4.1 Lithostratigraphic Description
The Fika Member of Pindiga Formation is exposed at Kware/Gabukka stream sections. Two composite sections were logged in detail (Fig. 3). The sections consist of nodules and concretions intercalated with gypsum. The gypsum may be diagenetic whereas the shale is black to grey in color.
4.2 Foraminiferal Analysis
The results of foraminiferal recoveries are generally good. Most of the samples were populated by arenaceous benthic species (Table 1). Calcareous species (planktic and benthic) were not recorded. This could be attributable to the depositional environments of the sediments. The benthic forminifera recorded include Miliammina pindigensis, Ammomarginulina emir, Ammobaculites bauchensis, Ammobaculites pindigensis, Ammobaculites gombensis, Haplophragmoides excavata, Haplophragmoides pindigensis, Haplophragmoides bauchensis and Reophax guineana. The foraminiferal dating of these outcrop samples were guided by the works of [5] and [20]. Though, planktic foraminiferal species are absent in the samples but the arenaceous benthic foraminiferal species whose stratigraphic distributions have been well established in the Nigerian Sedimentary basins were used to assign ages (Fig 4).
4.3 Palynological Analysis
The twenty outcrop samples studied indicate poor palynomorphs recovery. Few frequency of land derived sporomorphs such as Cyathidites minor, Tricolporopollenites sp., Cingulatisporites ornatus and Longapertites sp. were recorded especially in samples F9, F10, F14, F16 and F19. Few occurrences of Botryococcus braunii, fungal spores and diatom frustules were also recognized. The studied samples (F2-F21) recorded low frequency of palynomorphs (Table 3; Figs. 6 and 7).
4.2.1 Late Cretaceous
The foraminiferal assemblage that characterized these samples are mostly arenaceous species and this includes: Miliammina pindigensis, Ammomarginulina emir, Ammobaculites bauchensis, Ammobaculites pindigensis, Ammobaculites gombensis, Haplophragmoides excavata, Haplophragmoides pindigensis, Haplophragmoides bauchensis and Reophax guineana. (Fig. 4). This assemblage had been reported in some of the sedimentary basins in Nigeria. Haplophragmoides excavata has been reported in the Coniacian part of Nkalagu Formation and Nkporo Shale [20]. Originally described from the American Gulf Coast, this species has been reported from the Coniacian to Maastrichtian of North America [22]. The species has also been reported in the Paleocene sediment of the Niger Delta where it has its extinction level (Last stratigraphic appearance). Haplophragmoides bauchensis was reported in the Turonian to Conician sediments of Nkalagu Formation [20] and in the Turonian – Santonian outcrop section of the Pindiga Formation that was tentatively assigned to Turonian – Santonian age [20]. Haplophragmoides pindigensis, Haplophragmoides bauchensis Ammobaculites bauchensis, Ammobaculites pindigensis, Ammoastuta nigeriana, Miliammina pindigensis, Ammomarginulina emir and Reophax guineana (Fig. 5) were also reported in the outcrop section of the Pindiga Formation that was tentatively assigned to Turonian – Santonian in the valley near Pindiga village [20]. On the basis of above Foraminiferal associations, the studied interval of the Fika Member is assigned Late Cretaceous, not younger than Maastrichtian age.
For the palynomorphs, the recovered palynoflora is dominated by Late Cretaceous species such as Tricolporopollenites sp., Distaverrusporites simplex, Gleicheniidites sp., Inaperturopollenites sp., Cingulatisporites ornatus in association with few records of Rugulatisporites caperatus (Maastrichtian-Santonian), and Zlivisporites blanensis (Maastrichtian-Turonian). This assemblage suggests an age not younger than late Cretaceous (Maastrichtian-Turonian) for the studied samples. Longapertites sp. in association with Cingulatisporites ornatus, Zlivisporites blanensis, and Botryococcus braunii has been reported from Campanian to early Maastrichtian strata of the Gongola Sub-basin. The taxa assemblage in this study falls within longapertites sp zone of [11]. Based on the records of palynomorphs the Fika Member was deposited during Campanian-Maastrichtian age.
The foraminiferal assemblage of the Upper Benue Trough are not age diagnostic, hence, the aforementioned foraminera were tentatively assigned to Turonian– Santonian [21]. However, palynological analysis seems to provide a better stratigraphic interpretation of the Upper Benue Trough [25]. Concerning this, the Fika Member of Pindiga Formation palymorphs assemblages were correlated with Longapertites sp zone of [12] which was date Campanian– early Maastrichtian age , analysis of the Fika Member.
4.4 Palynofacies Analysis
The palynofacies types, percentage abundance and hydrocarbon generating potentials of the samples are shown in Table 4. The details of the palynofacies (kerogen) analysis based on Visual Kerogen Typing (VKT) which is indicative of the paleoenvironment and the Spore Colouration Index (SCI) and Thermal Alteration Index (TAI) that help in identification of hydrocarbon generating potential of the sediments.
