Mamu Formation Research Articles

Hydrocarbon generation potential of Campano-Maastrichtian to Paleogene shales in the Benin Flank of the Anambra Basin, SW Nigeria

The hydrocarbon generation potential of the Campano-Maastrichtian to Paleogene shales from the Benin Flank (located in SW Nigeria) of the Anambra Basin, has been previously investigated mostly by studying outcrops and by relatively few subsurface data-based studies. Thus, it is expedient undertaking an assessment of the hydrocarbon generation potential of this frontier area from subsurface samples. Campano-Maastrichtian to Paleogene shale samples obtained from Egoli-1 borehole in the Benin Flank of the Anambra Basin are studied by means of HAWK programmed pyrolysis, organic petrography, and mineralogy (XRD), with the aim to explore the petroleum-generating potential and the thermal maturity. The obtained results display a significant variation of the TOC content ranging from very low (<0.5%) to significant (>5%), indicating poor to excellent oil potential based on S2 values under the condition, of course, the studied formations reached the oil window. The shales of the Nsukka and Imo Formations display lower petroleum-generating potential than these of the Mamu Formation. All the studied samples are dominated by gas-prone (type III) and inert (type IV) kerogens, with few displaying mixed II/III (oil- and gas-prone). The organic-petrography observations support partly the results of the HAWK programmed pyrolysis, as they reveal an organic-richer Mamu Formation in comparison to the Nsukka and Imo Formations; huminite/vitrinite particles (both indigenous and recycled) along with variable contents of liptinite (mostly alginite and bituminite) and inertinite macerals (mostly inertodetrinite) are hosted in the shales. The latter display a typical composition for fine-grained clastic sediments; mostly kaolinite, illite/montmorillonite, quartz, and subordinately, anatase. In terms of thermal maturity, huminite/vitrinite reflectance data points to immature stage; however, the occurrence of solid bitumens, displaying equivalent vitrinite reflectance values within the oil window, points to an active petroleum system in sequences deeper than the examined ones.

Benthic foraminifera from the Campanian–Maastrichtian shales of the Mamu Formation at the Benin flank of the Anambra Basin, southern Nigeria: A paleoenvironmental assessment

Benthic foraminifera from the Campanian–Maastrichtian shales of the Mamu Formation at the Benin flank of the Anambra Basin, southern Nigeria: A paleoenvironmental assessment

Late maastrichtian- early eocene dinoflagellate cyst biostratigraphy and aleoenvironmental study of sediments in Akukwa-1 well in the Anambra Basin

Dinoflagellate cyst biostratigraphy and palaeoenvironment of deposition of sediments penetrated by Akukwa-1 Well was carried out with a total of 253ditch cuttings using the conventional maceration technique for recovering of acid insoluble organic-walled microfossils from sediments. Sedimentological description delineated five (5) lithological units within the well, which included those of Nkporo Shale, Mamu Formation, Ajali Sandstone, Nsukka Formation and Imo Shale. A total of 14 genera and 35 species of dinoflagellate cyst were identified. The recovered dinocysts were used to establish palaeoenvironment which ranged from marginal to shallow marine, the forms were also used to propose four (4) informal dinoflagellate cyst zonations labelled (A-D) based on the first and last occurrences of two or more species. They included Zone A (Apectodinium paniculatum zone), occurring at 7990ft(Maastrichtian), zone B(Paleocystodinium australinium zone) occurring between 7990ft -4000ft (Upper Maastrichtian), zone C(Apectodinium homomorphum zone) occurring between 4000ft -2550ft (Upper Maastrichtian), and zone D(Homotryblum palladium zone) occurring between 5550ft-1520ft(Paleocene- Early Eocene). The erected dinocyst zones were correlated with the existing biozones in tropical areas of Africa and northern South America.

