Petrological and Geochemical Evaluation of the Paleocene-Eocene Lithofacies in Dahomey Embayment , Southwestern Nigeria

The detail mapping and logging of the Ibese area within the Dahomey Basin has revealed the total thickness of the limestone to range between 12.0 m to 15.0 m. This study is based on qualitative lithologic, spectrochemical analyses via X-ray fluorescence and petrographic indices to understand the lithofacie, chemistry, viability of the sequences and depositional environment. This consists of milky to grey and nodular to massive fossiliferous limestone units while the sand content increases with depth. The composition of the major oxides in the thirty-seven (37) samples from six exploratory wells revealed that CaO (51% 56%), SiO2 (2.6% 10.56%), Fe2O3 (0.33% 0.94%), MgO (0.78% 1.02% and Al2O3 (0.72% 0.98%) were the most abundant elements. The trend of the geochemical indices show increase in the percentage composition of CaO, and MgO down the sequences with high values which defined the depth of diagenetic re-distribution. The percentage composition of Al2O3, Fe2O3 and K2O are significant indicating continental influence during diagenesis and diverse depositional episodes. The three different microfacies recognized based on depositional texture are shelly biomicrite, shelly biosparmicrite and alga biosparmicrite. The predominance of micrite as the cementing matrix revealed that the rock was deposited in a quiet shallow inner shelf environment occasioned by storm waves.


Introduction
The Dahomey Basin is an arcuate extensive basin located within the Gulf of Guinea, West Africa (Figure 1).It extends from the eastern part of Takoradi Figure 1.Map of Africa showing the location of Nigeria and the generalized geological map of the eastern.Dahomey Basin (modified after Gebhardt et al. [27]).
Height in Ghana through Togo, Republic of Benin and then terminates at the Okitipupa ridge, south western Nigeria separating it from the Niger Delta.It runs parallel to the coastal states of these countries covering a total area of approximately 49,209 km 2 [1].The states within the basin in the Nigerian sector include Lagos, Ogun, Ondo and Benin and the sediments' thicknesses increase toward the sea (Figure 1).The northern part of the basin in Nigeria is the exposure of the Abeokuta group which unconformably overlying the basement while towards the sea, there is a progressive increase in the thickness of chemical and biochemical sediments such as gypsum, glauconite, limestone and other associated marine deposits which is then capped by coastal sands [2] [3].Numerous works have been carried out on the Ewekoro and Akinbo formations which are exposed at different sections of eastern part of the Dahomey Basin [3] [4] [5].
Omatsola and Adegoke described the structure and stratigraphy of the Abeokuta formation [6].Ogbe proposed that the overlying finely laminated shale at the Ewekoro quarry is Akinbo Formation [7].Ekweozor and other workers studied the origin of tar sands, organic geochemical investigation of bitumen, properties and source rock evaluation of interbedded shale of southwestern Nigeria [8] [9].They concluded that the tar sand deposits represent the product of reservoir transformation of hydrocarbon by anaerobic micro-organisms.The study of the subsurface in Dahomey Basin of upper Cretaceous and lower Tertiary of Bodashe-1 and Ile paw wells was carried out and concluded that the sediments belong to the Araromi Formation [10], while the use of pebble morphometry associated with the siliciclastic sediments of Abeokuta group was used to deduce their depositional environment [11] [12].Boboye and Raji reported that the biostratigraphy and petrography of the sequences were deposited in quiet shallow marine environments and that the presence of Toweiuscallosus and Coccolithusformosus indicated that Oshosun and Ewekoro Formations were Ypresian and Thanetian to Ypresian age [3].This rock is a potential hydrocarbon reservoir for the overlying shale while this character of the rock is seen in the Benin Republic sector of the basin, where petroleum is produced at marginal level [3] [13] [14].The aspect geochemical evaluation, depositional environment and reserve estimation of limestone deposit were also reported around Orimogija and Okeluse areas of eastern Dahomey Embayment [15] [16].However, this study focus on the distribution of diverse lithofacies, genetic description of the Ewekoro Formation with emphasis on the limestone and what characterize the environment of its deposition, the extent of diagenesis, biological influence on the chemistry of the units, major elemental composition, post depositional processes and their viability in determining its economic values.

