Inorganic Geochemical Evaluation and Aspects of Rock Eval Pyrolysis of Eocene to Recent Sediments of Soso and Kay-1 Wells, Western Niger Delta, Nigeria

Abstract

The Niger Delta Basin situated in the Gulf or Guinea, has been penetrated by Soso and Kay 1 well in the western axis. A combined element and organic geochemistry was done to define source rock potential, organic Matter Type, conditions at deposition and their relationship with sediment provenance, Paleoenvironment and tectonic settings.Core samples were collected from ‘Soso and Kay 1 Wells’ in western Niger Delta (Nigeria) and were subjected to lithological description so as to establish their lithologic characteristics and profile. The core samples, comprising shales, silty-sands, silty-shales, sandy-shale, coal samples and silt sample, were analysed for major, trace and rare earth element using ICP-MS and selected shale samples were also subjected to Rock Eval Pyrolysis and TOC. The results for the major oxides composition revealed that the SiO₂ ranges from 57.4-89.04 wt. %, with an average of 72.5wt. %. The average concentration of TiO₂, Al₂O₃, Fe₂O₃ and MgO are 0.94 wt. %, 10.55wt.%, 5.38wt.% and 0.51wt.% respectively. The Discriminant Function Diagram showed that the sediments were deposited in an Active Continental Margin (ACM). The plots of Log (K₂O/Na₂O) against SiO₂ further confirmed deposition in ACM setting. Further Discriminant Function plots showed a mixed provenance of Intermediate Igneous Provenance and Mafic Field, while the lignite bearing sediments plots within the Quartzo-sedimentary province. The A-K-F ternary diagram indicates deposition in a transitional environment. The Chemical Index of Alteration (CIA) values ranges from 25.51 – 91.9 % while the Chemical Index of Weathering (CIW) ranges from 38.36 – 98.4 %. The Plagioclase Index of Alteration (PIA) ranges from 1.38 - 98.2% while the Ruxton Ratio (RR) ranges from 2.9 – 39 %. These values indicate that the samples had suffered low to moderate chemical alterations. The Chondrite normalised REE pattern showed a high LREE, a flat HREE and a negative Eu anomaly averages 0.89. This confirmed an ACM setting as it is diagnostic of a mixed provenance. Trace element proxies; V/Ni (2.17 and 2.68), V/Cr (0.57 and 1.01), Ni/Co (2.1 and 2.66) and V/ (V+Ni) (0.68 and 0.72) for Soso and Kay 1 wells respectively indicates an oxic to dysoxic conditions at deposition.The results revealed that the sediments were derived from a mixed provenance and were deposited within the ACM in a transitional environment amidst oxic to dysoxic depositional conditions yielding a Type II/III and Type III organic matter.

Keywords: Provenance, Tectonic Settings, Discriminant Function diagram.