standard tons of coal
1.6. WORLD COAL TRADE
1.7.3. COAL SEAMS IN THE LOWER COAL MEASURES
Enugu, Ezimo, Orukpa, Okaba and Ogboyoga are typical examples of coal in this category and some coal exposures in streams near Enugu occur in Ofam, Nveni, Nyaba, Olawba, Asata, Ogololo, Atoto-Ebedde, Obwetti, Obokwa, Discovery Creek, Iva, Kraal Creek, Prison Creek, tributaries of Prison Creek and Ekulu streams, Obuya stream, Eva or Afa stream and stream 213.4m north of beacon CP12 [29]. These streams drain the Enugu escarpment which rises about 233.3m above sea-level in some places. From borehole drilled cores it was understood that there are five persistent coal seams in Enugu area which are known as No's 1 to 5 from the bottom to the top. No.3 seam is the thickest seam in Enugu area, and it is the only coal at present being worked.
West and South of Enugu No.3 seam consists of two coals of roughly the same average thickness separated by from 50mm to 300mm of shale or sandy shale, the so-called "dirt band". The seam is l.Ametres in thickness, and passes downwards into
carbonaceous shale, sandy shale or sandstone and the junction with coal is usually sharp. The dirt band grades into the upper coal but these lower junction is always well defined. The band is absent in certain parts of the workings. Its position is usually taken by a thin band of durain, sharply differentiated from the coal below, but where this is absent the two coals become
indistinguishable.
The annual production of coal from Enugu collieries from 1916 to 1980 is shown in Table 15. Mining in Enugu collieries is by the pillar and stall method. The seam is worked "on board"
with the face parallel to the cleat. This facilitates the breaking off of large slabs.
67
TABLE
15: ENUGU COAL PRODUCTION (1 916-1980)
Year Production (long tons)
Year Production (long tons)'
1916 24,51 1 1946-47 633,852
1917 83,405 1947-48 551,706
1918 145,407 1948-49 610,283
1919 137,844 1949-50 526,613
1920 180,122 1950-51 583,487
1921-22 194,073 1951-52 566,393
. 1922-23 112,818 1952-53 • 613,374
1923-24 175,137 1953-54 679,437
1924-25 220,161 1954-55 : 675,918 1925-26 242,582 1955-56 • 750,058 1926-27 353,274 1956-57 : 790,030 1927-28 345,303 1957-58 : 846.526
1928-29 363,743 1958-59 905,397
1929-30 347,115 1959-60 684,800
1930-31 317,681 1960-61 565,681
1931-32 263,548 1961-62 596.502
1932-33 259,860 1962-63 • 615,681 . 1933-34 234,296 1963-64 : 600,230
1934-35 258,893 1964-65 698,502
1935-36 257,289 1965-66 • 730,183 1936-37 310,308 1967-69 ! Civil War
1970 • 11,703 1937-38 391,159 1970-71 • 24,000 1938-39 323,266 1971-72 : 176,000
1939-40 300,090 1972-73 334,940
1940-41 328,594 „ 1973-74 • 340,972 1941-42 402,640 1974-75 : 277,753 1942-43 463,978 , J1 975-76 • 270.935 1943-44 528,421 1 \ *1976-77 282,727
1944-45 668,158 1977-78 267,158
1945-46 505,565 1978-79 204,837
1979-80 ■."/ 140,487' V
S o n ' l l • C fC V
l- M-* fO ' *• ^
< lr' C o a J f ) fYThe roof of the seam is strong and stands up well for long periods without support. Most of the coal is hand-hewn and the high extraction rate of 98 percent is obtained. Efforts, however, are on to mechanize the coal mining.
Ezimo Coals
Ezimo lies at the foot of the Enugu escarpment and it is approximately 64km north of Enugu. Coal outcrops in the Ezimo area could be seen in streams like Iyinzu, Iyiocha, Iyiozuma, Oshenye, Ogele, Ikpeke, Aboine and Enerve. The main seam at Ezimo outcrops for a distance of about 25km along the
escarpment. In the sourthern and central part of the area the coal is from 1.2 to 1.5metres thick, but north of the Oshenye stream a parting develops in the seam. In the Enerve the seam has thinned to 0.3metre. The coal is friable and partially weathered and is overlain by ferruginous sediments.