Stratigraphy of the Dahomey Basin
The Dahomey Basin is described by an extensive wedge of Cretaceous to Recent sediments which lies unconformably on the basement [1].Theses rocks are upper Cretaceous Post Santonian which also outcrops in the River Niger, Bida Basin and the eastern sector part of the Anambra Basin down to the Okitipupa high.In the Dahomey Basin the three stage tectonic evolution allows the stratigraphic sequence to be divided into three sequences namely Precambrian to Triassic intracratonic rocks and Jurassic to lower Cretaceous, Continental to marginal marine rocks representing the pre-transform stage, the lower Cretaceous to late Albian rocks representing the syn-transform stage and Cenomanian to Holocene rocks representing the post-transform stage [9].
The lower Ise Formation which overlies the basement contains conglomerates, coarse to medium grain sandstones and shales that were deposited in continental and deltaic environments.The conglomerates are imbricated and at some locations, ironstones occur [10].The conglomerates are poorly sorted, micaceous and more or less clayey [1].This formation which is part of the Abeokuta group can be classified under the groups of rocks formed in the pre-transform stage between Upper Jurassic to Neocomian, [9] [11] [12].This age is more representative on the offshore part of the basin which may reach about 2000 m thick though this has not been reached by drilling but geological interpretation have been inferred from various seismic data [13].This formation also is represented in the Neocomian.Cross bedding is noticeable while the rock is soft and friable [1].
Overlying the Ise Formation of the Abeokuta group is the Afowo Formation which is made up coarse to medium grained sands and interbedded shales, siltstones and claystone with the sandy facies bearing tar while the shales are organic rich [12] [18] (Figure 2).
The lower part of this formation which overlie the Ise Formation is transitional with mixed brackish to marginal horizons that alternate with well sorted, sub-rounded sands indicating a littoral or estuarine near shore environment of deposition [2].Brownfield and other workers described fairly sorted sandstone interbedded with shale beds overlying a shale and siltstone sequence deposited as a reworked fan delta in a marginal marine to inner shelf environment [19].This unit corresponds to the beginning of the post transform stage in the basin and is dated as Turonian.The upper part of this formation grades into Maastrichtian in different parts of the basin [19].

Location of the Study Area and Geology
The study area is located in Ibese, southwestern Nigeria about 40km southwest of Abeokuta where quarrying activities have revealed rocks of diverse ages and lithologies within the basin (Figure 1).The geology as exposed at different road cuts and quarry exposures show the occurrence of lateritic sandy horizon which is the topmost layer.The underlying lithology in this sequence is sandy shale with lateritic concretions which is succeeded by the occurrence of shale.At other sections where weathering is minimal, this layer show glauconitic nodules followed by the occurrence of partially weathered poor fossiliferous limestone units succeeded by a fossiliferous horizon which extends to a significant depth before terminating on marl.Intercalating the marl units are sandy shale and poorly sorted sandstones.

Field Study
The field mapping was carried out in order to determine different rock units and their association within the area.The samples constitute limestone, shale and sandstone.A systematic mapping and sampling was adopted at five meters' intervals.The lithostratigraphic descriptive geology of the study area is well described in the quarry site where exposures of the rocks can be studied (Figures 3-9).The rocks are structurally undeformed and strike east west running parallel to the coastline.The rocks association were defined from core samples with the uppermost unit extending up to about 0.7 m.

Laboratory Analyses
The samples were disaggregated and subjected to lithological description and petrography in the laboratory while the samples for the XRF analysis were pulverized into a fine powder.Between 10 mg -15 mg of the samples were subjected to X-ray fluorescence Spectrometric analysis which was done using the Rigaku XRF Spectrometer with a rhodium (Rh) target.This runs at 50 KV/50 mA with full vacuum and a 25 mm mask for the analysis of various major oxides with the detectable limits for the major elements which were 0.01% for SiO 2 , Al 2 O 3 , CaO and 0.04% for Fe 2 O 3 both at 100% upper limit.The petrographic slides were prepared at the thin section laboratory, where thin sections approximately 30 µm thick of the samples were produced for microscopic study.

Lithostratigraphy
The lithofacies that constitute the study area are sandstone, clay, shale, limestone and marl.The sandstones are fine grain with the ferruginized sandy clay units      significantly difference from the upper units.The clay and shale units are lateritized occurring as concretions ranging from brown to reddish brown and grey in colour respectively.The shale units are rich in glauconite and the limestone units constitute the weathered, poorly fossiliferous partly to the telltale of weathering on the layer and unweathered units which grades from the upper layer in a diffuse manner.The diagenetic process has had its toll on this layer due to pore spaces (mouldic and vuggy porosities) which shows casts of the disintegrated fossils (Figures 3-9).The marl units are described as the impure limestone.The deeper section of this unit shows contact between the basal weathered marl and a fresh top layer with the former occurring in the form of lateritic clay concretions.

