Evaluation of the Hydrocarbon Potentials of shale exposures at Okpekpe in Edo North

This study evaluates the source rock characteristics of rock exposures along a newly exposed road cut in Okpekpe. An integrated technique of organic geochemical analysis and biostratigraphy evaluation were adopted to determine the source rock quality, Maturation index, kerogen types, depositional environment andsediment age. Results of organic geochemistry gave total organic carbon (TOC) value between 0.81 to 3.04 w.t% (2.08wt.% average) indicative of a good source potentials. The plot of Total Sulphur Content (TSC) against TOC suggests a transitional depositional environment for the samples while the plot of hydrogen index (HI) against oxygen index (OI) shows that the samples are capable of generating mixed type II/III kerogen. Palynological analysis revealed that the basal section of the exposure is characterized by the occurrences of typical and moderately rich Late Cretaceous – Early Tertiary palynomorphs. While the upper section is poorly rich in palynomorph abundance but with spot occurrences. The palynomophs asssemlages is of Late Maastrichtian Early Paleocene and the outcrop is characterized by the presence of terrestrial pollens and spores indicating a continental to transitional depositional environment, typical of the Mamu Formation of the Anambra Basin.


Introduction
described source rock as fine grained argillaceous deposits, rich enough in organic matter and capable of generating hydrocarbon. Prospectivity for hydrocarbon is dependent on various factors such as the presence of viable structures and a viable petroleum system, of which the presence of source rock is a key component.
Understanding the characteristics of the source rock shed light on its quality and hydrocarbon generating potentials . In localities where the formation of the study source rock needed to be tied to regional geology, Biostratigraphy characteristics play a major role in age and environment of deposition determination, which will ultimately help in correlating the rock exposure to it corresponding regional equivalent when compared to literature.
Only few documented reports are available on this region of the Anambra Basin. (Edegbai and Emofurieta 2015;, Most recorded work of the Anambra basin are restricted to the center of the Basin (Burke et al., 1972;Ladipo, 1988;Nwajide and Reijers, 1996;Nton and Bankole, 2013;Ola and Adeoti 2017).
Hence the motivation behind this study; which intends to characterize the encountered shale exposures to understand its hydrocarbon generation potential and define its regional equivalent by comparing analytical result obtained with recorded literatures.
The study area is located in Okpekpe community, Edo North at coordinates N07° 11' 02.0'' and E006° 28' 06.5''. It is situated within the Anambra Basin, specifically at the Benin flank of the basin (Figure 1). Generally, the sediment of this region includes extension of the Ajali Formation, the transgressive Nkporo group and the regressive Coal measures of the late cretaceous sea (Nwajide, 2005).

Materials and Methods
Five outcrop samples were obtained from the study area and subjected to both organic geochemical analysis and Palynological evaluation after sample preparation in the laboratory. The slide preparation method for Palynological study was in accordance with standard methods as described in literature (Traverse, 1988;Wood et al., 1996).
A Frequency count of stratigraphically important forms was determined for each of the samples. Necessary photomicrographs of important structures and forms were taken using a digital camera attached to the microscope for detailed study and identification of the fossils forms and characteristic.
The organic geochemical evaluation technique adopted include: Turbidimetrically barium sulphate precipitate for total sulphur content determination (TSC), The Walkley Black Wet Oxidation method using a soxhlet extractor, for Total organic carbon (TOC) estimation and the Rock-Eval Pyrolysis technique. The Rock-Eval Pyrolysis technique is used for the anhydrous pyrolysis of source rocks that enables the chemical composition of kerogen, and hence its hydrocarbon potential to be determined (Espitalie et. al., 1985;Peters, 1986). Parameters estimated mathematically from results of the pyrolysis process for further evaluation include; TMAX ,S1, S2, S3, , HI, PI and OI.  (Abubakar, 2014).

Results
The geochemical analysis results of the samples are presented in the table 1. TOC is the total amount of insoluble organic material or kerogen present in the rock, and it is expressed as a percentage in weight (TOC wt.%). It is a source quality index that qualifies a rock sample as either being a potential source of hydrocarbon or not.
TSC represent the total amount of sulphur in the sample. The sulphur content in a rock sample is associated with fauna abundant as we move from the continent to the deep marine. Therefore, it can be used as an index to predict the environment in which the sample was deposited.
The SOM represent the extractable organic constituent in the sample. S1 captures the free hydrocarbon constituent in the analyzed samples i.e the organic matter that has been completely transformed to hydrocarbon. S2 is defined by the generated hydrocarbon that are released by thermal cracking of nonvolatile constituent of organic matter, which give an insight to the potential amount of producible hydrocarbon in the rock if sediment burial and maturation continues.S3 is the total quantity of generated CO2, that account for the oxygen richness of kerogen, and is rely upon to estimate the oxygen index.
Tmax is the temperature at which the greatest amount of hydrocarbon is released from kerogen during the pyrolysis process. This happens at the top of S2 peak, and it is an index of sediment maturity, which is dependent on the kerogen type.
HI is the ratio of S2 hydrogen to TOC in grams. HI is a source rock hydrocarbon richness index. OI is the ratio of S3 hydrogen to Total organic carbon.
GP represent the total free hydrocarbon generated from pyrolysis of the samples and the existing hydrocarbon in the samples. PI represent the ratio of the free hydrocarbon to the potential hydrocarbon yield. This parameter is an index that account for organic matter evolution.  The plot of hydrogen Index versus Oxygen Index is a graphical means of highlighting the hydrocarbon type the source rock is expected to generate (Figure 2). The Hydrogen Index is dependent on the kerogen type present in a source rock i.e the higher the hydrogen index, the greater the tendency for the source rock to yield oil and vice versa for gas. Figure 3 present the graphical plot of Total Sulphur Content against Total Organic Carbon. This plot is a means of interpreting the possible environment of deposition of the analyzed rock sample. The results of Palynological analysis is presented in Figure 4 and summarized in table 2. Tissot and Welte (1984) and Hunt (1979), proposed 0.5wt% as the minimum threshold value for a rock to be regarded as a petroleum source rock. As presented in Table 2, the TOC of the samples ranges from 0.81 to 3.04 w.t% (2.08wt.% average), which exceeds the threshold value of 0.5wt.%. This suggests that the samples are good to very good source rocks and is similar to report of earlier workers that study the source potentials of the Mamu Formation and the Nkporo Shales (Babatunde, 2010).