69
Orukpa Coals
Orukpa village is about 96 Kilometres north of Enugu by road. The Orukpa seam outcrops west of the town, near the base of the Enugu escarpment. Coal outcrops in Orukpa area show along streams like Addu, Iyari, Ojeche, Achimodo and Akpawku.
At outcrops the seam is from 0.19 to 2.4metres thick.
Okaba Coals
At a point 7° 28', latitude North of the Equator and 7° 45* longitude East of Greewich in Nigeria situates the town of Okaba located in Ankpa local Government area of Benue State[29].
Here the Igala plateau which is about 500m high has a steep slope through a range of about 100m and then slopes gently for about 3km Eastward to a height of 300m where Okaba situates
16km from Ankpa town. The area is Igala speaking. Enugu escarpment rises from here. An outcrop of Okaba coal was discovered in 1930 in the Otokpa stream. Pitting revealed the deposit to be 2.3m in thickness. Between September 1954 and February 1955 seventeen boreholes were drilled and all
intersected the seam.
The depths of the boreholes range from 12.8m to 131.8m. The coals are overlain sharply at the top and bordered at the base by progressive gradation and carbonaceous shale, shale or sandy
shale. The reserve is estimated at a total of 74.2million tons.
Ogboyoga Coals
Ogboyoga is situated in hilly country where the trend of the Enugu escarpment and the Lower Coal Measures outcrop changes from north-south to east-west. The escarpment is much indented by steep-sided valleys. It was in 1930 that coal was first discovered in this area and later in 1950 many more outcrops were located near Ogboyoga and further south around Odokpono.
Streams like Ocakuca, Ede, Edili, Ajuna, Ikelu, Ucomo, Agbajitodo, Umutayaji, Orikpi, Odokpono, Ajafo, Ebene and Adawa revealed the outcrops at different locations. In most of the area investigated the coal is over lmetre thick. There is no evidence of large scale faulting, but small irregularities in the seam contours in the north-east and near Odokpo stream may be the expression of minor faults.
Nigerian Coal reserves (in million tonnes) in some of the areas described in the foregoing is indicated in Table 16 •
71
Table 16: Nigerian Coal reserves (in million tonnes)
Nigerian Coal Corporation Mines
Indicated reserves
Inferred reserves
Enugu 42.7 120.2
Ezimo 29.5 17.3
Orukpa 50.9 70.1
Okaba 54.9 190.3
Ogboyoga 83.3 310.5
Inyi 10.3 _ _
TOTAL 271.6 707.4
1.7.4 Coal Seams In the Upper Coal Measures
The coals of the Upper Coal Measures show considerable variation in thickness and quality along the strike, especially
in the Awlaw-Owerri area. A pit on the bank of the Iyiofia stream near Owerri penetrated a seam with a very irregular roof.
In the small section exposed the coal is from 1.2 to 1.5metres thick and the presence of irregular coal fragments in the overlying sandstone suggests a local uncomformity. In the Omuma stream three exposures of the same seam, a small distance apart are respectively 1.7, 0.6, and 0.8 metres thick.
Impersistent bands of shaly coal and shale are also common in the seams.
Inyi Coals
The main coal seam in Inyi is extremely variable both in thickness and in quality. Sandstone usually rests directly on the coal, but a thin layer of shale sometimes intervenes. The floor of the seam nearly always has shales.
73
1.7.5. Other Coal Formation S'
As far back as the early part of this century it was known that coal outcrops and subsequent pitting revealed occurrences of coal in Okwoga (Benue State), Lafia (Plateau State), Jamata, Agbaja Plateau, Koton Karfi (Kwara State), Doho, Kurnu-Pindiga - Putuk (Bauchi State), Lamja, Lediye, Chikila, (Gongola State) and Gindi akwati (Plateau State). They were observed however, to be tertiary coals or of the uppermost cretaceous age-like those of the Lower Coal Measures.
Afuji (Bendel State) and Ute (Ondo State) have shown coal- outcrops that indicate good qualities of the coal.
1.8. Geology of Lafia Coal Deposit
Lafia town lies 08.30N latitude and 08.34E longitude which is some 85km North of Makurdi and about 150km South-south
west of Jos, the State capital. Lafia coal deposit extends in a broad belt 30 kilometres wide to the south-east of Lafia town
(Figure 7). The deposit may overall cover an area of approximately 2,500 square kilometres.
75
Detailed investigations have been restricted to an area
covering 50km2 around the village of Obi, which itself lies in the south-eastern part of the deposit (Figure 8).
