ABSTRACT
Reservoir quality assessment and characterization of sandstone units was carried out across the Afikpo area of the Southern Benue Trough. The study involved field investigations and laboratory studies/analyses. Field samples were subjected to Grain Size Analysis (GSA), Sand Equivalent test and Methylene Blue test. Studies revealed that the sandstones are friable, cross-bedded and show coarsening upward motif. Results from lithofacies analysis indicate seven (7) lithofacies deposited in a low to high energy environment. They are (1) Dark Gray Shale facies (Micaceous Dark Gray Shale Facies and Fossiliferous Dark Gray Shale Facies), (2) Bioturbated Sandstone Facies, (3) Wave Rippled Sandstone Facies, (4) Cross Stratified Sandstone Facies, (5) Horizontal/Laminated Sandstone Facies (6) Heterolithic Sandstone Facies, and (7) Pebbly/Conglomeritic Sandstone Facies. The Micaceous Dark Gray Shale Facies belongs to the Eze-Aku Group while other liithofacies are of the Nkporo Group. Granulometric analysis of sand samples indicates sediments that are poorly to well sorted, generally positively to very positively skewed and mesokurtic to leptokurtic. Results from bivariate plots indicates river sand deposition while multivariate plots indicate the sediments are fluvial and deposited in a shallow marine environment. Out of 18 locations sampled, 13 locations had Sand Equivalent values ranging between 90% – 99%, 4 locations had values between 80% – 89% and one location had 77%. These results indicate that a high number of the samples contained little clay because the higher the Sand Equivalent values, the cleaner the sands. The Methylene BlueF test revealed that the clays contained in the samples are not susceptible to moisture (do not swell) as they had Methylene BlueF values ranges of 1.7g/kg and 3.3g/kg. These values do not exceed the Methylene BlueF limit which is 10g/kg. Permeability results range from 35.07mD to 4112.56mD indicating moderate to excellent reservoir while porosity values indicate poor to good with value range of 7.0 – 15.7%. Regression analyses showed a good correlation between porosity and permeability with R2 (coefficient of correlation) being 74.6%. There was a weak correlation between Sand Equivalent and Methylene BlueF Tests with R2 being 39.5%. Thus, combining results derived from the various analyses, sandstones within the study area possess good reservoir qualities especially to house gas hydrocarbon.
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND INFORMATION
The study area is located in the Southern Benue Trough (Fig. 1), between latitudes 5°49′N and 5°54′N, and longitudes 7°54′E and 8°00′E. Benue Trough is an intra-continental rift basin characterized by tectonic and magmatic activities that occurred during Cretaceous times. The Benue Trough was affected by Santonian tectonic activity which deformed the “Benue Trough” and inverted the main depocenter of the Abakaliki Trough and subsequently created the Anambra and Afikpo Basins to the north-west and south-east respectively (Murat, 1972; Benkhelil and Guiraud, 1980; Benkhelil, 2001). The Santonian tectonics differentiated the sedimentary successions into pre and post Santonian packages. The post-Santonian successions are Campanian – Maastrichtian in age (Reyment, 1965); they occur both in the Anambra and Afikpo Basins respectively. In the Afikpo Basin, the Campanian – Maastrichtian succession comprises the Nkporo, Mamu, Ajali and Nsukka Formations.
Sandstones within the Afikpo area occur as ridges which consist of sands that are occasionally pebbly with few heterolithic beds as well as a lot of clays/fines. These ridges form northeast-southwest-trending topographic prominences while the shales underlie the swales i.e. the depressed areas. The sandstone ridges are dry and often barren of vegetation, while the swales are swamps. The ridges show extensive and deep weathering and laterization, such that exposures of fresh rock are available only along new roadcuts, ditches/gullies, quarries and some stream channels. The ridges are asymmetrical, with their gentler, coarser flanks facing the southeast and east, while the steeper flanks face the west.
1.2 LOCATION AND ACCESSIBILITY OF THE STUDY AREA
The study area lies within the Afikpo Basin and covers about 44km2. It is limited by latitudes 5°49′N and 5°54′N, and longitudes 7°54′E and 8°00′E. The area is bounded in the north by Ibii and the Cross River which runs north-to-south direction on the eastern border and is the main drainage system in the area, south by Unwana and west by Edda and Amasiri villages. The locations studied are Ndibe, Edobi village, Mac Gregor, Mgbom, Ugwuagu, Ozizza, Ngodo, Kpoghirikpo and Akpughuru (Fig. 2).
Access to the study area is through a network of roads which are main, secondary and minor roads. Some of the access routes are untared which during the rainy season are essentially not motorable thereby making geological fieldworks difficult. Others are footpaths which connect most parts of the study area. Exposures in the southern part of the study area were accessed through the Afikpo-Unwana Road, those in the northeastern part were accessed through Ndibe Beach Road, those in the north through Ngodo Road and those in the northwestern part through Afikpo-Amasiri-Abakaliki Road.
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