ABSTRACT
Anambra Basin is one of the basins in Nigeria, harbouring the largest deposit of sub-bituminous coal. Sub-bituminous coal exposures have been encountered in the Eha-Alumona/Orba environs. This study therefore aims at delineating the coal seam and estimating the coal tonnage available. Integration of geological and geophysical interpretations was used in study of a coal sequence in MaastritchianMamu Formation at. Outcrop study in the field shows that the coal seam was deposited in a swampy environment, followed by a tidally-influenced shallow marine environment which led to the deposition of heterolitic layers of shale and siltstone, then deposition of fluvial sediments of the Ajali Formation. Core samples were obtained from two wells drilled in the area. Well 1 penetrated thick layers of Ajali Sandstone, then heterolithic beds of shale and siltstone before encountering the coal seam at an interval of 41 -41.6 m. Thickness of coal at this depth is 0.6 m, which is just equal to the thickness of the coal seam at Iyi Coal spring 417 m southeast. Well 2 penetrated much of the Mamu Formation to a depth of 61 m without any seam. Geophysical survey was then carried out to investigate coal seam continuity. Four Vertical Electrodes Sounding (YES) were carried out. Results show that resistivity of the shale and mudstone beds masked the influence of the relatively thin coal seam. YES 1 and YES
4 showed Type KQ and KA curves. Geo-electric sections matched significantly with Well 1 core data. Coal seam occurs at the fourth layer at depths 40.7 -56.6 m. The thickness of the seam cannot be deduced. YES 2 and YES 3 showed Type H curves. Geo-electric sections from YES 2 and YES 3 were tied to Well 2 core data and no Coal seam was penetrated in Well 2. Three dipole-dipole profiles were done to further analyse the lateral variation in resistivity in order to understand the attitude of the seam laterally. Profile 1 show that the seam occurs at depth of 30 m with resistivity values ranging from 4114 -12,986 ohm.m, while Profile 3 revealed resistivity
contours equivalent to that of Profile 1 but with coal seam at depth of 24 m. Low resistivity values of Profile 2 signifies absence of coal seam.
Resistivity depth slice maps of the three dipole-dipole profiles showed high resistivity (about
18,000 ohm.m) at the northwestern comer of the area which suggest areal occurrence of coal seam. NW-SE resistivity profile on these slice showed unusual uniformity in resistivity values across the seam which is proves lateral homogeneity.
The seam tonnage within the study area was estimated accordingly while observing the
Australian Guidelines for the Estimation and Classification of Inventory Coal. An estimate of
8,719 tonnes was recorded within an area of 10,380 m overlain by the Ajali Sandstone.
CHAPTER ONE
INTRODUCTION
1.1. Background of The Study
Nigeria is endowed with large coal deposits most of which are reported to be within the Benue Trough. Many of the coal fields discovered in Nigeria are situated in the Benue Trough. The Benue Trough ofNigeria which is subdivided into Lower, Middle and Upper portions contains a thick folded sedimentary pile ranging in age from Albian to Recent (Kogbe, 1976). Anambra Basin in the Lower Benue Trough is a major coal producing basin in Nigeria where intensive exploration and exploitation activities have been going on sinceas a result of the discovery of commercial coal in Udi near Enugu in 1909 by the Mineral Survey of Southern Nigeria (Famuboni, 1996). The coal deposits of the Anambra Basin, located in south eastern Nigeria, appear to contain the largest and most economically viable coal resources. This basin covers an area of approximately 1.5 million hectares and has been subdivided into seven coal mining districts (MMSD, 2010). Some of these coal districts include the Benue district (Omkpa-Ezimo) and Enugu district. Over the years, exploration for coal in Nigeria has been around Udi, Enugu, Orukpa, Okaba, Ogboyoga-Odukpono and west ofEnugu Escarpment (Michael, et al., 2008).