The deposit is part of a vast synclinorium with the coal
bearing measures contained within this structural basin, along with other rocks of the Cretaceous age. The whole succession is folded into numerous synclines and anticlines which have their axes orientated in a general NE - SW direction. To the north
west of the basin, the Cretaceous rocks out-crop against the Precambrian Basement, and to the south-east, the structure changes to an anticlinorium composed of rocks lower down in the Cretaceous succession than the coal-bearing measures.
The Obi area is located on one of these groups of folds within the basin. The Giza anticline represents the Central
*
structural feature, and it was on the crest of this that drilling for core-samples was intensified (Figure 8).
The area covering the deposit is relatively flat, varying in evaluation from 120m around the numerous seasonal streams that intersect the area to 250m above sea-level on the areas of higher ground associated with outcrops of sandstone which lie within the sequence. A well defined, very low escarpment
created by sandstones older than the coal-bearing measures forms the south-eastern limit of the deposit.
In the western half of the deposit, the main drainage directions is to the south, with both seasonal and persistent water courses draining into the River Guma, which itself joins
the River Benue east of Makurdi. To the East, the River Biu, supplied by numerous seasonal streams, flows in an easternly direction to join the River Dep. The area is prone to poor drainage during the rainy season, particularly away from the limited sandstone ridges, (Figure 8) near the seasonal streams.
1 . 9 . Stratigraphic Seccession of Lafia Coal
The coal-bearing measures are represented by predominantly areanaceous and argillaceous sediments of the Upper Cretaceous age. The coal measures are about 550m thick and contain at least 36 coal seams of variable thickness, quality, and lateral extent. The coal bearing succession is over-lain by a marine argillaceous series of 1000m which, in turn, is overlain by a thick areanaceous sequence, the Lafia sandstone. Beneath the coal measures the sequence has not been proved by drilling but it is though to consist of a thick section of Keana sandstone.
«
77
Arufu limestone and the Uoma formation of arenaceous/
argillaceous measures. These are assumed to rest uncomformably on the Pre-cambrian basement (Figure 9). The coal seams are concentrated much deeper down within the measures. A zone of maximum coal occurrence of 60m has been identified in the lower
part of the section and it is this location that contains the coal seams which are though to be of commercial value. It is primarily in this zone, around Obi area that detailed exploration was intensified.
Seams 12, 13, 18, 19, 20 and 24 were considered in this Ph.D work as deserving further attention and examination because of their thickness, persistence and macroscopic visual quality appearance. The choice of Lafia/Obi Coal otherwise referred to as Lafia Coal in this study is explained later on in this thesis.
1.10. Exploration of Lafia Coal Deposit
The search for coking coal started in 1973 in and around Lafia and Obi towns. It started with a general search for
outcrops of coal in the locality. Before long, a major group was found in the valley of the River Dep in the extreme east of the area (Figure 8).
GE O LO GI C A L P L A N OF L A F I A A R E A WI TH B O R E H O L E L O C A T IO N S O U T S I D E T H E OBI. A R E A
**—* - - J r '
_______' ...+ >
m
1 :
79
The whole of Lafia area was then surveyed using vertical sounding techniques to delineate potential areas represented by argillaceous successions. Earlier drilling in the western portion of the area which had arenaceous sequence gave no useful result other than that the search should continue and be re-located. Obi area gave encouraging coal-bearing drilled cores and hence efforts were concentrated here. About 100
boreholes were drilled in Obi area while about 17 were scattered around Lafia. Two shafts were sunk to depths of 28m and 32m
respectively for the purposes of obtaining large samples for pilot scale tests. The deepest borehole reached 520m while others are much shallower. The spacing of boreholes external
to the obi area was generally less than 1 per 50km2 , but within the area it rose to a maximum of about 8 per km2 on the crest of the Giza anticline (Figure 8).
The boreholes sunk in Lafia area reached only about 350m depth because of the limited capacity of the drilling rigs available at that time (Figures 10 and 11). Boreholes LB-2, LB-3, LB-9, LB-10, LB-11, LB-15, LB-18, LB-20 and LB-22 showed no coal. Only traces of coal were shown by boreholes LB-14 and LB-17. In borehole LB-1, two thin coal horizons were intersected, but it was not possible to correlate them in any way to the 36 seams discovered later.