Geochemistry
The major elements from the six wells within the Ewekoro Formation put the average percentages of major oxides across the wells as CaO (51% -56%), SiO 2 (2.6% -10.56%),Fe 2 O 3 (0.33% -0.94%), MgO (0.78% -1.02% and Al 2 O 3 (0.72% -0.98%) (Appendix: Tables 1-6).The percentage concentration of CaO increases down the formation to approximately 13.0 m depth, where the highest value was recorded which tapper down the formation to the limestone-marl contact.The average percentage composition of CaO is 52.6% which indicate the dominancy of this oxide.This corroborate with the petrographic studies with the occurrence of carbonate bioclasts within the lime mud matrix [3] [19] [20].The high ratio observed in calcium and magnesium defines a high calcite purity which is significance in the cement production.The presence of magnesium in carbonate rocks is a function of the temperature of deposition and time due to its removal by interstitial solution while dolomitization process could also contribute to high magnesium content [21].
The increase in the amount of CaO at different depths clearly defines diverse carbonate lithologic units.At 13 m depth, there is a significant increase in the amount of MgO, this significant enrichment is attributed to increase in the amount of coralline algae and/or had an initial dolomitization where the water in the basin may be partially isolated under conditions of aridity and hence enriched in magnesium by continual inflow of sea water and subsequent precipitation of carbonate.Enrichment in this manner would form a heavy brine solution that moved down through porous sediments below and dolomitize those carbonates with which it came in contact with [21] [22] [23].This depth of replacement serves as the datum of skeletal accumulations at water thermodynamic condition.At depth ranging from 12 m to about 14 m, there is an increase in fossil content with large primary porosity which creates the avenue for replacement.At other deeper depths there is an inversion of aragonite to calcium and also consequent shedding of carbonate sediments from above [3].
The geochemistry of well-1 defined the depth of carbonate diagenetic redistribution complemented by the amount of SiO 2 measured from various horizons (Appendix: Table 2, Figure 10 and Figure 11).At 12 m to 14 m depth, in the upper limestone horizon with an average of 0.40 is an indication of continental influence [22].The moderate amount of both seems to reflect in the ferruginous syn-sedimentary and post depositional inputs of these sediments by surface meteoric waters flushing rapidly through ferromagnesian rich minerals (shale and glauconite) overlying the limestone.The rocks are stained at some parts with reddish brown colouration when exposed and it is seen that some of the allochems exhibit some iron oxide stained under the microscope [3].However, the results have shown that the limestone sequence bounded by thick shales, correspond to higher value of Fe/Mg ratio, due to the occurrence of shaley glauconitic bands and leaching of smectite clays into the unit.The result of the major oxides in well-2 has shown that the depth at which Mg occur in this well tend to become shallower to that measured in Well-1 suggesting the possible increase in water level thus causing the deposition of the carbonate at a shallower depth.The depth of SiO 2 still remain at 13.5 m indicating a common diagenetic depth for both horizons.Silica percentage (SiO 2 ) is low and range between 2.4% to 10.38% (average of 4.46%).This is an indication that the continental areas have a mild effect on the limestone through diagenesis [21].The presence of alumina with average value of 14.43% (shale) and 0.66% (limestone) confirms the presence of little alumino silicates in the adjacent areas that had not been completely weathered prior diagenesis and also that which dripped on the upper limestone horizons with high alumina value of 2.44% in the limestone [21].The modal composition of Fe 2 O 3 is low relative to others hence suggesting low availability of oxygen to oxidize Fe 2+ present in heavy minerals to Fe 3+ during diagenesis with little effect of post depositional processes on the overlying glauconite (Appendix: Table 2, Figure 11 and Figure 12).
The amount of MgO relative to depth in well-3 defined the depth of carbonate primary replacement complemented by the amount of SiO 2 measured from various horizons which showed different peaks (Figure 11).Though there exists fluctuations at diverse horizons but the depth of high MgO and SiO 2 concentration is put at 12.0 m to 14.0 m.MgO peaked showing the depth of favoured carbonate deposition of aragonite -calcite inversion which also correspond with an increase in the amount of SiO 2 .Two peaks were recorded for this horizon (Figure 11).An average of 64.91% and 4.89% were measured for the shale and limestone respectively.Relative to previous wells, a high percentage of SiO 2 was recorded at 13.0 m depth which indicate the presence of authigenic quartz and minor detrital quartz [21] (Appendix: Table 3).An average of 11.97% and 0.96% of alumina were recorded for the shale and limestone.A local enrichment of 4.2% was measured at the upper limestone horizons while a gradual decrease in  The ratio of MgO with depth in well-4 defined the depth of carbonate diagenetic re-distribution which is complemented by the amount of SiO 2 measured from various horizons with different peak (Figure 10).At 12 m to 14 m depth MgO peaked showing the depth of favoured carbonate deposition or aragonite-calcite inversion which also correspond with an increase in the amount of SiO 2 .SiO 2 percentage is fairly high and ranged from 1.45% to 10.56% (average of 3.87%).The rock can be described as chemically pure indicating that the continental areas have little effect on the limestone [24] (Appendix: Table 4).
Feldspar containing silica had been weathered during transportation prior the commencement of diagenesis.The values of Al 2 O 3 in the shale (11.8%) and limestone (0.81%) lithofacies confirms the presence of little alumino silicates in the adjacent areas that are partially weathered priorthe diagenetic processes in the overlying shale facie.Significant shale contamination is well defined by the increase in percentage of K 2 O (0.11%) measured at the shale-limestone contact [21].The percentage composition of Fe 2 O 3 measured in the shale (7.3%) increased compared to other oxides thus helps to infer the amount of oxygen available to oxidize Fe 2+ present in heavy minerals to Fe 3+ .The increase in the amount of Fe 2 O 3 in this horizon describes the intense chemical weathering of the continental areas (Appendix: Table 4).
In well-5 at 13.0 m depth, there is a sharp increase in both MgO and SiO 2 (Figure 12).This graphical behaviour is suggestive of an increase in the thickness of the deposit towards the continent, which showed a decrease in the extent of continental inputs as the deposits progrades onshore.Silica (SiO 2 ) percentage ranged from 1.4% to 10.2% (average of 5.1%) (Appendix: Table 5).This is an indication of the Continental areas having little effect on the limestone [22].
Alumina, (Al 2 O 3 ), increases in this horizon confirms the presence of alumino-sili-cates in the adjacent formations.The effect of shale contaminante in the limestone units is minimal.This corroborates with the average percentage of K 2 O (0.1%).The amounts of Fe decreases down the formation making it obvious that it is post depositional.This is due to the percolation of meteoric waters through shale and glauconitic layers into the underlying limestone units.