In southern Nigeria, coal-bearing formations occur within the Upper Cretaceous and Tertiary sedimentary successions (Akande, et al., 1992). The Cretaceous Anambra basin consists of rhythmic elastic sequences of sandstones, shales, siltstones, mudstones, sandy shales with interbedded coal seams (Ogala, 2011). These Cretaceous sediments in the basin reach a thickness of 6000m with approximately 2000m of these sediments deposited in the basin during the Campanian-Maastrichtian. The coal seams in this basin are sub-bituminous and occur principally
at two levels, the Lower Coal Measures (Mamu Formation) and the Upper Coal Measures
(Nsukka Formation) (Ogala, 2011)
In the Lower Benue Trough, lignite and sub-bituminous coals occur within the Mamu Formation (Middle Campanian-Late Maastrichtian). In the Middle Benue Trough, high-volatile bituminous coals are found within the Awgu Formation (Middle Turonian-Early Santonian). In the Upper Benue Trough contains lignites and sub-bituminous coals in the Gombe Sandstone Formation (Early Campanian-Late Maastrichtian) (Felix &Yomi, 2013). Subbituminous coals are distributed in sequences of Mamu and Nsukka Formations (Maastrichtian), while lignite deposits occur in the Oligocene-Miocene Ogwashi-Asaba Formation (Reyment, 1965, Akande, et al.,
1992). Together, the coal and lignite resources have an estimated reserve of 1.5 billion tons
(Orajaka et al., 1990) and 300 million tons respectively (Ogala, 2011)
Prior to the discovery of oil at Oloibiri in Bayelsa state in 1956, coal played a significant role in Nigeria’s economic development (Felix &Yomi, 2013). These coal resources that have not been well explored and exploited and theseranges from bituminous to lignite (MMSD, 2010).
Total dependence on oil and oil-derived foreign exchange in planning the nation’s economy resulted in the relegation of coal to the background. Hence, coal production has steadily declined in the last few decades, but because of the need for more energy resource for national development and economic growth, coal industry is recovering world-wide and Nigeria is not left out.
The use of geophysical method in the study of coal resources is continually being used in delineating and evaluating coal resources. It is also used in investigating flooding in mines and abandoned mines within Nigeria and in other parts of the world were coal is present. Several
methods have been adopted in these studies but the most effective and often used has been the electrical methods which we have employed in this research. This project attempts to delineate and estimate the amount of coal in the Eha-Alumona and Orba environs.
1.2. Objective of Study
The main objective of this project is to delineate and estimate the amount of coal resources in the study area and the significance of geological factors in their extraction. In summary, the project is intended to
• Identify the various coal seams in the study area.
• Estimate the coal resources available, and
• Determine the significance of geological factors of their extraction.
1.3. Location and Accessibility of the Study Area
The study area is located within the Anambra Basin, a sub-basin within the Benue Trough Complex. It covers an area of about 85km within latitudes 6°44’55.71″N and 649’10.94″N and longitudes 7″29’40.57″E and 7°35’29.68″E. This is within Eha-Alumona in NsukkaLGA and Orba in UdenuLGA, all situated in Enugu State, Nigeria. It can be accessed through the minor roads/dirt tracksleading either from Orba or Eha-AlumonaJunctions along the Enugu- Makurdi Road. Paths linking villages, streams and farmlands also exist in the study area.
1.4. Physiography of the Study Area
The physiography of the study area involves the climate, vegetation, drainage and topography of the study area.
1.4.1. Climate
The climatic condition of the study area is that of the typical rainforest with the rainy season spanning from late March to October while dry season last from November to early March, with the NE trade winds (Harmattan winds) intervening between December and January. The mean annual rainfall ranges between 1875mm and 2500mm (Inyang, 1975) and the average temperature is about 28°C.
1.4.2. Vegetation
The vegetation of the area consists of slightly thick forest of the Guinea (tropical) Savannah with sparse nucleated village settlementswith their farmlands. Guinea (tropical) Savannah is the vegetation belt of the Enugu area and it marks the northern limit of the rainforest and south limit of the Guinea savannah. Also, large portion of the study area is grown by Cashew (AnacardiumOccidentale) which thrives well on the reddish Ajali Sandstone and GmelinaAborea plantation. Plate 1-3 show team walking across traverses cut and cleared of vegetation for the geo-electrical investigation.
1.4.3. Topography
The Topography of the area is an undulating one; with elevation ranging from more than 450m in the North West to 180m around streams. The terrain is severally scarred in the western part by deep gullies caused by erosion of the friable loose Ajali Sandstone. Gullies as deep as 200m were encountered.
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1.4.4. Drainage
The study area is drained by rivers and streams running down from the Enugu escarpment at the west to the east and southeast. The drainage pattern of the study area is dendritic and scarring the Ajali sands of the western region and flowing at a
relative lesser gradien t over
the
more
resistan
| # |
-<; if
‘f
‘ ~
t Traverses cut/cleared through the veg./
le
Plate 1 travers
ough 1 km thick
6
Plate3: Typical River flowing west-east across the study area.Amanyi River is the largest encountered. mudstone and claystones of the Mamu Formation in the east. The rivers encountered include theAmanyi, IyiOtu, Kokoro etc. These rivers are generally shallow. The Amanyi River is
shownin plate3.
