of the trough (shared with the Bornu Basin). It contains gas reserves. -Gombe formation which is a sandstone formation and heterogeneous sequence within the Upper Benue Trough consisting principally of shales with sands, clays and intercalations of coal.

Figure 7: Formations of the Upper Benue Trough

-Keri-Keri Formation It is represented by the gently dipping continental conglomerates, sandstones, siltstones and clays which overstep into the Gombe Formation. Due to the faulted and folded nature of the Gombe Formation, the continental clastics of the Kerri-Kerri Formation reaches a thickness of over 320 meters as a result of sediments infill into the tectonic structures.( Adegoke et al,)assigned Paleocene age to the Formation using pollen evidence.

-Yola-Garua Formation which is located on the eastern part of the upper trough.

-Pindinga/Gongila Formation which is dominantly a marine shale facies with limestone at the base. The formation is believed to be deposited under marine conditions that prevailed during the early – late

Turning and Coniacian times in the Northern Benue Trough. Recent research has shown some deposits of Bentonitic clays in the region

-Bima Sandstone: This is the oldest formation in the upper trough. The Bima Sandstone was derived from the granitic Basement Complex. It consists of feldspathic-sandstones and clays which pass upwards into medium to coarse grained sandstones with less feldspar. Guiraud M. (1990) subdivided the sandstone into three members namely; the Upper Bima (B3), the Middle Bima (B2) and the Lower Bima (B1). The palynological analysis of the outcropping beds of the Bima Sandstone dated the formation Late Aptian - Early Albian.

-Yolde Formation This consists of a variable sequence of sandstones and shales. The sandstones are thin-bedded at the base, followed subsequently by alternations of sandy mudstones and shelly limestone. THE BIMA SANDSTONE, GONGOLA SUB-BASIN AND FIKA SHALE FORMATON ARE ALSO IN TRANSITION TO THE BORNO BASIN The Middle Benue Trough which consists of the - Keana Formations: This consists of calcareous shales, sandstones, siltstones, and silty shales overlying the Asu River sediments in the Lower Benue Basins.

Figure 8: Formations of the Middle Benue Trough

- Awe Formation consisting of transitional beds of sandstones and carbonaceous shales which overlies the Asu River Group.

-Gboko Formation: cretaceous deposits and sedimentary rocks. It comprises of large amounts of Limestone, It even has a cement company in the region. It is Located at the lowest portion of the Middle Trough with transition into the southern Trough.

-Lafia Sandstone: which comprises of ferruginised coarse to fine grained sandstones, flaggy mudstones and lignites bands within the fluvial successions. It is layer of rock which lies over all formations in the basin

-Eze-Aku Formations consisting of calcareous shales, micaceous fine to medium grained sandstones and shelly limestone in the Middle Benue Basin. : This consists of calcareous shales, siltstones, and silty shales overlying the Asu River sediments in the Lower Benue Basins. A flaggy, laminated oil shale member of this formation (the Lokpanta Shale) cropped out on the south side of the Abakaliki Basin. This marine oil shale member is traceable to the Calabar flank areas of southern Nigeria.

-Asu River Group: This constitutes the oldest sediments in the Lower Benue Basins. Sediments of the Asu River Group consist of alternating shales and siltstones with occurrences of limestone and arkosic sandstones in these basins

-Wukari Formation: contains undiffentiated marine and non-marine sediments of the middle trough. It is sometimes confused as being part of the southern trough due to similar sediments composition.

The Lower or Southern Benue Trough: which is the most important area of the Benue Trough due to richness in organic matter. Due to geographic positioning, the lower trough has in recent times be described as the Anambra Basin due to stratigraphic similarities.

The sediments in this region are part of the Asu River group which deposited in the middle Albian age. (Reyment 1965). Major formations here include

-Awgu Formation: The Awgu Formation overlies the Makurdi Formation and outcrops to the southwest of Awajir and west of Gungul in the extreme west central part of the Gboko area. The formation consists of bluish to grey shales and siltstones. At the type locality between the towns of Awgu and Ndeaboh in the present Anambra state the formation consists mainly of marine shales and limestone.