Shelly Biomicrite
This represents the topmost portion of the limestone layers.The rock is micrite (mud) supported and contain a substantial amount of bioclasts (grain).At different sections, the rock defines about 1/4 of spar.Bioclasts described from this layer include pelecypods which take a higher percentage of bioclasts population, gastropods, echinoids, cup corals exhibiting radial structure and coralline algae.Internal micritization of some grains were observed especially the mollusks, gastropods, echinoids and brachiopods.Micritic corrosion of the surface of coralline algae and pelecypods gave rise to more degenerated structures.Internal recrystallization of bioclasts is well defined.Non bioclastic components are intraclasts, quartz grains and occasional pellets.

Shelly Biosparmicrite
This microfacies succeeds the top biomicrite due to the increase in intraclasts, spars crystal increase outward from shell boundaries, however, the compaction increases grain to grain contact.

Alga Biosparmicrite
This microfacies is the lowermost section of the limestone sequence in Ibese.It is synonymous to the alga bioclastic wackestone-packstone [25].This facies is characterized by abundant stromatolitic algae which defines sediment binding.
Other bioclasts include coiled gastropods, micritized echinoids, foraminifera, mollusks and spherulitic corals.Non bioclastic components include pellets and intraclasts which make about 1/3 of various portions of this microfacies.The radial pattern show internal rim cement (microspore calcite cement) within micritic ground mass.Other depositional texture that can be inferred from this layer is oncolitic grainstone defined by corals and algae complemented by increase in the amount of skeletal debris.Grain dissolution and interparticle porosity are significant as they reflect the degree of diagenetic process, hence affecting their physical properties.