64930N 64930N
6490N 6490N
64830N 64830N
Legend
• Settlement
Accessibility
—— Footpath minor road
Drainage
Contours
6480N 648 0 N
1150
64730N 64730N
7
Fig 1: Map of the study area showing relief, drainage, settlement and accessibility. Note: Contours are in feet.
6‘510N 6510N
6500N
6490N
––~(
6470′N
Legend
Drainage
6«8N_Contours_Clip
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Value
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6460N
Fig 2: Digital Elevation Map (DEM) showing contours and Drainage
0.5
Kilometers
730OF 7310′E 7320€
Fig 3: Triangulation Irregular Network (TIN)created from Digital Elevation Map (DEM) of the study area mapped in
Figure 2.
Elevation
I 419.778–447
I 392.556- 419.773
I 365.333- 392.556
l 338.111– 365.333
310.889– 338.111
283.667 – 310.889
256.444 – 283.667
I 229.222 – 256.444
202–229.222
1.5. Previous Work
The stratigraphy of the southern Benue Trough has been described by many authors (e.g. Simpson,1954; Reyment, 1965; Agagu et al., 1985), but a more recent description has been provided by Nwajide (2005). Several authors have laid emphasis on the geology of the petroliferousAnambra Basin because of the good exposures of the Cretaceous successions and exploratory wells drilled by oil companies compared with the other areas of the Benue Trough (Akande, et al., 1992) There are extensive studies of the Cretaceous-Tertiary sediments of the Southern Benue Trough, for which Anambra Basin is part of and since the discovery of coal in thebasin in 1909, several quantitative and qualitative research works has been done on the basin to delineate, evaluate or analyse the quality (composition, rank, depositional environments and technological properties) of the coal seams.
A great contribution to the understanding of the relationships of the coal-bearing sediments was made by Tattam (1944) who showed that the coals did not at all occur in one formation, as was previously thought, but rather belonged to two distinct formations, separated by a great thickness of sandstone, namely the Lower Coal Measures and the Upper Coal Measures. Akandeet.al (1992) recognized that in southern Nigeria, coal-bearing formations occur within the upper Cretaceous and Tertiary sedimentary successions.
Reyment (1965) recognized that sub-bituminous coals are distributed in sequences of Mamu and Nsukka Formations (Maastrichtian), while lignite deposits occur in the Oligocene-Miocene Ogwashi-Asaba Formation.Chiaghanam et al (2013) noted that the shales and mudstones of the Mamu Formation are dark blue or grey and frequently alternate with the sandstone to form a characteristically striped rock and coal seams vary in thickness from a few inches to 12ft. (Reyment 1965, Simpson 1956 and Whiteman 1982).
Petters and Ekweozor (1982), Orajaka et al. (1990), Akande et al. (1992) and Ogala (2011) have reported that the Nigeria Coal and Lignite resources have an estimated reserve of 1.5 billion tons and 300 million tons, respectively, although most of these coal depositslieunexplored and abandoned.
Fatoye and Gideon (2013) reported the occurrence of coal in several part of the country and this includesEzimo, Enugu State which is about 5 km from the study area. They noted that Iyi coal deposit is the only deposit that occurs in the Upper Coal Measures while other deposits occur in the Lower Coal Measures. They also noted that it is thinner and poorer in quality than the coal of the Lower Measures.
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Akande et al. (1992) characterised the coals of the Upper Cretaceous and Tertiary from southern Nigeria using their maceralcompositions such as huminites, inertinites and liptinite. They postulated that the coal of the Anambra Basin was deposited in environments ranging from forest swamp to fen swamp.
Chiaghanam et al. (2013) carried out lithofacies and palynological analysis of the lithofacies in Mamu Formation. They suggests that the coarsening upward sequence is indicative of decrease in sea level, and the intercalation of Siltstone/Sandstone and Shale in the lithofacies of Enugu Shale with its high abundance and diversity of Palynomorphs suggest a shallow-marine environment with Tidal influence.