English

Vertical electrical sounding (VES) and 2D electrical resistivity tomography (ERT) employing the Schlumberger and Wenner configurations respectively were combined for groundwater exploration in parts of Enugu metropolis. The area is underlain by Enugu Shale and Mamu Formation. A total of twenty (20) VES and four (4) 2D ERT traverses were acquired in the study area. VES and 2D ERT data were processed and interpreted using INTERPEX 1X1D and RES2DINV respectively. Sounding curves shows dominant decrease in resistivity with depth while 2D ERT inverse model shows lateral variation in resistivity of the rock layers with lower resistivity at increasing depth. VES geoelectric layer sections show top sideritic clay - clay/shale sequence. Resistivity curve analysis also shows predominance of Q and K curve types. 2D map of aquifer layer resistivity, thickness and depth were constructed. Aquifer layer resistivity range from 2.00 to 190 ?m, while the thickness range from 10 to 75 m and a depth range of 23 to 150 m. The 2D ERT shows layer resistivity range of 0.1 to 250 ?m. Based on the combination of the two techniques for groundwater exploration in the study area, a water table depth of 18 m with a maximum drill depth of about 100 m was determined. However, it is recommended that the selection of point of drill should be on the low saturated regolith as observed on the 2D ERT sections.&nbsp;&nbsp; Key words: Aquifer, groundwater, VES, 2D ERT, resistivity.

Palynomorphs Content and Paleodepositional Environments of Mamu Formation in Okpekpe-1, Imiegba-1 and Imiegba-2, Benin Flank of Anambra Basin, Nigeria

Twenty surface samples were taken from road cuttings along Okpekpe and Imiegba areas located in Benin Flank of Anambra Basin, Nigeria with an aim to establish the biozones, age and paleodepositional environments of the Mamu Formation. Using palynological laboratory standard procedures, the shale surface samples collected from the Mamu Formation in the study area were prepared and analyzed. The recovered marker pollens, spores and dinoflagellate cysts from the study area are Milforadia jardinei, Milforadia sp., Longapertites marginatus, Longapertites sp., Retidiporites sp., Monocolpites marginatus,Cyathidites minor, Cyathidites spp., Cingulatisporites ornatus, Distaverrusporites simplex, Laevigatosporites spp., Foveotriletes margaritae, Subtilisphaera sp., Andalusiella sp., Paleocystodinium australinium and microforaminiferal wall linings. Four biozones were erected for the shale intervals in the study area, namely: Longapertites sp., Monocolpites marginatus, Cyathidites minor, and Paleocystodinium australinium biozone, respectively, dated to Upper Maastrichtian to Early Paleocene age, which are lateral equivalents to Spinizonocolpites baculatus biozone of the Upper Benue Trough. The presence of the dinoflagellates cysts such as Paleocystodinium australinium, Subtilisphaera sp. and Andalusiella sp., microforaminiferal wall linings, pollens and spores established that the shales were deposited in nearshore to shallow marine environments. The presence of Cingulatisporites ornatus and Distaverrusporites simplex are indicative of marine regression that was followed by fluvial continental influence in anoxic bottom conditions that in turn favored the formation of coal seams in shales in the study area. The paucity of palynomorphs in the study area is inferred to have occurred because of the dry climatic condition that prevailed during the Upper Maastrichtian to Early Paleocene.

Depositional conditions of the Upper Cretaceous shales from the Anambra Basin, SE Nigeria: Constraints from mineralogy and geochemistry

Thirty-four core shale samples from the Upper Cretaceous Mamu Formation, southeastern Nigeria, were studied using XRD and geochemical analyses to constrain their provenance, tectonic setting, palaeoweathering and palaeoredox conditions during deposition. X-ray diffraction analysis revealed the presence of clay minerals (kaolinite, illite and illite-smectite mixed layer) and non-clay minerals (quartz and feldspar). The dominance of kaolinite and the high kaolinite/illite (K/I) contents indicate warm and humid climatic conditions during sediment accumulation. The total organic carbon (TOC) content of the shales ranges between 0.31 and 55.34%. The high Chemical Index of Weathering (CIW; mean∼98.8%), Chemical Index of Alteration (CIA; mean∼92%) and Plagioclase Index of Alteration (PIA; mean∼98.7%) for the shales, including a high content of kaolinite, indicate intense chemical weathering in the source area. The positive correlations between V, Cr, Ni, Co, Ga, Cd, Be, Sc, TiO2, MgO with Al2O3 and Ba with K2O suggest that these elements in the shales are mainly held in clay minerals. The elevated contents of Zr and the high Zr/Cr and Zr/Sc ratios imply that the shales were derived from predominantly felsic source rocks. However, TiO2/Zr and Y/Ni ratios suggest a derivation from an admixture of complex felsic and mafic basement rocks. The low average enrichment factor values of Mo (MoEF), V (VEF) and U (UEF), along with low concentrations of redox-sensitive trace element ratios (Mo/U, Ni/Co, V/Cr and Cu/Zn) and high TOC contents infer oxic conditions during deposition. Furthermore, Fe2O3–S-TOC relationships, along with DOPT, V/(V + Ni) and Fe2O3/Al2O3 ratios, suggest dominantly oxic to slightly dysoxic/anoxic conditions during the accumulation of the Mamu Formation shales. Binary plots of K2O/Na2O ratio versus SiO2 indicate a passive margin tectonic setting for the shales.

