CHAPTER ONE
INTRODUCTION
- Location and Accessibility
The study area is bounded by latitude 50 42׀N and 50 47׀N and longitude 70 30׀E and 70 35׀E within the Anambra Basin. It covers the whole of Umuasua and part of Ovim in Isiukwuato LGA in Abia State.
The area extent is about 86.49km2. The area is accessible through some minor and major roads. The most pronounced road is the Enugu-Port-Harcourt expressway, and Uturu–Akara road. Serveral minor roads and pathways as shown in fig. 1 links to the study area.
Fig. 1: Accessibilty Map of the study area
- SCOPE OF WORK
The scope of work is to demarcate geologic boundaries and describe the various lithology, a detailed observations of geologic features made during the study is used to interpret the depositional environment, attempts are also made using the samples collected for geochemical analysis to know the hydrocarbon potential of the study area.
- AIM AND OBJECTIVES
The aim of this work is to locate and evaluate the potential source rock within the Anambra Basin.
- GEOMORPHOLOGY
- TOPOGRAPHY
The topography of the area was pronounced as a result of weathering and erosion arising from different lithologic units. The northern part of the study area (Ovim) is dominated by the alternation of very fine–grained laminated sandstone and siltstone. The Formation occupies the highest relief and is underlained by medium coarse –grained friable sandstone. The fribiable sandstone increases the capacity of infiltrations in areas where the overlaying Ajali Formation has been eroded.
The Nkporo Shale is located at the southeastern part, and this part is made up of the coastal plain, sands and shales, which are completely flat.
The study area is within the dip slope of Enugu cuesta. The Enugu cuesta is characterized by a gentle dip slope and a step scarp slope towards the north and is a prominent linear asymmetric feature.
- CLIMATE
The study area lies within the tropical rain forest of Nigeria. It experiences two district seasons, the rainy and dry seasons. The rainy season last from (March –November) while the dry season last from (December- February).
The mean annual rainfall in the study area is about 187mm to 2,500mm. (lnyang, 1975). In the dry season, the total rainfall is less than 60mm. The average temperature within the study area is about 280C, and is usually at its peak from February- April towards the end of dry season but it hardly exceeds 350C. The August break occurs and last for several weeks. The mean annual pressure ranges from 1010 milibars to 1012.9 milibars. The “August break” is associated with an inversion in the tropical maritime air mass, which enables the air mass to rise mass cause conventional rainfall even though the humidity is quite high.
The study area is covered by low rain forest vegetation which is influenced by the climatic condition, although the area is dominated by a secondary plant cover, the vegetation generally is thick and comprises of a tree like rubber tree, oil palms, and cashew tress used to cheek erosion. Plants of different species, which are tree types, characterize the forest.
- DRAINAGE
The study area has various rivers and streams such as the River Okwogor, River Ide, which are the major tributaries of River Oke. The above mention river empty into the Imo River, the river exhibits a dendritic pattern with a flat line nature of the terrain. Fig shows the drainage pattern of the study area.
- METHODS OF STUDY
The methods used in this work are as follows:
- The measurement of outcrops to obtain data for lithological sequence, sedimentary structure and texture.
- The use of brunton compass for the measurement of attitudes of the beds (Strike directions, dip, direction, and dip amount).
- Detailed study of sedimentary features.
- Stratigraphic and structural analysis and identification of the depositional environment.
- LITERATURE REVIEW
Several bulletins and papers have been published by the Nigeria researchers from various institutions and firms.
Wilson and Bam (1925) made a detailed geologic survey in the study area during the construction of the eastern railroad. Shell D’ Archy (now the shell Bp Development Company of Nigeria between 1938 and 1941 and 1946 to 1951) drilled many boreholes in search of petroleum in the Nigeria cretaceous sediment. Based on their study, the “Umuahia” map (sheet NO 79) was published by the geological survey of Nigeria. Reyment (1965), Wilson (1925) and some other workers demarcated the boundary between the Imo shale and Ameki Formation to be paleocene-Eocene. Short and Stauble (1967) and Berggen 1960, (Adegoke 1967) assigned the upper part of the two shale to lower Eocene.
Reyment (1965) attempted a bisotratigraphic correlation of terrigenous deposites of the southen Nigeria sedimentary basin. He used ammonites. Reyment (1965) suggestsed that Nkporo Shale was deposited throughout the mastrichtian and that the Mamu, Ajali and Nsukka Formation are time equivalent of the upper part of the Nkporo Formation. He used foraminiferas, pelecypods to distinguish the data of some Formation in the basin. The Formations include the Nkporo Shale and the Ajali Formation.
Short and Stauble (1967) suggestsed three sedimentary cycles in the Southern Nigeria sedimentary basin, from the study of the tectonic event. The first cycle (Albian- Cenomanian) was confined to the Benue trough while the second (Campanian- Peleocene) filled the Anambra Basin and Afikpo Syncline. It is believed that the three major tectonic events separated from the second cycle. The 3rd cycle formed the Niger Delta, which is separated from the second cycle by an angular unconformity. The mapped area belongs to the second cycles.
Burke et al (1970) discussed the tectonic event of southern Nigeria and noted that sediments of the second cycle were derived from Abakaliki anticlinorium. Also, Agagu, O.K 1978 carried out a survey on Geology and Petroleum potentials of Santonian to Maastrichtian sediments in Anambra Basin, eastern Nigeria.
Avboubo A. A and Ayoola O. (1981), there did a research on petroleum prospects of Southern Nigeria Anambra Basin.
Table 1: Correlation Chart for Early Cretaceous Tertiary Strata in the Southern Nigeria (After Nwajide, 1990)
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