Babatunde(2010) suggests that the Maastrichtian coals of the Mamu Formation with high Total Organic Carbon (TOC) value of 56.05 wt. % are of the type III/IV kerogen and the thermal maturity of the coal is of low level conversion, while Ogala(2011) gave the TOC to range from
0.07 to 61.42 wt. % confirming the same kerogen type. He suggested further that the genetic potential (GP) and hydrogen index (HI) values range from 0.05 to 332 mg HC/g rock and 40 to
771 mg HC/g respectively, which indicate that the sediments have gas and oil generating potential though vitrinite reflectance (R) of 0.47 to 0.78% and Maximum Temperature (Tmax) of 338 and 441C suggestthermally immature to marginally mature with respect to petroleum generation.
Adedosu et al. (2007) analysed sub-bituminous coal from Ribadu, Okaba, Orukpa, Okpara, Ogboyoga and lignite from Asaba for concentration of trace metals using the energy dispersive x-ray fluorescence spectrophotometer (EDXRF)and subsequently determined the origin, organic matter input, maturity and mineralogical importance of the coals.
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Mamah et al. (2014) utilized several geologic models such as digital/terrain model for topographic assessment, delineation of resource target areas to include the subcropline to aid mining activities via establishing a wash-out elevation and zones and also corelogsfor subsurface delineation and evaluation of Okobo-Enjema coal resources in Kogi State within the Lower Coal Measures ofMamu Formation.
Resistivity technique is a useful tool routinely used under a variety of field conditions and geological settings in environmental geology and geotechnical engineering (Dahlin, 1996; Parasnis, 1997; Beresnev et al., 2002; Vchery and Hobbs, 2003; Godio and Naldi, 2003)
Verma and Bhuin (1979) used the electrical resistivity method to study coal seams in the Jhariaand Raniganj coalfield, India, respectively. They adopted various electrode configurations such as Wenner, two-electrode and half- Schlumberger, for the horizontal profiling and the Schlumberger for the VES.Comparative studies were carried out and the result showed that Wennergives a broad response, two- electrode gives less noisy result with sharp peaks over the coal seams and the half Schlumberger gives good outline of the seam on the resistivity curve. Resistivity of coal seam obtained in the studies was up to 1140 0.m.
Aweto and Adaikpoh (2014) adopted the square array method which was originally developed as an alternative to Wenner or Schlumberger array. It resolves dipping subsurface, bedding or foliation by isolating current direction. They were able to delineate the geologic sequence at the Ezimo coal fieldwhich is similar to that observed at the study area at hand. Resistivity acquired for the coal seam ranges from 4900 -90590.m. From borehole records, they gave seam thicknesses that range from 0.1 -2 m at the coal fields. Also, suggested that the seams are not extensive having that >70% of the points sounded indicated anomaly traceable to the coal occurrence.
Utom (2012) adopted the Schlumberger array method in investigating the water quality predicting aquifer parameters and acid mine drainage at Okpara coal mine, Enugu. Among his findings include the resistivity of coal seams1326 -5831 Q.m,probable shallow aquifer resistivity28 – 527 n.m, thickness 2.1-22.5 m and depth to seam 3.1 -28.3 m.
Dahlin& Zhou (2004) have investigated the resolution capacity and efficiency of 10 electrode arrays in a numerical modelling study. They find that the dipole-dipole, pole-dipole and the gradient arrays have the best resolution. However these arrays are most sensible to noise, whereas the often used Wenner array and the gamma array are less sensible to noise. Despite the sensitivity to noise, they recommend for 2D resistivity surveying collected with a high data density, the gradient, pole-dipole and dipole-dipole arrays as well as Schlumberger array, because of their resolution capacity.
Sheets (2002) under the USGS reported the survey at two sites along State Route 32 in Jackson and Vinton Counties Ohio using Electrical resistivity method. The surveys were done to determine whether the electrical resistivity method could identify areas where coal was mined, leaving air- or water-filled voids. He reported that these voids can be local sources of potable water or acid mine drainage and that they could also result in potentially dangerous collapse of roads or buildings that overlie the voids. He also noted that resistivity response of air- or water• filled voids compared to the surrounding bedrock may allow electrical resistivity surveys to delineate areas underlain by such voids.
This material content is developed to serve as a GUIDE for students to conduct academic research
GEOLOGICAL AND GEOPHYSICAL INVESTIGATION OF A MAASTRICHTIAN COAL SEAM AT EHA-ALUMONA/ORBA ENVIRONS OF ENUGU STATE
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