Occurrence of some campanian-maastrichtian organic-walled microfossils from Enugu Shale, Anambra Basin, Southeastern nigeria: Implications for age and paleoenvironments

Detailed geological field mapping and sampling of the Enugu Formation in the Anambra Basin has been carried out in order to re-examine the age of sediments and reconstruct their paleoenvironments of deposition. A total of ten (10) outcrop samples of shale were subjected to palynological laboratory examination, using conventional method of acid demineralization and maceration techniques for recovering acid-insoluble organic-walled microfossils from sediments. Two main lithological units were distinguished: - carbonaceous fissile shale and siltstone. A late Campanian - Earliest Maastrichtian age was assigned based on index palynomorphs marker taxa Longapertites marginatus (overwhelming abundance), Monocolpites marginatus, Zlivisporis blanensis, and Echitriporites trianguliformis. The age designation was strengthened by the occurrence of a well-known stratigraphic age-diagnostic organic-walled microplankton Coronifera tubulosa, Senegalinium spp. and Andalusiella polymorpha. Palynomorphs of environmental value include Cyathidites minor, (a tree fern of wet, forested, tropical to temperate regions, usually most developed in mountainous / highland terrains under moist and equable climate); Spinizonocolpites baculatus/echinatus, Longapertites marginatus, Mauritidites crassibaculatus and Moncolpites marginatus, which are palm pollen that inhabit similar brackish water as the mangrove. A non-marine to marginal marine depositional setting has therefore been proposed for the Enugu Formation.

Optical (visual) Kerogen assessment of Enugu Shale, Anambra Basin, Southeastern Nigeria: Implications for source rock potential and thermal maturation

Palynological analysis was carried out on Ten (10) samples from outcrops of the Campanian Enugu Formation, a component lithostratigraphic unit of the Anambra Basin, using the acid maceration techniques for recovering acidinsoluble organic-walled microfossils. Two main lithological units were encountered, which include: carbonaceous shale and siltstone. Result from kerogen laboratory examination reveals two (2) main groups of palynofacies association namely; palynofacies (A and B), based on the change in particulate organic matter constituents of organic residue extract. Palynofacies A is characterized by abundant opaques debris with common terrestrial phytoclasts, which occupy the southwestern and northwestern parts of the studied area, whereas palynofacies B dominates in the northeastern part, characterised by abundant phytoclasts followed by frequent opaques debris. Kerogen type III with gas-prone material is suggested for both palynofacies. The examined exine of spore/ pollen grain are pale yellow – yellow, with Thermal Alteration Index TAI of 1+ to 2- and Vitrinite Reflectane (R o) (0.3 % - 0.4 %) in palynofacies A, and yellow – yellow brown, with Thermal Alteration Index TAI of 2- to 2, and Vitrinite Reflectane (R o) of 0.3% - 0.5% in palynofacies B. These revealed source rock that is thermally immature to slightly mature but has potential to generate mainly gas. The kerogen data generated using transmitted light microscopy correlated well with geochemical data obtained using rock-eval pyrolysis method, and this shows the method a reliable tool for assessing petroleum potential in any given sedimentary basins.

Geochemical Characteristics of the Campano-Maastrictian Sediments of the Anambra Basin, Southeastern Nigeria - Implication For Provenance, Paleodepositional Environment, Maturity and Tectonic Setting