Stratigraphy and Paleoenvironment(s) of the Late Cretaceous deposits in Ohafia area, Afikpo Basin, Southeastern Nigeria

Lithofacies and biofacies data extracted from three outcropping sections of Nkporo and Mamu Formations’ sediments in Ohafia area were used for paleoenvironmental and sequence stratigraphic interpretations. The five (5) lithofacies distinguished from the sections were Dark-Grey to Black Shale Facies (A), Oolitic Limestone Facies (B), Calcareous Siltstone Facies (C), Sandstone Facies (D) and Heterolithic Facies (E). The attributes of these lithofacies, including lithotype, rootlets, bioturbation, reducing conditions and microfossil contents, were used to discriminate the units into various environments of deposition and paleodepths. The oolithic limestone facies for instance was interpreted as deposits in shallow coastal areas with high turbulence, especially, in warm shallow marine environments. Paleobathymetry of the lithofacies ranged from non-marine through coastal deltaic, shallow inner neritic, inner neritic to middle neritic paleodepths. Micropalaeontological analysis of the sediments revealed a complete absence of planktonic foraminifera while agglutinated benthic assemblages (dominated by Haplophragmoides spp., Textularia spp. and Anomalinoides spp.), calcareous benthic foraminifera (Gavelinella guineana and Planulina nacatochensis), Gastropods, Ostracods, and Arenaceous indeterminate were recovered. The recognition of Bolivina afra-Haplophragmoides talokaense assemblage zone in the Mamu Formation conferred Maastrichtian age to the deposits. The presence of palynological assemblages like the Adallussiella polymorpha were used to assign Late Campanian age for the Nkporo Formation. Sequence stratigraphic analysis using the integration of foraminiferal, paleontological and lithofacies data identified two incomplete depositional sequences. The first sequence identified in the Nkporo Formation, comprised a Highstand systems tract (HST) bounded at the top by a sequence boundary (SB) dated 75 Ma. The SB also marked the beginning of the second depositional sequence that occurred in the successions of the Mamu Formation and comprised of Transgressive systems tract (TST), bounded at the top by a maximum flooding surface (MFS) dated 69.8 Ma. The results of this study have indicated the presence marker beds and constrained stratigraphic surfaces which can be used to map the identified play elements (source, reservoirs, traps and seals) in the study area. The surfaces are thus essential ingredients in basin-wide correlations for improved hydrocarbon exploration and development in the Afikpo and other adjoining inland basins of Nigeria.