81
Only in boreholes LB-7, LB-13, LB-16, LB-19 and LB-21 were recognizable seams intersected. Borehole LB-3, which was located about 8 km south-south-east of Obi Village, on the road to Agaza, went down to 225m depth, intersecting seams 32, 31, 26, 22 and 21. None of these seams were of any
commercial value.
In borehole LB-13, located close to LB-7, being less than 1km to the south-east, the position of the borehole on the anticline was more fortunate. The hole intersected the "zone of maximum coal occurrence", but unfortunately, the section of seam
12 which later proved of commercial value in Obi area was disturbed by faulting and was therefore difficult to evaluate at that time.
Borehole LB-16 located approximately 12kra west of the village of Keana, once again intersected seams 13 and 12 in
the zone of maximum coal occurrence. Here the section of the seam 13 was very thin and that of seam 12 thick at 2.85m, but very dirty, and along with the remaining seams, can be considered
of little commercial value. This also applied in general terms to borehole LB-19, located 14km east of Obi village, where once again no horizons of commercial value were recorded.
Borehole Lb-21 was drilled at the extreme north-eastern part of the deposit near the River Dep where outcrops of coal had been noted. Here seams 13 and 12 appear to be of the same
w o r k a b l e t h i c k n e s s , at n n
cc-1.11. Contours of Coal-seam floor level for reserve evaluation
In the majority of boreholes drilled in the area, accurate checks were carried out on coal-horizon floor levels using down
hole logging equipment. Contouring of these levels was done on 50m intervals, based on the assumption that the throw and position of the major faults are proven. In areas of low borehole density,
(i.e. less than 1 hole per 10km2) where, because of depth, drilling did not reach the main coal horizons, contouring was obviously very approximate.
Based on the limitations of approximate fault position
orientations and lateral extents, a tentative structure delineating structurally bounded blocks of coal has been prepared for use in calculating geological and workable reserves in seams 12 and 13
(Figures 10 and 11 and Tables 17 and 18). With regard to faulting, a number of assumptions have been made which provide the most
optimistic picture and the maximum reserve figures. The estimates of geological reserves are given in Table 17 whilst Table 18
gives the workable and recoverable reserves.
83
Figure
85
Table 17: GEOLOGICAL RESERVES OF SEAMS 12 AND 13 (tonnes)
Block Total Coal Thickness
Total
<0.5 m 0.5 - 1.0 m 1.0 - 1.5 m 1.5 - 2.0 m Seam 12
A - 81,600 828,000 288,000 1,198,000
B - 752,400 - - 752,400
C 80,600 920,400 3,854,000 811,500 5,666,500
D 699,500 3,694,800 5,686,000 - 10,080,300
E 99,200 2,257,200 3,200,000 - 5,556.400
Total 879,300 7,706,400 13,568,000 1,099,900 23,253,600 Seam 13
A - 593,000 - - 593,000
B 407,700 - - - 407,700
C 286,100 2,061,000 - - 2,347,100
D 758,400 2,394,000 - - 3,152,400
E 1,335,700 1,487,000 - - 2,822,700
Total 2,787,900 6,535,000 - - 9,322,900
Note: T otal g eo lo g ica l reserves = 32.6 m illio n tonnes, which ar present can only be c la s s ifie d as " inferred" . This
c la s s ific a tio n is the same as that given in v /o Technoexport's
report on the L afia C o a l-fie ld ..November 1976, ie . C lass C.
Table 18: WORKABLE AND RECOVERABLE RESERVES OF SEAMS 12 AND 13
(tonnes)
B l o c k T o t a l C o a l T h i c k n e s s T o t a l
0.5 - 0.75 m 0.75 - 1.0 m >1.0 m W o r k a b l e R e s e r v e s
R e c o v e r a b l e R e s e r v e s
Seam 12
A 235,500 518,800 754,300 490,300
B 774,700 - - 774,700 503,600
C 1,065,000 1,908,700 - 2,973,700 1,932,900
D 1,567,000 1,401,200 2,968,200 1,929,300
E 1,100,000 1,057,300 57,600 2,214,900 1,134,800x
T o t a l 4,742,200 4,886,000 57,600 9,685,800 5,990,900
Seam 13
A 379,000 73,200 - 452,200
-B - - - -
-C 1,618,600 - - 1,618,600
-D 1,481,300 - - 1,481,300
-E 1,688,400 141,800 - 1,830,200 428,900x
T o t a l 5,167,300 215,000 - 5,382,300 428,900