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.
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.
1.3 AIMS AND OBJECTIVES
This research work is aimed at assessing the reservoir quality and characterizing the sandstone units in Afikpo area, Southeastern Nigeria. To achieve these aims, the following are the objectives:
To assess the reservoir quality of the sandstone bodies within the research area by studying the porosity, permeability, quantity of clay present and type of clay.
To interpret the depositional environment within the study area.
To integrate all available data such as Grain Size Analysis, Sand Equivalent Test, Methylene Blue Test, Facies analysis, porosity and permeability, quantity and type of clay present to model reservoir architecture, connectivity and flow properties.
1.4 SCOPE AND METHODOLOGY
Systematic study of the area was carried out in three phases: preliminary studies of literature and materials to get acquainted with the study area and visits to outcrops, deskwork and field studies, and laboratory analyses.
1.4.1 PRELIMINARY STUDY
This is the first step for a good geological mapping and involved collection and evaluation of all existing data on the study area. Such data include geological data from archival records, maps, photographs, reports and publications. An understanding of the history and culture of the people also helped in facilitating the work. This preparatory task involved geological interpretation of air photos to help in delineating rock units, tectonic structures and morphological features which facilitated the planning and execution of the actual field activities and also access into the mapping area. With these data at hand, a preliminary study of the area was made from the 2nd to 4th October, 2012. Since the locations (outcrops) to be studied are sites on private land, a visit was first made to H.R.H Ezeogo John O. Ekuma Izuegu 1 of Amizu Autonomous Community, Afikpo and Chief Gabriel A. Agwo the Onikara of Amizu Autonomous Community, Afikpo to seek permission to enter the lands and study outcrops. With the consent of the council, the cultural code for the area was obeyed which included no permission to access certain sites and certainly not allowed the removal of samples without special permission. Also, there were no removals of samples in some sites in other not to destroy the features exposed for other geologists who may wish to visit the site.
Outcrops were located with the help of field guide/base map of the area, information from tipper drivers and other logistic arrangements were made for detailed mapping phase.
1.4.2 DETAILED MAPPING
This phase involved deskwork and field study. Detailed mapping and logging of the sections were carried out in four phases: from 10th to 14th October, 2012, 22nd to 29th October, 2012, 13th to 17th March, 2013 and lastly 31th July to 4th August, 2013. This was done to get the characteristics of the various lithologic units, physical and sedimentary structures, and other details necessary in this research. During these periods, samples were collected from locations and detailed examination of the lithostratigraphic profiles of the exposures. Series of equipment were used which include:
Relevant topographical maps.
Handheld Global Positioning System (GPS) model GPSmap 76CS x, which uses ultra high-frequency radio wave signals from satellite to trigonometrically derive positions to within a few metres laterally.
The compass-clinometer was used to measure:
o The orientation of geological planes and lineations with respect to north.
o The angle of dip of geological features with respect to the horizontal.
o It was also used together with topographic map to accurately determine outcrops.
Field notebook.
Field tape for measuring vertical and horizontal extents of outcrops.
Geological hammer
Masking tape.
Sample bags.
Digital camera for photographing of outcrops and important sedimentary structures.
1.4.3 LABORATORY ANALYSES
This involved processing the samples collected in other to extract necessary information from
them. Sand Equivalent test was used in determining the cleanliness of the sandstones from clay, Methylene Blue test was used to determine the swelling of the clay while granulometric analysis was useful in the analysis and interpretation of textural data. Results from these analyses were presented in form of tables, graphs, photographs, logs and figures.
1.5 LITERATURE REVIEW
The area of study is within some part of the Southern Benue Trough, Southeastern Nigeria. The Benue Trough of Nigeria, located in the West African Continental Margin is about 80-150km wide long, and extends in a NE-SW direction from the Niger Delta in the Gulf of Guinea to the Chad Basin in the interior of the West African Precambrian Shield. The rocks of the Southern Nigeria sedimentary basin are mostly Palaeogene rocks. The rocks comprise the Niger Delta, Benin Embayment (ex-Dahomey), Anambra Basin, Abakaliki Fold-Belt, Afikpo Syncline and the Calabar Flank.
Reyment (1965) undertook the first detailed study of the stratigraphy of the southern Nigerian sedimentary basin and he proposed many of the lithostratigraphic units in the region. He recorded some paleontological evidence that showed that the Turonian was well developed and enriched by lots of ammonite and other fauna.
Murat (1972) and Burke (1996) observed that the Southern Nigeria sedimentary basin followed the breakup of the South American and African continents in the Early Cretaceous. Various lines of geomorphologic, structural, stratigraphic and palaeontological evidence have been presented to support a rift model (King, 1950; Bullard et al., 1965; Reyment, 1969; Burke et al., 1971; Fairhead and Green, 1989; Benkhelil, 1989; Guiraud and Bellion, 1995).
Hoque (1977) described the Eze-Aku Formation as being dominantly texturally and compositionally immature and attributed this to the proximity of provenance to the deposition of the basin. He also described the second cycle of sandstone as texturally immature and compositionally mature and thus the Afikpo Sandstone as quartz arenite and Eze-Aku Formation as feldspathic arkose.
Nwajide (1979) and Arua (1986) suggested environments that ranged from nearshore (barrier ridge-lagoonal complex) to intertidal and subtidal zones of the shelf environments.
Petters (1980) believe that the Eze-Aku Formation and Awgu Formation are equivalent because they are indistinguishable in the field both in lithology and faunal assemblages.
Zaborski (1983) gave detailed description of the ammonite fauna from southern Nigeria. He assigned Campano-Maastricthian age to the Nkporo Shale and Eze-Aku as being Early Turonian in age.
Fayose and Ola (1990) suggested that the sediments were deposited in marine waters between the depths of 10m and 1000m.
This material content is developed to serve as a GUIDE for students to conduct academic research
RESERVOIR QUALITY ASSESSMENT AND CHARACTERIZATION OF SANDSTONE UNITS IN AFIKPO AREA SOUTHEASTERN NIGERIA
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