Organic Geochemical Characterisation of the Campano-Maastrichtian Sediments in Anambra Basin, SE Nigeria: Implications for Paleodepositional Conditions, Provenance and Petroleum Generation Potential

Abstract

A total of 24 rock samples comprising mainly shales, siltstones and shale heteroliths, collected from the Nkporo, Owelli and Mamu formations of Anambra Basin were subjected to bulk and molecular geochemical analyses (total organic carbon content determination, pyrolysis analysis, bitumen extraction and gas chromatography) to provide further insight on the quantity, quality and thermal maturity of organic matter within the sediments, the source input and paleodepositional conditions of the organic matter and the hydrocarbon generation potential. The total organic carbon (TOC) contents and hydrogen index (HI) of the analysed sediments from the Nkporo, Owelli and Mamu formations range from 0.60 to 2.02 wt.%, 0.25 to 2.75 wt.% and 0.52 to 4.78 wt.%, respectively, while the hydrogen index (HI) range between 07 and 359 mg HC/g TOC for the Nkporo samples, 39 and 408 mg HC/g TOC for the Owelli samples, and 10 and 226 mg HC/g TOC for the Mamu sediments. This indicate that the organic matter has fair to good hydrocarbon generation potential. The kerogen is predominantly Type II-III for the Nkporo shales and some of Owelli samples, while the organic matter in the Mamu Formation and the rest of Owelli samples is mainly Type III and Type IV kerogens. This indicate that majority of the analysed sediments from Nkporo Formation are oil and gas prone, while the others are mainly gas prone. Pyrolysis T_max values in the Nkporo, Owelli and Mamu samples are in the range of 418–437 ˚C, 415–430 ˚C and 424–430 ˚C, respectively, indicating that the sediments are thermally immature to early mature for hydrocarbon generation. This is supported by biomarker maturity parameters. Molecular geochemical data further suggested that the organic matter within the sediments was derived from mixed aquatic algae and land plant source input and was deposited under suboxic paleodepositional conditions.

Keywords: Paleoenvironment; Suboxic; Geochemistry; Organic matter; Nkporo Shale