English

2D electrical resistivity imaging technique has been used to investigate the nature and dimensions of the fracture prone zones in Isu-Awaa, Enugu State, Nigeria. The study area lies within Latitude 06&deg; 14I 0II N to 06&deg; 17I 0II N and Longitudes 007&deg; 23I 0II E to 007&deg; 28I 0II E, with an area extent of about 51 Km2. The area is underlain by three main geological units (Enugu Shale, Owelli Sandstone and Mamu Formation). The study area is topographically undulating with high ridges surrounding the area. 2D electrical resistivity data was acquired with Wenner configuration, employing the roll-along technique to image the subsurface. A total of four (4) number 2D electrical resistivity imaging traverses were carried out. Data was processed and interpreted using RES2DINV. Measured and calculated apparent resistivity pseudo sections were convolved to generate the inverse model resistivity section, which was the diagnostic interpretative model used. Discontinuities in the layering are possible fracture zones. Major discontinuities were observed at distances of 48 and 87, 52.5, 106, 8.0 and 26 m at traverses A, B, C and D respectively. Fracture trend azimuth rose diagram, shows pre-dominant trend to the SE direction with minority to the SW and NW, and relicts to the NE directions. This conforms to the regional lineaments map of Nigeria. The use of 2D electrical resistivity imaging tool has proved useful in the mapping of fracture prone zones in the study area. Key words: Electrical resistivity imaging, fracture lineaments, Rose diagram.

Lithofacies evaluation of the mamu formation, southeastern nigeria: implications for depositional setting and maastrichtian sea regression

Lithofacies analysis was carried out on the sediments and sedimentary rocks of the Mamu formation for the purpose of interpretation of the depositional setting and the Maastrichtian sea history. Both field and laboratory methods were employed in the study. Nine (9) lithofacies were identified and grouped into three lithofacies associations based on their stacking patterns. The three facies associations include the lagoonal/swamp, tidal flat (dominantly inter-tidal), and the shoreface. Vertical succession indicated deposition in a prograding deltaic system (marginal marine) influenced by both fluvial and marine (tide and wave) processes. The general coarsening upward pattern displayed by the lithofacies and facies association suggests sediment deposition during the regressive phase of the ancient sea.

Stratigraphic palynology and micropaleontological study of the Cretaceous-Paleogene succession of the Nani-1 well of the Anambra and Niger Delta basins: Implications for hydrocarbon exploration

Integrated palynological and micropaleontological analyses were carried out on the Nani-1 well drilled into the Anambra and Niger Delta basins of Nigeria. The study is aimed at reconstructing biozones and paleoenvironments of deposition, as well as deducing the sequence stratigraphic framework of the Cretaceous-Paleogene successions of an area containing key elements for hydrocarbon exploration. Encountered lithofacies succession of sandstone, siltstone, dark-grey to black shale, and coal beds comprises six lithostratigraphic units, namely the Nkporo Group (Campanian-Maastrichtian), Mamu Formation (Middle Maastrichtian), Ajali Formation (Middle-Late Maastrichtian), Nsukka Formation (Late Maastrichtian), Imo Formation (Danian-Thanetian) and Ameki Group (Eocene). Fourteen dinoflagellate zones (A-N) and seven pollen and spore subzones (A1-G1) are recognized based on the first occurrence of two or more species. Two major stratigraphic events are documented: (1) dinoflagellate cyst (dinocyst) zone G at 880 m depth marked by first appearance of Danea aff. californica and Danea cf. mutabilis in association with Cordosphaeridium fibrospinosum, and the last occurrence of Danea aff. californica and Phelodinium magnificum place the Cretaceous-Paleogene (K/Pg) boundary in the lowermost part of the Imo Formation; and (2) a hiatus characterized by a starved section observed at 380 m directly overlain by the Early Eocene assemblages of the Ameki Group indicates a sequence boundary at Imo/Ameki Boundary. Thus, a chronostratigraphic chart is developed for parts of the Anambra Basin and the Niger Delta. Six depositional environments which include marginal to shallow marine, marshy lagoon to estuarine, shallow neritic and tidally influenced fluvial channel are identified, and palynostratigraphic evidences indicate that fresh water palynomorphs are present in the Lower Maastrichtian strata. Four sequence boundaries and four maximum flooding surfaces are also identified within the Maastrichtian–Early Eocene sediments. The paleoenvironmental reconstruction shows that possible hydrocarbon reservoirs in the basin are the marine sandstone facies of the Nkporo Group and Mamu Formation. Their shale members constitute potential source rocks, while the overlying shale units of the Nsukka and Imo formations may provide regional seals for hydrocarbon habitation.