Source Quality and Kerogen type
Source rocks genetic potential that is below 2mgHC/g are indicative of minor oil content but with some potential gas, while those with genetic potential of 2 -6mgHC/g have some reasonable oil potential (Tissot and Welte, 1987;Akande et. al., 2005). The Genetic potential values for the samples ranges from 0.33 to 13.38 mgHC/g (5.12 mgHC/g average), which indicates that they have infinitesimal oil but some gas potential.
The kerogen type present in a source rock determines its Hydrogen Index. Many organisms contribute to the organic matter present in petroleum source rocks and they differ in their organic matter and total hydrogen contents. The preserved organic matter exhibits parallel diversity that are further modified overtime into gas or oil (Dow, 1977). Laughrey (2009) proposed that source rocks with HI greater than 600 mgHC/g will generate oil, while those with HI between 200 and 600 mgHC/g will generate wet gas (oil and gas). Rocks with HI values between 50 and 200 mgHC/g will generate gas and those with HI values less than 50 mgHC/g are inert.
The HI value for the studied samples ranges from 31 to 422 mgHC/g (190.6 mgHC/g average) suggesting that the source rock is gas and oil prone. The plot of HI against OI in the modified Van Krevelen diagram in figure 2, also reveals that studied samples are predominantly type III/II kerogen. This implies that the samples are majorly oil-gas prone source rocks and is similar to reports of Akaegbogbi (2000).

Source Rock Maturation
Maturation is the process of chemical change in sedimentary organic matter due to burial, i.e. the action of increasing temperature and pressure over geological time (Miles, 1989).
The concentration and distribution of the Hydrocarbon contained in a particular source depends on both the type of the organic matter and its degree of thermal maturation. Peter and Cassa (1994) proposed that source rocks with TMax values of less than 435oC are Immature, while those with Tmax value of between 435-470oC are Mature and those above 470oC are post mature.
For the studied samples, The value ranges from 417 to 4700C (430oC average). This suggests that majority of the samples are immature and are similar to the report of Ogala (2011) for the Maastrichtian Mamu Formation of the Anambra Basin.

Paleo-environment Interpretation
The paleo-depositional environment of a rock can be determined by the abundance of ancient life forms recoverable from sample analysis, believed to have thrived in the same environment where the rock was deposited.
From the Palynological evaluation results (Figure 4 and Table 2, The outcrop sediments were predominantly dominated by land derived pollens and spores. According to Schrank (1984), an assemblage of palynomorph with a high content of pollens and spores indicates a terrestrial influence and vice versa.
This depositional environment is also supported by the paucity of foraminiferal species over these samples. The nonrecovery of foraminifera is also attributable to the lithology of the samples, deposited in a continental environment where bottom conditions were not conducive for the preservation of foraminiferal species.
The cross plot of TSC versus TOC parameters derived from the organic geochemical analysis in figure 3, exclusively shows a normal marine depositional environment for the samples, which tends towards a terrestrial environment. Both results from palynological and geochemical analysis is suggestive of the proximal end of the transitional depositional environment for the studied samples.

Biozonation and age determination
From the results in Figure 4 and (1978), this zone is equivalent to the Zone III-IV of Legoux (1978).
The studied sections of the outcrop ranges in age from Late Maastrichtian-Early Paleocene and the predominance of terrestrial pollen and spores, paucity of dinocysts and forams indicates that the paleo-depositional environment is Continental to transitional. This corresponds to the depositional environment established in literature of the Mamu Formation (Obaje, 2009).

Conclusion
The results of organic geochemical and palynological evaluation of the source potentials of the studied exposure in Okpekpe revealed that the shales have a good to very good source potentials and are typically made of a type III/II kerogen constituent capable of generating gas and oil when attained maturation.
Both analysis results revealed that the rock was most likely deposited close to the terrestrial end of a transitional environment. Palynological evaluation showed that the studied rock exposure ranges in age from Late Maastrichtian-Early Paleocene. This was based on index spores, pollens and dinoflagellates. These include Longapertites marginatus, Cingulatisporites ornatus, Proxapertites cursus, Echitriporites trianguliformis, Cyathidites minor, Rugulatisporites caperatus, all of which are dated to be of Maastrichtian -Paleocene age which agrees with the age of the Mamu Formation in the Anambra basin.