-Abakaliki Uplift /rift: This is located at the lowest point in the trough (It is sometimes considered as part of the Niger Delta Basin). It contains cretaceous sediments which are the oldest sediments in south eastern Nigeria. It is made up of poorly bedded shales, limestone and large amount of sandstones which were deposited in south-eastern Nigeria. It has a failed rift zone beside it which also joins the failed arm of the Benue trough.

Figure 9: Formations of the Lower Benue Trough

Figure 10: Geological map of the Lower Benue Trough regions and adjoining areas. Cenozoic basalts; Nsukka Formation; Ajali Sandstone; Mamu Formation; Otobi/Ekeh Sandstone; Enugu Shale; Owelli Sandstone; Afikpo Sandstone; Nkporo Shale; Agbani Sandstone; Awgu Formation; Makurdi Formation; Agala Sandstone; Amaseri Sandstone; Ezeaku Formation;

-ANAMBRA BASIN which is divided into 2 major basins

(a) Enugu / Nkporo Formation: Post deformational successions are represented by the Enugu / Nkporo Formation (lateral equivalents) which consists of a sequence of bluish to dark grey shale and mudstone locally with sandy shales, thin sandstones and shelly limestone beds. The shaly facies grade laterally to sandstones of the Owelli and Afikpo Formations in the Anambra Basin. The Enugu / Nkporo Formations are essentially marine sediments of the third transgressive cycle. These, in most parts of the Anambra Basin is overlain by the Lower Maastrichtian sandstones, shales, siltstones and mudstones and the interbedded coal seams of the deltaic Mamu Formation. The deltaic facies grade laterally into the overlying marginal marine sandstones of the Ajali and Nsukka Formations.

(b) Ajali Sandstone: consists of overlying marginal marine sandstones and deltaic facies.

-Imo/Nsukka Shale: consists of overlying marine shales and sandstones from the Asaba Formation which were deposited in the Paleocene. It also has facies assemblages like-Tidally-influenced fluvial facies- Estuarine bay fill delta-Estuarine and marine shale facies; and-progradational shoreface-foreshore facies Assemblages.

Figure 11: Stratigraphic successions in the Benue trough and Chad (Bornu Basin)...Obaje N. G. (2004)

STRUCTURAL/TECTONIC HISTORY OF THE BENUE TROUGH

The mechanism of the origin of The Benue Trough has been much debated and remains quite controversial. Before the advent of Plate Tectonics, discussions of the area concentrated on the description of the sediments and the faults and folds mapped in them by the Geological survey of Nigeria and especially following oil exploration by Shell-British Petroleum Development Company.

However there are basically two models of the tectonic origin of Benue trough-Plate Tectonic Theory of Origin-Spreading Related

-Plate Tectonic Theory of Origin-Strike slip Related

PLATE TECTONIC THEORY OF ORIGIN-SPREADING RELATED It has been noted that since the Triassic, an extensional regime has governed the tectonic evolution of the entire African Continent except the Atlas Mountains areas.

At the start of the Cretaceous era, the Benue trough accumulated sediments deposited by rivers and lakes. During the Late Early to Middle Cretaceous, the basin subsided rapidly and was covered by the sea. Sea floor sediment accumulated, especially in the southern Abakaliki Rift, under oxygen-deficient bottom conditions.

In the Upper Cretaceous, the Benue Trough probably formed the main link between the Gulf of Guinea and the Tethys Ocean via the Chad and Iullemmeden basins. Towards the end of this period the basin rose above sea level, and extensive coal forming swamps developed, particularly in the Anambra Basin.

The trough is estimated to contain 5,000 m of Cretaceous sediments and volcanic rocks

A common explanation of the Trough’s formation is by Burke (1976) who described the Trough as the failed third arm (aulacogen) of a three armed rift system elated to the development of hotspots as shown in (Fig 11). The remaining two arms continued to spread as the continent of South America broke away from Africa during the tectonic break-up of the Gondwana supercontinent. It is important to note that this separation of the continental plates are also what led to the formations of Bida, Anambra basins of Nigeria.

