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1
A RESEARCH CARRIED OUT
BY
AGBAJE TITUS MAYOWA
Email address: [email protected].
AT THE UNIVERSITY OF ILORIN, KWARA STATE, NIGERIA
COAL DEPOSIT IN MAMU
FORMATION, ANAMBRA BASIN
2
OUTLINE
INTRODUCTION……………………………………………………………………………………………… 3
GEOLOGY OF THE BASIN………………………………………………………………………………… 4
STRATIGRAPHY OF ANAMBRA BASIN…………………………………………………………….. 6
EVOLUTION HISTORY OF ANAMBRA BASIN……………………………………………………. 9
PALEOENVIRONMENTAL CHARCATERISTICS OF MAMU FORMATION……………. 10
EXPLORATION HISTORY OF ANAMBRA BASIN……………………………………………….. 11
ECONOMIC GEOLOGY OF COAL IN MAMU FORMATION……………………………….. 13
MINING AND MARKET…………………………………………………………………………………… 14
REFERENCES………………………………………………………………………………………………….. 15
3
INTRODUCTION
Coal, is organic in origin; however, it contains some inorganic matters called
Macerals. Most of the Macerals contained in coal occur in the form of mineral
inclusions, and they constitute the bulk of the non-combustible portion of coal
which, on burning is left behind as ash (Diesel, 1992).
Nigeria is endowed with a large coal deposits most of which are reported to be
within the Benue Trough (Carter et al., 1963 and Obaje et al., 1994). The Benue
Trough of Nigeria which is subdivided into Lower, Middle and Upper portions
contains a thick folded sedimentary pile ranging in age from Albian to Recent
(Kogbe, 1976; Petters, 1982, and Ojoh, 1992). The coal deposits of the Anambra
Basin, located in southeastern Nigeria, appear to contain the largest and most
economically viable coal resources. This basin covers an area of approximately 1.5
million hectares and is constrained by the Niger River on the west, the Benue
River on the north and the Enugu Escarpment on the east. The coal is
predominantly in one seam that outcrops along the eastern side of the basin at
the base of the Enugu Escarpment and dips gently toward the center of the basin.
De-Swardt and Casey (1963) later reported the occurrence of coals in the Nsukka
Formation (formerly called the ‘Upper Coal Measures’), located 4 miles north of
Okaba town. Lignites and sub-bituminous coals are distributed within the coal
measures of the Maastrichtian Mamu and Nsukka Formations in the Lower Benue
Trough (Akande et al., 1992a). The Nigerian coals are sub-bituminous (black coals)
of Campanian – Maastrichtian age, and Lignites (brown coals) of Tertiary age.
4
GEOLOGY OF THE BASIN The geology and stratigraphic descriptions of sediments in the Benue Trough of
Nigeria have been generally discussed and widely reviewed by many authors
(Carter et al., 1963; Reyment, 1965; Petters, 1982; Offodile, 1976; Benkhell, 1989;
Obaje et al., 1994; Omada and Ike, 1996; Obaje, 2009).
Lignites and sub-bituminous coals are distributed within the coal measures of the
Maastrichtian Mamu and Nsukka Formations in the Lower Benue Trough (Akande
et al., 1992a) and in the Campanian – Maastrichtian Gombe Sandstone Formation
in the Upper Benue Trough. The coals in the Lower Benue outcrop mainly in
Enugu area where four mines: Iva Valley, Onyeama, Okpara and Ribadu are being
worked by the Nigerian Coal Corporation.
5
Figure 2: Geology sketch map of Anambra basin
6
STRATIGRAPHY OF ANAMBRA BASIN
Figure 3: Stratigraphic successions in the Anambra Basin
Sedimentation in the Lower Benue Trough commenced with the marine Albian
Asu River Group, although some pyroclastics of Aptian – Early Albian ages have
been sparingly reported (Ojoh, 1992). The Asu River Group in the Lower Benue
Trough comprises the shales, limestones and sandstone lenses of the Abakaliki
Formation in the Abakaliki area and the Mfamosing Limestone in the Calabar
Flank (Petters, 1982). The marine Cenomanian – Turonian Nkalagu Formation
(black shales, limestones, siltstones) and the interfingering regressive sandstones
of the Agala and Agbani Formations rest on the Asu River Group. Mid-Santonian
deformation in the Benue Trough displaced the major depositional axis westward
which led to the formation of the Anambra Basin. Post-deformational
7
sedimentation in the Lower Benue Trough, therefore, constitutes the Anambra
Basin.