Electrical Resistivity Sounding for Groundwater Investigation around Enugu Metropolis and the Environs, Southeast Nigeria

This report evaluates the use of electrical method and borehole data to investigate the subsurface to delineate the groundwater potential in Enugu metropolis and the environs, south-eastern Nigeria other than rely only on resistivity method which could lead to interpretation error. Integrating these 2 data sets is key in this study. The study area is located in the Anambra Basin and is underlain by Nkporo/Enugu Shale which is overlain by the Mamu Formation. It is bounded by Latitudes 6°2 0'00"N to 6°30'00"N and Longitudes 7°25'00"E to 7°35'00"E and covers surface area of about 342 m2. Thirty-one vertical electrical soundings (VES) were carried out across the area using the Schlumberger electrode array with current electrode separation from 2 to 500 m to identify the depths and resistivity values of the identified geo-electric layers. Through data analysis using WinResist software, the apparent resistivity, thicknesses and depths and the thicknesses of the aquifers were generated. The resistivity and depths were modelled to generate resistivity map and depth map. The resistivity of the aquiferous zone within the study area varied from 20.55 - 427.8 ohm-m at depths of between 10.7 - 40.05 m. Depth to the water table appears to be shallow at the south western part of the map. The interpreted geo-electric layers show a sequence of lateritic top soil, shale, sand and shale. The frequency distribution of the VES curves generated shows the presence of 3 to 5 layers with HK type as the highest. Also, a 2D model was generated using the correlation of VES to VES data and borehole data to VES data to show the underlying stratigraphy beneath the study area as well as the direction of ground water flow. Result of the VES curve analysis reveals that the sub-surface is underlain by three lithological layers namely: lateritic top soil, shale, sand and shales with NW direction of groundwater flow from the 2D model. Groundwater prospective zones can be seen along NW, SW and central parts of the study area which have low resistivity values.

Litho-Bio Facies Analysis and Systematic Paleontology of Sediments in Amansiodo-1 Well, Anambra Basin, Southeastern Nigeria

Sixty-two (62) ditch cutting samples were obtained from Amansiodo-1 well within the Anambra Basin, Nigeria, at depth intervals between 16 - 2191 m and analyzed for lithofacies, Foraminiferal, and systematically described. A total of thirty (30) benthonic foraminiferal species were retrieved: twenty-six (26) arenaceous and four (4) calcareous. The age of the well is determined to be Maastrichtian base on the appearance of benthonic arenaceous <i>Haplophragmoides saheliense (</i>Petters) species. Facies analysis of the samples indicated ten (10) lithofacies and two (2) facies associations. The facies comprise a base to top, the shale facies, shale/sandstone facies, shale/mudstone facies, mudstone/coal facies, sandstone/clay facies, mudstone/clay facies, very coarse sandstone facies, coarse sandstone facies, sandstone/clay facies and siltstone/clay facies, association FA-1 (subtidal) and FA-2 (Intertidal) corresponding to the Mamu Formation in the Anambra Basin. The studied sediments are assigned a shallow marine depositional environment based on the triangular plot of foraminiferal suborder test type (arenaceous, porcelaneous, and hyaline) and lithofacies distribution. The paleosalinity studies suggest a transition from brackish to an open marine depositional environment. Also, paleo-oxygenation of the sediments is anoxic based on foraminiferal distribution and the ratio of epifaunal to infaunal benthonic foraminifera respectively.

Elemental geochemistry and organic facies of selected cretaceous coals from the Benue Trough basin in Nigeria: Implication for paleodepositional environments