Depositional Palaeoenvironment
Based on the sedimentological indices, the shale units of Akinbo Formation are fine with no sand, silt or carbonates, grey, flaggy and slabby indicating low energy quiet water setting.The palaeobathymetry of the fossil assemblage defined from the earlier studies indicated benthic dwellers except for some shell fragments whose origin could not be ascertain [3]  quel to this, there has been decrease in the total porosity of the rocks where the topmost biomicrite defines mouldic, fenestrael, intraparticle and little interparticle porosities.The shelly biosparmicrite and alga biosparmicrite microfacies are characterized by increase in grains' components which increase the grain to grain contacts of the rock units thus enhances interparticle porosity [27].
The magnesium content of the carbonate rock is low when compared to calcium, while the average percentage composition across the wells ranges between 0.78% -1.02%.This low composition is attributed to an increase in organism complexities (advanced phyla) based on this premise, a shallow marine environment of deposition is suggested.The presence of SiO 2 , K 2 O and Al 2 O 3 is an indication of clastic input and degree of diagenesis on the lithofacies [21] [22] [28].The downhole increment of SiO 2 in the formation indicates an increase in the continental influence.In well-1, the relative increase in the amount of SiO 2 defines the degree of continental influence which suggest that the paleobathymetry during the formation of the carbonate to be within the range of 12.0 m to 14.0 m depth suggesting a shallow water environment (Figure 10 and Figure 11).The average Fe/Mg in well-3 measured in the limestone (1.24) indicated gradual increase suggesting a continental influence.The moderate amount of both reflect the ferruginous syn-sedimentary and post-depositional inputs of these sediments by suface meteoric waters flushing rapidly through ferromagnesian rich minerals overlying the limestone.

Conclusion
Detailed mapping and logging of the study area showed the total thickness of the limestone ranged from 12.0 m to 15.0 m.The limestone consists of milky to grey and nodular to massive fossiliferous units.It is evident that the sand units increase with depth.The major element percentages revealed an increase in CaO with depth and an average of 56% and also a corresponding increase in MgO with average of 0.86% which show the calcitic nature of the deposit.The six wells showed significant peaks in MgO and SiO 2 at depth approximately 13.0 m.The increase in the Mg component suggests diagenetic processes and the significant occurrence of Al 2 O 3 and Fe 2 O 3 in the sequence is an indication of continental influence in the diagenetic history of the sedimentation.Three different microfacies were recognized based on depositional texture, they are shelly biomicrite, shelly biosparmicrite and alga biosparmicrite.The predominance of micrite as the cementing matrix revealed that the rock was deposited in a quiet shallow inner shelf environment occasioned by storm waves.The elemental composition of the rock units suggested the suitability of deposit for cement production however; further studies could look into the lime saturation factor (LSF), silica modulus (SR) and alumina modulus (AR) in making up for corrections to meet production specifications.

Figure 3 .
Figure 3.A lithostratigraphic section as exposed in Ibese.

Figure 10 .
Figure 10.A lithofacies correlation of study wells showing diverse horizons.

Al 2 O
3 with depth was recorded.This indicated the rate of Al 2 O 3 percolating into the limestone from the shale unit.The K 2 O value (0.06%) measured from the top of the limestone unit confirm with the shaly contamination.Average values of Fe 2 O 3 (7.864%and 0.675%) were recorded for the shale and limestone while a peak of 8.36% at the shale-limestone contact suggest the partial oxidation of glauconite from Fe 2+ to Fe 3+ .

Figure 12 .
Figure 12.Plots of MgO and SiO 2 percentage against depth for the study wells.
[20] [22] [26].As the processes of micritization and sparitization are diagenetic processes which may involve the recrystallization of solid solutions of carbonate into coarse calcitic crystals or the mechanical degradation of shells into carbonate mud which may eventually alter the initial depositional fabric of the deposit, se-O. A. Boboye, O. J. Omotosho

Table 1 .
Major elemental geochemistry of the Akinbo and Ewekoro Formations in Well 1.

Table 2 .
Major elemental geochemistry of the Akinbo and Ewekoro Formations in Well-2.

Table 3 .
Major elemental geochemistry of the Akinbo and Ewekoro Formation in Well-3.

Table 4 .
Major elemental geochemistry of the Akinbo and Ewekoro Formations in Well-4.

Table 5 .
Major elemental geochemistry of the Akinbo and Ewekoro Formations in Well-5.