The present study investigates the Anambra Basin shales to determine the provenance and maturity of the sediments using standard geochemical techniques. Twelve (12) representative samples recovered from shale sequences of The Mamu Formation and Nkporo Group of The Anambra Basin were studied to determine the sediment provenance, paleoenvironment, diagenetic conditions, maturity as well as the tectonic setting. To consider in detail and establish the inherent constituents of the Major minerals, Trace and Rare Earth elements, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analyses techniques was employed. The detrital minerals determined are Al2O3 (18.27% and 21.16%), TiO2 (1.73% and 1.63%) and Fe2O3 (2.78% \ and 2.85%), for Nkporo Group and Mamu Formation respectively. The enrichment of SiO2, Al2O3 and TiO2 (1.14, 1.94, 3.67 respectively) supported by Chemical Index of Alteration (CIA) of 93.54 & 39.55 and Rb/Sr ratio of 0.57 & 0.40, indicate that the Anambra Basin sediments are matured. TiO2/AL2O3 binary plots, Th/Co Vs La/Sc crossplots, Th-Sc-Zr triplots and Cr, Ni concentration suggest mixed provenance of felsic to mafic source rocks for these sediments. From the log (K2O/Na2O) Vs SiO2 crossplots, a passive margin tectonic setting was determined for these sediments.

Geology and Geochemical Assessment of Maastrichtian Coal Quality in Abocho area, Northern Anambra Basin, Nigeria

Detailed geologic mapping and geochemical analysis of coal samples around Abocho area, northern Anambra Basin, Nigeria was conducted in order to assess the quality of the coals in the area. Proximate and ultimate analyses were carried out on coal samples from the Mamu Formation to determine its chemical characteristics. Physical analysis was also carried out on the coal samples to determine the specific gravity, density and hardness. Geochemical analysis was also carried out on the associated rocks in the study area (Abocho), particularly Shales and Clays to determine their major oxides composition. The area is composed of the Maastrichtian Mamu Formation overlain by the Ajali Sandstone of the same age both dipping between 16°E and 19°E. The geologic mapping of Abocho area revealed two mappable lithologic units: The Mamu Formation and the Ajali Sandstone. The Proximate analyses indicates that the coal contains an average 7.15%, 35.53%, 36.24% of moisture content, volatile matter and fixed carbon respectively. These burns to generate 4,339 kcal/kg calorific value with 20.80% ash yield. The result of the ultimate analysis shows 57.81% organic carbon, 4.15% hydrogen, 8.41% oxygen, 1.39% nitrogen and 0.3% Sulphur. The physical analysis revealed that, the coal has an average specific gravity of 1.5g/cm, average density of 1.4g/cm3 and average hardness of 1.2. These characteristics qualify the coal to be ranked as high volatile sub-bituminous to marginal lignite. The coal is thus, suitable for combustion, gasification, electric power generation and industrial uses. Geochemical results show that the Shale contains 60% Silica (SiO2) and 26%Alumina (Al2O3) constituting 86% of bulk chemical composition. The Clay contains 70% Silica (SiO2) and 25% Alumina (Al2O3), constituting 95% of bulk chemical composition. The occurrences of CaO, NaO and K2O which are the major component of feldspar in clay suggests the clay to be of granitic origin possibly from Oban massif, east of the Anambra Basin. It also suggests low feldspar content.

Depth to magnetic basement in the Anambra Basin, Benue Trough of Nigeria from aeromagnetic data: A prelude for hydrocarbon exploration

Data transformation, regional-residual separation, trend analysis, and Analytic Signal (AS) depth estimation were applied to aeromagnetic data covering the Anambra Basin, which is a major depocentre in the Benue Trough, southeast Nigeria with the primary objectives of accentuating attributes of magnetic sources and determining if sufficient sediment thickness exists for hydrocarbon generation, maturation, and expulsion. The application of data transformation techniques (such as map projection, merging, and reduction-to-pole) and regional-residual ensured the computation of a crustal magnetic field that would be suitable for magnetic analyses. Results indicate that the magnetic basement in the basin forms an undulating surface overlain by sediments with average thickness ranging between 4 km and 7.5 km, while maximum thickness reaches 8 km in some areas. This depth range suggests promising prospect for source-facies maturation and expulsion. We expect that areas in the study area with these appreciable sediment thicknesses, good preservation of graben-fill, and suitable areal closures or fault structures would be favorable for hydrocarbon prospectivity.