Benue Trough basin is geologically subdivided into three segments. It consists of marine and continental sediments that are affected by folding movements. In the upper section, the continental coal-bearing Campano-Maastrichtian-age is found within Gombe Sandstone and the Cenozoic Kerri-Kerri Formation of the Tertiary. In the middle segment, the late Turonian/early Santonian coal-bearing Awgu Formation lies conformably on the Ezeaku Formation. In the lower part, shales at Nkporo mark marine incursion and deposition of the Maastrichtian Mamu Formation coal measures. Cretaceous coal samples taken from the three segments of Benue Trough basin were examined using ultimate and proximate, organic petrographic, x-ray fluorescence (XRF) and laser ablation-induced coupled plasma spectroscopy (LA-ICPMS) analytical techniques. Results of trace element ratios, Ce/Ce* values (“where Ce* is the theoretical Ce value derived from a chondrite-normalized rare earth element (REE) pattern”), authigenic uranium values, and low hydrogen content indicate deposition under oxic to suboxic conditions. However, the V/Ni and V/Sc ratios indicate under oxic, suboxic to reducing conditions. The Y/Ho values for selected coals are lower than the range for seawater, indicating terrigenous influence. The Sr/Ba values indicate low influence of saline water during deposition and a strong continental rather than marine influence. High value of vitrinite and low proportion of liptinite/intertinite suggest that peat in the study areas were situated on the irregular basin floor where sediment thicknesses vary from place to place thus the depositional environments shifted causing mineral matter filling in coal. Based on the tissue preservation index (TPI) and gelification index (GI), LFB, CHK, OKB, LMZ, IMG; and ENG (high GI and high TPI) were deposited in telmatic facies; wet-forest swamps with rapid organic matter accumulation. This trend indicates mild humification and strong gelification of plant tissues which ascribed to high rate of subsidence with wood- and bark-derived telovitrinite. The MGA peats were deposited in terrestrial facies in a dry-forest swamp with wood-derived telo-inertinite. The coal samples code-named IMG and ENG were deposited in planar margin mires. However, coal samples code-named LFB, CHK and LMZ were deposited in planar central mires. Lastly, OKB and MGA were deposited in transitional-wet and transitional-dry mire. The low V and Ni concentrations, low H/C, high O/C, high C/N, and very low N/C ratios <0.05 implies Type III land-plant organic matter that is not oil-prone. The depositional environments interpretations of investigated coal seams indicate that the coals were deposited within a telmatic, limnic, limno-telmatic and open march zone. Generally, a combination of results from inorganic geochemistry and organic petrology is required for precise and all-inclusive determination of coal depositional environments.

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.

Palynological Studies of Upper Creataceous-Paleocene Rocks in Auchi Sheet 266, Benin Flank, Western Extension of the Anambra Basin, Southwestern Nigeria

Out crop samples from Mamu and Nsuka Formation at different locations and ditch cutting samples gotten from a water borehole at Ugbekpe-Ekperi road at different intervals collected from Benin Flank, Western extension of the Anambra Basin, were used for sedimentologic and palynology studies with the aim of establishing palynological assemblage zones, age and paleoenvironments of the sediments. Out crops and Ditch cutting samples were lithologically described using a transmitted light microscope in order to obtain their lithofacies and latter subjected to palynological slides preparation to identify the presence of palynomorphs. Lithofacies units identified were clay, sandy clay, silt, sandstone, shale and sandy shale facies. From the analyzed palynological slides, ninety one (91) palynomorphs species were identified in the outcrop samples, eighty three (83) were Miospores (pollen and spores) and seven (7) were Dinocysts while the borehole samples recorded fifty five (55) palynomorph species, forty three (43) were Miospores while fifteen (15) were Dinocyst. Clusters analysis were used to establish four (4) Palynological Assemblage zones which are Palynological Assemblage zones 1, 2, 3 and 4. Late Maastrichtian to Early Paleocene age is suggested for Nsukka Formation because of the presence / abundance of Spinizonocolpites baculatus with admixture of Apectodinium homomorphum and Homotryblium tenuispinosum. The occurrence of Monocolpites marginatus, Retidiporites magdalenensis, and non-occurrence of Spinizonocolpites baculatus in the outcrop samples suggests that Mamu Formation did not penetrate Late Maastrichtian. Thus Early to Middle Maastrichtian is suggested for Mamu formation. The occurrence of mangrove palynomorphs Zonocostites ramonae, fresh water algae Botryococcus braunii and Concentricyst circulus indicates sedimentation in coastal to shallow marine environment close to mangrove vegetation.