Figure 12: RRR Triple Junction with failed arm (aulacogen)

PLATE TECTONIC THEORY OF ORIGIN-STRIKE SLIP RELATED

Benkhelil (1989) defined the Benue Trough as a set of juxtaposed pull-apart basins generated along pre-existing N60˚E strike-slip faults during the Lower Cretaceous, having definitely identified transcurrent plate motion in the origin of the Benue Trough due to geological mappings of outcrops, combined with magnetic, SLAR imagery and gravity data interpretation.

The strike-slip (transcurrent) faults are believed to be connected to the oceanic fracture zones and reactivated during the separation of the South American and African plates (as shown in Figure 12). This model originated from the fact that most of the major faults identified in the Benue Trough are transcurrent faults rather than normal faults of rift systems. Identified normal faults (mostly N120˚E trending) in the Benue Trough are seen to control the grabens but are always linked to major sinistral N60˚E strike-slip faults.

Figure 13: Magnetic discontinuities showing axially placed structural high associated with transcurrent faulting and location of some sub-basins in the Chad Basin, Benue Trough, Anambra Basin, Niger Delta and Nigerian Continental Margin

RESOURCES/PETROLEUM POTENTIALS OF THE BENUE TROUGH Mineral resources in this basin include high concentrations of coal in the Anambra basin (which has been a major coal producing basin in Nigeria where intensive exploration and exploitation activities have been on since 1916 owing to the discovery of commercial coal in Udi near Enugu in 1909 by the Mineral Survey of Sothern Nigeria),Limestone, clay and other resources like lead-zinc, barytes, gypsum, phosphate, glass sand, fluorspar, salt, ironstone, uranium, sulphur, graphite, cassiterite, manganese, mica and silver.

PETROLEUM POTENTIALS OF THE UPPER TROUGH Organic geochemical studies of Cretaceous formations in the Gongola Basin, northern Nigeria, show TOC values that are generally higher than 0.5%. An exploratory well drilled in the Gongola Sub Basin Region of the upper region in 2003 found no oil, but it found 33BCF of gas in the Kolmani River area. Core samples obtained from this region showed that it contained Type II Kerogen which has potential for oil. Research has shown that the source rocks of Gongila and Pindinga Formations are thermally immature. The organic richness of the Fika formation is too high, it contains richly bonded hydrocarbons due to high anoxic conditions which are frackable.

PETROLEUM POTENTIALS OF THE MIDDLE TROUGH Shales in this region have TOC (TOTAL ORGANIC CARBON) of 0.19% to 0.56%. But classification of a possible source rock requires a TOC of 1% and above. No exploratory wells have been drilled in this region, but there is likely to be in the future exploratory wells drilled in the Awgu formation.

PETROLEUM POTENTIALS OF THE LOWER TROUGH Early hydrocarbon exploration of the Anambra Basin is owed to the abundant Maastrichtian coal deposits. Several efforts to explore its petroleum potentials were unrewarding, resulting to neglect from both research groups and oil companies. Nevertheless, with the increasing quest for petroleum development of Nigeria inland basins, the advent of improved exploration techniques, integrated basin analytical methods, and ultimately the global energy demand, petroleum exploration and prospecting activities within the Anambra Basin has taken a new dimension. Analysis done on the Geothermal gradients of this region revealed that most parts of the Asu River Group have the potential of generating oil and gas, while parts of Nkporo Shale, Awgu Formation, and Ezeaku Formation have the potential of generating mainly oil

CONCLUSION In conclusion we suggest that hydraulic or shale fracking of the Fika Shale Formation be carried out in order to ascertain the viability of the oil prospect in that region. More seismic analysis should be done on the basin especially in the Lower trough/ Anambra basin so as to get further detailed seismic reflection and geochemical data. As of now the petroleum potential of the Benue Trough might not be too promising but one thing that can’t be taken away from it is its high amount of mineral resource concentration in the area.

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