Based on [16] and [23] the major classes of palynomaceral classes identified in the samples include; Palynomaceral 1 (PM-1) / Coaly (Plant materials that are predominantly dense black, dark brown, or orange-brown, often exhibiting irregular shapes), Palynomaceral 2 (PM-2)/Woody (Plant materials with a structured color ranging from brown to orange), Palynomaceral 3 (PM-3) / Woody (Light-colored, structured material occasionally containing stomata), Palynomaceral 4 (PM-4) / Coaly (black equi-dimensional, blade or needle shaped material) and Structureless Organic Matter that composed of bacterially reworked biomass preserved in anoxia.
4.4.1 Visual Kerogen Typing (VKT)
The samples (Table 2) are generally characterized by abundant occurrence of small to medium sized PM-1 (brackish water environment) and small sized PM-4 (commonly found in brackish water to fluvio-marine environment) and PM-2 kerogen types, with relatively common records of SOM, and PM-3 (Fig. 8). This admixture suggests a predominantly fluvio-marine paleoenvironment of deposition. This interpretation is further corroborated by the occurrence of occurrence of Tricolporopollenites sp., pteridophytic spores such as Cyathidites minor, Cingulatisporites ornatus and palmae species such as Longapertites marginatus and Longapertites sp.
4.4.2 Spore Colouration Index (SCI) / Thermal Alteration Index (TAI)
The samples show colour variation from dark brown to black which is approximately 8.5-10 on the SCI scale of [17]. This correlates with the TAI value of 3+ to 5 and estimated Ro value of 1.2-˃2.00 of [23]. This suggests the samples fall within a super mature horizon in the basin making it predominantly gas prone.
4.5 Paleoenvironment
The interpretation of the paleoenvironment of the outcrop samples were inferred mainly from foraminiferal assemblage. The depositional environment of the samples are predominantly marginal marine to shallow Inner Neritic settings based on the recorded foraminiferal assemblage dominated by arenaceous benthic species such as Haplophragmoides excavata, Haplophragmoides bauchensis, Haplophragmoides pindigenis, Ammobaculites bauchensis, Ammobaculites pindigensis, Ammobaculites benuensis, Ammobaculites gombensis, Ammomarginulina emir, Ammoastuta nigeriana and Reophax guineana. The above foraminiferal association indicates marginal marine (fluvio marine) to shallow Inner Neritic environments [4], [18], [20]. The dominance of arenaceous foraminiferal species in shallow water limestone and micaceous shales in the Benue Trough have been used by [21] to suggest shallow water habitat for the Nkalagu limestone exposed at the quarry site. Similarly [21] has used the dominance of arenaceous species of Haplophragmoides, Ammobaculites, Reophax and Miliammina which he used in concluding that the paleoenvironment of Pindiga Formation of the Gongola Sub-basin depict a paralic to a very shallow neritic environment. The dominance of agglutinated foraminiferal species also suggests restricted, low oxygen bottom water conditions. On the basis of the arenaceous foraminiferal assemblage recorded in the studied interval of the Fika Member indicates marginal marine to proximal Inner Neritic settings. This is supported by dominance of PM-1(brackish water environment) and small sized PM-4 (commonly found in brackish water to fluvio-marine environment) and PM-2 kerogen types, with relatively common records of SOM, and PM-3that generally suggest fluvio-marine paleoenvironment.
4.6 Biostratigraphy and Age
The sample is characterized by co-occurrences of Haplophragmoides excavata, Haplophragmoides bauchensis, Haplophragmoides pindigensis, Ammobaculites bauchensis and Ammoastuta nigeriana suggesting a Turonian – Santonian age.
4.7 Thermal Maturation and Hydrocarbon Potential
The samples from Fika Member (Table 5) exhibit a complete spectrum of Foraminiferal Color Index (FCI) values, ranging from light grey to light brownish-grey. When assessing maturity level, the fossil color's darkness, determined using the Munsell Colour Chart, is the most crucial parameter among the three. To practically evaluate this darkness, specimens are juxtaposed with standard Munsell color chips for direct comparison. Subsequently, the darkness value is directly converted into an FCI number as outlined in the table 5.
Agglutinated foraminifera dominated the assemblage of the samples studied. These agglutinated foraminiferal colours ranged from light grey to light brownish grey to grey with corresponding FCI values of 2 to 3, these correspond to temperature range (TAI values) of 60 to 75 indicating low thermal maturity for both oil and gas generation. The samples generally show colour variation from dark brown to black which is approximately 8.5-10 on the SCI scale correlating with TAI value of 3+ to 5 and estimated Ro value of 1.2-˃2.00 [23] suggesting a mature horizon in the basin making it predominantly gas prone.