English

Tropical landscapes are generally known to store much carbon in its varied ecosystems, but information required to prioritize and conserve areas with much soil carbon concentration is scanty. This study provides insight on the patterns of carbon storage and its dynamics in an extensive humid zone in SE Nigeria. Soil carbon data was downloaded from soil grids of 250 m carbon stocks for the first 30 cm of the soil profile and compared with the land use/cover, geology and digital elevation model of the region. Carbon estimates varied over the four epochs used and ranged from 29-177, 29-172, 12-172 and 4-177 tons/ha in 1992, 2001, 2010 and 2018, respectively. Even though land cover variation was quite distinct in the zone (between the northern zone with much cover and the southern zone with less), it was not seen as the major factor that influenced soil carbon in the zone. Much of the explanations on carbon concentration and variability were mainly from the geology of the zone which is categorized into six major classes. But three out of the categories were seen to dominate the carbon store: Ajali formation, Mamu formation and Nsukka formation. This exogenous factor (geology), which is not uniform, was seen as the major variable to consider in deciding areas that will be prioritized for carbon conservation in the zone. Proper land use strategies and policy frameworks that would help to maintain proper baselines and further enhance adequate carbon conservation were equally recommended. Key words: Carbon management, conservation, land use change, modified ecosystems, prioritization, tropical.

SEDIMENTARY ARCHITECTURAL ELEMENTS AND SANDY BRAIDED FLUVIAL SUCCESSIONS IN AJALI SANDSTONE RIDGES, WESTERN AFIKPO BASIN, UTURU, NIGERIA

Lithofacies, bounding surfaces and sedimentary architectural elements exposed in two ridges at Uturu being quarried for construction sands were analysed to determine the paleoenvironment of deposition and the factors that control the deposition of sand units. Mainly outcropped is Ajali Formation overlying locally exposed Mamu Formation in Western Afikpo basin. Lithofacies identified include: Trough cross bedded medium- to coarse-grained (St), Planar cross bedded fine- to coarse-grained sandstone (Sp), Small scale planar cross bedded sandstone (SSp), Lenticular mudstone (Fm), Heterolithic sandstone/mudstone (Fsm), Horizontal stratified sandstone (Sh), Cross ripple laminated sandstone (Sr), Reddish muddy sand (Fl), Siltstone (SSm) and Shale (Fsh). The associations of lithofacies and bounding surfaces gave four fluvial and one marine architectural element. The fluvial elements which mainly characterized the Ajali Formation include: Channel-fill (CH), Macroforms Accretion (MA). Flood-Plain Fines (FF) and Channel Abandonment Fines (CAF). Offshore-shoreface fines (OSF) element defined marine Mamu Formation. The profiles of the ridges show dominance of MA followed by CH while FF is limited in occurrence and in some zones pinch-out to lenticular inter-bar mudstone. The MA is characterized by planar cross beddings, reactivation surfaces, internal grading, steep dipping ferruginized accretion surfaces and abrupt flat top which indicate mid-channel bars deposition in typical sandy braided fluvial depositional system. Generally, there is vertical aggradation/ amalgamation of channel deposits and dominance of sheet alluvial architecture. Low rate of channel avulsion, moderate rate of lateral migration and aggradation, variable discharge rate and high rate of sediment supply and subsidence were considered as factors that controlled the deposition and preservations of sand units. This study provided an understanding of mesoscopic heterogeneities and compartmentalization style inherent in hydrocarbon bearing sandy braided reservoirs which can be used as analog model for its development.

Hydrocarbon Generation Potential of the Campano-Maastrichtian Dark Mudstone Lithofacies, Benin Flank, South West Anambra Basin, Nigeria

