Petroleum geology of outcropping sediments along 3 Imiegba road , in Etsako east local government area 4 of Edo state , Southern Anambra Basin flank , 5 Nigeria : Inference from sedimentology and organic 6 geochemistry

The outcropping sediments along Imiegba road have been studied using their sedimentological and 10 organic geochemical (Total organic carbon, TOC and Soluble organic matter, SOM) parameters. A 11 total of sixteen (16) samples were collected and analyzed for the study. Based on sedimentological 12 and field evidences, the main lithofacies identified from the study area are sandstone, shale and 13 claystone. The sandstones are fine to medium-grained and friable. Also the result of the textural 14 analyses show that the sandstones are sub-rounded, moderately to poorly sorted, strongly coarsely 15 skewed and mesokurtic. The calculated permeability values ranging from 307.18-724.85 Md showed 16 that they possess good permeability. Based on the high permeability values of the sands, the 17 sandstones were inferred as good to excellent potentials for fluid transmission making them good 18 reservoir for hydrocarbon. 19


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The studied area "Imiegba" is located in Etsako East local Government area of Edo state, Benin flank 28 of the southern Anambra Basin, Nigeria and falls within the co ordinates N 07 0 11 1 27.0 11 and E006 0 29 26 1 48 11 (Figure, 1). This study was aimed at the determination of the hydrocarbon potentials as well 30 as the reservoir quality of the outcropping sediments along Imiegba road using Sedimentological, and 31 Organic Geochemical analysis. The objectives of the study were to determine the lithofacies 32 characteristics of the sediments of the study area, the hydrocarbon potential from geochemical 33 parameters such as TOC and SOM, and characterize the reservoir properties (porosity and 34 permeability) from the textural analysis. Abakaliki fold belt. The basin is roughly triangular in shape and covers an area of about 40,000 44 square kilometres with sediment thickness increasing southwards to a maximum thickness of 45 12,000m in the central part of Niger Delta [2]. The basin lies between latitudes 5. 00N  Based on field study, four (4) lithofacies-sandstone, siltstone, claystone and shales were 138 encountered in the studied location (Figure 3). The Percentile Values for the Granulometric Analysis  139  are presented in table 2, and the Graphical Representation of the percentiles are shown in figure 4,  140 while the summary of the textural attributes of the sediments interpreted from the sieve analysis 141 results are given in Table,  usually have low porosity. The finer sediments in poorly sorted sediments typically fill the pore spaces 149 thereby impeding porosity. Similarly, permeability is a secondary property of rocks and depends on its 150 primary properties such as texture, composition and structure. In particular, permeability depends on 151 parameters such as grain size, shape and size of pores (porosity), sorting, packing and/or 152 compaction. 153 From the calculated sorting values (Table 3) which generally indicate moderate to poor sorting, a 154 moderate to poor porosity was inferred for the sediments. However, the sandstones as observed from 155 the outcrops were mainly friable to moderately-consolidated. It is therefore, inferred that porosity-loss 156 in the sediments for this study due to compaction has been minimal. This inference is based on the 157 fact that porosity reduces with compaction. Empirical determination of permeability using [11] formula 158 for the sand facies of the study area shows that the sediments have good permeability (Table 4) Dark coloured siltstone with patches of brown colouration.
Light grey mudstone.
Fine -medium grained, moderately sorted, pink to reddish brown sandstone Fine grain, poorly sorted sandstone. Pink in colour, indicating high quantity of K-Feldspar.
Black shale interbeded with reddish brown sandstone, highly fossilized (contain pelecepods, mollusks and trilobites).  From the above interpretations (Table 3), the sandstone facies are fine to medium-grained, sub-172 angular to sub-rounded, moderately to poorly sorted sands with the sorting values ranging from 0.67 -173 1.92. The sediments are strongly coarse, skewed and ranges from mesokurtic to leptokurtic. It also 174 showed that the porosity and permeability of the sandstone facies of the study outcrop is high, which 175 indicates a good reservoir quality. 176 177 The organic matter richness of source rock is usually determined using the total organic carbon 183 content, which is the total amount of organic material (kerogen) present in the rock, expressed as a 184 percentage by weight (TOC wt.%). 185 The higher the TOC value the better the chance and potential for hydrocarbon generation. According   The transformation ratio (TR) defined by SOM/TOC was used as a maturity index. It is a measure of 210 the transformation of kerogen into hydrocarbon. [15], stated that TR values between 0.002-0.016 211 indicate no hydrocarbon generation. The mean transformation ratio of 0.79±0.1 was far above the 212 threshold value required for hydrocarbon source generation, implying that the organic matter in the 213 shales is mature and in adequate concentration for hydrocarbon generation ( figure 7).

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Based on sedimentological and field evidences, the main lithofacies identified from the study area are 219 sandstone, shale and mudstone. The sandstones are fine to medium-grained and friable. The result 220 of the textural analyses showed that the sandstones are sub-rounded, moderately to poorly sorted, 221 strongly coarsely skewed and mesokurtic. The calculated permeability values ranging from 307.18-222 724.85 Md showed that they possess good permeability. Based on the high permeability values of the 223 sands, the sandstones were inferred as good to excellent potentials for fluid transmission 224 consequently, a good reservoir for hydrocarbon. 225 The TOC values ranges from 0.17-1.42 wt (%) with most of the samples above the threshold of 0.5 226 wt% with SOM greater than 500ppm indicating that the shales have poor to good organic matter 227 quantity and in adequate concentration for petroleum generation. 228 In Conclusion, the result of the various analyses carried out showed that the sandstones are 229 moderately to poorly sorted with high permeability, indicating a good reservoir quality for the 230 sandstone, while the geochemical parameters showed that hydrocarbon generation potential of the 231 shale samples is fair, therefore can source hydrocarbon. 232