NANKA FORMATION
The Eocene Nanka Sands mark the return to regressive conditions. The Nanka
Formation offers an excellent opportunity to study tidal deposits. Well-exposed,
strongly asymmetrical sand waves suggest the predominance of flood-tidal
currents over weak ebb reverse currents. The presence of the latter are only
suggested by the bundling of lamine separated from each other by mud drapes
reflecting neap tides. A good outcrop of the Nanka Formation is the Umunya
section, 18 km from the Niger Bridge at Onitsha on the Enugu – Onitsha
Expressway.
NSUKKA FORMATION
The Nsukka Formation and the Imo Shale mark the onset of another transgression
in the Anambra Basin during the Paleocene. The shales contain significant amount
of organic matter and may be a potential source for the hydrocarbons in the
northern part of the Niger Delta (Reijers and Nwajide, 1998). In the Anambra
Basin, they are only locally expected to reach maturity levels for hydrocarbon
expulsion.
AJALI SANDSTONE
The Ajali Formation (Middle to Late Maastrichtian) a sandy tidal deposit lies above
and the Late Maastrichtian to Danian Nsukka Formation, also a paralic coaly
sequence completes the succession (Obianuju, 2005).The fluviodeltaic sandstones
of the Ajali and Owelli Formations lie on the Mamu Formation and constitute its
lateral equivalents in most places. The converging littoral drift cells governed the
sedimentation and are reflected in the tidal sand waves which are characteristic
for the Ajali Sandstone.
8
MAMU FORMATION
The coal-bearing Mamu Formation and the Ajali Sandstone accumulated during
this epoch of overall regression of the Nkporo cycle. The Mamu Formation occurs
as a narrow strip trending north–south from the Calabar Flank, swinging west
around the Ankpa plateau and terminating at Idah near the River Niger The Ajali
Sandstone marks the height of the regression at a time when the coastline was
still concave. The Mamu Formation is best exposed at the Miliken Hills in Enugu,
with well-preserved sections along the road cuts from the King Petrol Station up
the Miliken Hills and at the left bank of River Ekulu near the bridge to Onyeama
mine.
NKPORO/ENUGU SHALE
Sedimentation in the Anambra Basin thus commenced with the Campanian-
Maastrichtian marine and paralic shales of the Enugu and Nkporo Formations,
overlain by the coal measures of the Mamu Formation. In the Paleocene, the
marine shales of the Imo and Nsukka Formations were deposited, overlain by the
tidal Nanka Sandstone of Eocene age. Down dip, towards the Niger Delta, the
Akata Shale and the Agbada Formation constitute the Paleogene equivalents of
the Anambra Basin. The Enugu and the Nkporo Shales represent the brackish
marsh and fossiliferous Pro-delta facies of the Late Campanian-Early
Maastrichtian depositional cycle (Reijers and Nwajide, 1998). Deposition of the
sediments of the Nkporo/Enugu Formations reflects a funnel-shaped shallow
marine setting that graded into channeled low-energy marshes. The best
exposure of the Nkporo Shale is at the village of Leru (Lopauku), 72 km south of
Enugu on the Enugu – Portharcourt express road, while that of Enugu Shale is at
Enugu, near the Onitsha-Road Flyover.
9
EVOLUTIONAL HISTORY OF ANAMBRA BASIN
The Benue Trough was formed by rifting of the central West African basement,
beginning at the start of the Cretaceous period. At first, the 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 Abakiliki 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
(predecessor of the Mediterranean Sea) 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 that it is an aulacogen, an
abandoned arm of a three-armed radial rift system. The other two arms
continued to spread during the break-up of Gondwana, as South America
separated from Africa. The two continents seem to have started to split apart at
what are now their southern tips, with the rift extending up the modern
coastlines to the Benue Trough, then later split along what is now the southern
coast of West Africa and the north eastern coast of South America. As the
continents were wedged apart, the trough opened up. When separation was
complete, the southern part of Africa swung back to some extent, with the
sediments in the Benue Trough compressed and folded. During the Santonian
age, around 84 million years ago, the basin underwent intense compression and
folding, forming over 100 anticlines and synclines. The deposits in the Benue
Trough were displaced westwards at this time, causing subsidence of the
Anambra basin.