The dark mudstone lithofacies of Mamu Formation was deposited during the Campano-Maastrichtian flooding episode. It is laterally heterogeneous, and has been subdivided into marsh, bay and central basin subenvironments in order of proximality. Arising from recommendation from a previous study, we evaluated its hydrocarbon generating potential using multidisciplinary tools involving visual kerogen analysis, as well as bulk and isotope geochemistry. Seventy-seven sample materials were taken from 3-outcrop sites at Uzebba, Okpekpe and Imiegba locations, Benin flank, SW Anambra Basin, Nigeria. The results show that bulk of the samples have good organic richness. Kerogen quality is dominantly of gas prone Type III kerogen. However, visual kerogen analysis indicates the presence of an oil prone Type II/III kerogen in the central basin subenvironments. An immature thermal maturity is inferred based on spore colour index (SCI) of less than 6 on the SCI chart (thermal alteration index of &lt;2.5). In addition, we hypothesize that the dark mudstone lithofacies possesses biogenic gas potential based on its organic richness, kerogen quality and thermal maturity. Shale gas prospectivity is further enhanced by the low dip of the Mamu Formation, shallow burial as well as high silica content. Worth mentioning is the proximal marsh mudstone (Uzebba location) with suitable microfabric, very high silica as well as &gt;10m of combined (continuous) outcropping and subcropping thickness&#x0D; Keywords: Kerogen, palynofacies, stable isotope geochemistry, shale gas,

Mineralogical and pore structure of organic-rich deltaic shales and sub-bituminous coals from early Maastrichtian Mamu Formation, Anambra Basin, Nigeria

The Mamu Formation source rocks in the Anambra Basin are targets for shale gas reservoir. However, fundamental data on the critical pore structures for assessing the shale reservoir capabilities are lacking. In this study, the source rocks were investigated for their mineralogical compositions and pore structure characteristics by X-ray diffraction, accelerated surface area and porosimetry and field emission scanning electron microscopy (FE-SEM). The bulk mineralogy of the samples is dominated by kaolinite and quartz with average percentage composition of 52.0 and 41.5, respectively, suggesting that the rocks are potential candidates for fracking. The total average pore volume and specific surface area in the samples are 0.0421 cm3/g and 22.43 m2/g, respectively. The shale samples show higher amounts of total pore volumes relative to the sub-bituminous coals. No obvious organic pore is observed in the shale samples, but some inherited primary organic matter pores of plant origin exist in the sub-bituminous coals. Therefore, inorganic minerals pores are the major contributors to the pore development in the immature shales. The FE-SEM results reveal that the pore network in the shales is dominated by intraparticle pores and fractures. The observed well-developed fractures within the Mamu shales that may have resulted from tectonic events are capable of positively influencing the petrophysical properties of shales in the basin and by extension assist in shale gas exploration and development in the entire Lower Benue Trough.

Geochemical evaluation of Campanian-Maastritchian clay-shale sediments of Patti formation, Southern Bida and Mamu Formation, northern Anambra basins

Two basins (Southern Bida and Northern Anambra Basins) were investigated to deduce weathering, paleooxygenation, provenance, depositional environment and tectonic setting, as well as to establish a relationship between the two basins. The obtained high values of calculated weathering indices such as Chemical index of alteration (CIA &gt; 90), Chemical Index of Weathering (CIW &gt; 90), Plagioclase Index of Alteration (PIA &gt; 90) and the Al2O3-(CaO + Na2O)-K2O ternary relationship for the clay – shale sediments from both basins indicate intense weathering in the source area. Important geochemical ratios such as V/Cr, Cu/Zn, Ni/Co, (Cu+Mo)/Zn, revealed predominantly oxic conditions for the clay – shale sediments from both basins, although, a more reducing or an anoxic condition cannot be ruled out for the clay – shale sediments from the Southern Bida basin due to high ratios of U/Th (1.93-5.67) and Cu/Zn (0.19-5.00). In addition, the Sr/Ba ratios (0.16–3.50) for the clay-shales from the Southern Bida basin indicated an alternated marine and continental paleo-depositional settings and only continental setting (Sr/Ba ratios = 0.22 – 0.50) for the Northern Anambra basin. The Th/Sc, La/Sc, Th/Co and the LREE/HREE ratios showed a derivation of the shale and clay deposits from similar felsic-rich source rock while the log of (K2O/Na2O) vs SiO2, revealed a Passive Margin tectonic setting for the two Basins. There is insignificant differences between the geochemical classifications, weathering, source rock/provenance and tectonic settings of clay-shale sediments of both sedimentary basins, however, there exist slight disparity in their salinity conditions and redox settings.&#x0D; Keywords: Geochemistry, Clay-shale, Provenance, Tectonic Setting, Northern Anambra and Southern Bida Basins