10
PALEOENVIRONMENTAL CHARACTERISTICS OF MAMU
FORMATION
The Campanian Maastrichtian stratigraphic succession in the Anambra Basin
begins with Nkporo Formation, which are predominantly marine shales of
Campanian age overlain by the Mamu Formation (Early to Late Maastrichtian),
paralic sandstones, mudstones and coals.
Umeji (2002) subdivided the Mamu Formation into three litho logic units which
are:
i). black carbonaceous marine shales which are overlain by more sandy units
(shore face deposits), showing typical heterolithic wave rippled, flaser bedded,
fine, white sandstones interlaminated with dark or grey mudstones;
ii). the coal-bearing facies;
iii). an upper unit composed of fine to medium-grained sandstone, with climbing
ripple-lamination. The studied section lies within the upper part of the coal-
bearing facies.
The sandstones are fine to medium grained and yellow in color. The Shale and
Mudstones are dark blue or grey and frequently alternated with the Sandstone to
form a characteristically striped rock. Coal seams vary in thickness from a few
inches to 12ft (Reyment 1965, Simpson 1956 and Whiteman 1982).
11
EXPLORATION HISTORY OF COAL IN ANAMBRA BASIN
Anambra Basin in Lower Benue Trough is a major coal producing basin in Nigeria
where intensive exploration and exploitation activities have been on since 1916.
Coal was first discovered in 1909 in Udi near Enugu by the Mineral Survey of
Southern, Nigeria. It was discovered in the Mamu Formation (formerly called the
\lower coal measures) of the Anambra Sedimentary Basin (Simpson, 1954). In
1950, the Nigerian Coal Corporation (NCC) was formed and given the
responsibility for exploration, development and mining the coal resources. The
NCC is 100% owned by the Federal Government and is headquartered in Enugu.
NCC has operated two underground mines, Okpara and Onyeama, and two
surface mines, Orukpa and Okaba, located on the eastern edge of the Anambra
Coal Basin.
Between 1950 and 1959, coal production in the Enugu mines increased annually
from 583,487 tonnes to a peak of 905,397 tonnes. After 1959, production
decreased significantly each year including the Civil War period of 1966 to 1970
when no coal production was reported.
Production in the 1980s was less than 100,000 tonnes annually and decreased
further in the 1990s. Much of this production was utilized by the railroad and
some smaller tonnages were exported. NCC has not operated any coal mines for
several years.
12
Figure 4a: Part of Coal Measure in Mamu formation, Lower Benue Trough
exposed in Okaba Area.
Figure 4b: Coal in Okaba Area, Lower Coal Measure
13
ECONOMIC GEOLOGY OF COAL IN MAMU FORMATION
A large coal reserve probably in excess of 1,000 Million tonnes is believed to occur
within the Mamu formation at depth over 600m in Amangiodo area of Enugu
State. Nigeria estimates its coal reserves at more than 2 Billion tonnes with
approximately 650 Million tonnes proven.
COMBUSTION
The calorific value of an air dried sample of Nigerian coal is usually between 7,000
and 8,000 kca/kg (Orajaka et al., 1990). Mackowsky (1982) in his work found out
that the calorific value of Onyeama, Orukpa, Okaba and Gombe coals ranges
between 7,000 – 8,000 kcal/kg, while that of the Obi/Lafia coal ranges between
7,500 – 8,500 kcal/kg. These are high and optimum calorific values for
combustion. It can be used as a domestic fuel. It gives a gas of high calorific value
and produces exceptional yields during low-temperature carbonization processes.
The combustibility, grind ability, calorific values and ash properties are genetically
linked to one another and they all depend on the coal rank and Maceral
composition (Mackowsky, 1982). And based on combustion properties (calorific
value, grind ability and ash properties) the most optimum coals for combustion
are those from the upper part of the Onyeama mine, lower part of the Orukpa
mine, Okaba whole mine and the sub-bituminous series of the Gombe coal
deposits.
14
GASIFICATION
Coal-based vapor fuels are produced through the process of gasification.
Gasification may be accomplished either at the site of the coalmine or in
processing plants. In processing plants, the coal is heated in the presence of
steam and oxygen to produce synthesis gas, a mixture of carbon monoxide,
hydrogen, and methane. This synthesis gas (syngas) can then be converted into
transportation fuels like gasoline and diesel through the Fischer – Tropsch
process. Currently, this technology is being used by the SASOL Chemical Company
of South Africa to make gasoline from coal and natural gas. Alternatively, the
hydrogen obtained from gasification can be used for various purposes such as
powering a hydrogen economy, making ammonia, or upgrading fossil fuels.
On-site gasification is accomplished by controlled, incomplete burning of an
underground coal bed while adding air and steam. To do this, workers ignite the
coal bed, pump air and steam underground into the burning coal, and then pump
the resulting gases from the ground. Once the gases are withdrawn, they may be
burned to produce heat or generate electricity. Or they may be used in synthetic
gases to produce chemicals or to help create liquid fuels.
In these regards, Enugu, Orukpa and Okaba coals in the Lower Benue Trough are
optimum for gasification while the high-volatile bituminous coals in the Middle
Benue Trough (Obi/Lafia coals) are sub-optimum for this purpose.
LIQUEFACTION
Liquefaction processes convert coal into a liquid fuel that has a composition
similar to that of crude petroleum. Coal can be liquefied either by direct or
indirect processes. However, because coal is a hydrogen-deficient hydrocarbon,
15
any process used to convert coal to liquid or other alternative fuels must add
hydrogen. Four general methods are used for liquefaction:
(1) Pyrolysis and hydro carbonization, in which coal is heated in the absence of air
or in a stream of hydrogen;
(2) Solvent extraction, in which coal hydrocarbons are selectively dissolved and
hydrogen is added to produce the desired liquids;
(3) Catalytic liquefaction, in which hydrogenation takes place in the presence of a
catalyst; and
(4) Indirect liquefaction, in which carbon monoxide and hydrogen are combined
in the presence of a catalyst.
MINING AND MARKET
Coal seams can be mined by surface/underground method. The choice of mining
method is dictated by both technical and economic factors. The most important
technical factors are the thickness of the coal seam, the depth of the coal seam,
the inclination of the seam and the surface topography.
Coal is the most widely available and well-distributed fossil fuel in the world and
is the second largest primary source of energy after crude oil in consumption
terms, and the largest in terms of reserves. Global consumption of coal is forecast
to increase significantly between 2010 and 2020.
The mined coal will be largely sold to international parties, however, there is
increasing domestic demand for coal. Nigeria has over 160 million people, the
required energy consumption has been estimated to be over 120,000 MW,
however, and at present Nigeria can only supplies 3,500 MW. Nigeria is also
changing from hydropower generation to coal fired power plants.
16
REFERENCES
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Composition of Selected Upper Cretaceous and Tertiary Coal of Southern Nigeria.
International Journal Coal Geology 20, 209 – 224.
Benkhell, J. (1989). The Origin and Evolution of the Cretaceous Benue Trough,
Nigeria. Journal Africa Earth Sciences 8, 251 – 282.
Carter, J. D., Barber, W., Tait, E. A. & Jones, G. P. (1963). The Geology of Parts of
Adamawa, Bauchi and Borno Provinces in Northeastern Nigeria. Bulletin
Geological Survey Nigeria, 30, 1 – 108.
Diesel, C. F. K., (1992). Coal Bearing Depositional Systems. Springer Verlag, Berlin,
721pp.
De-Swardt, A. M. J. & Casey, O. P. (1963). The Coal Resources of Nigeria.
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Kogbe, K. C. (1976). Paleogeographic History of Nigeria from the Albian Times. In:
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Institute, University Uppsala, Special Publication 4: pp 1–166.
17
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