Hydrocarbon Generation Potential of the Campano-Maastrichtian Dark Mudstone Lithofacies, Benin Flank, South West Anambra Basin, Nigeria

Aj Edegbai,Wo Emofurieta

doi:10.4314/jasem.v24i11.8

Abstract

The dark mudstone lithofacies of Mamu Formation was deposited during the Campano-Maastrichtian flooding episode. It is laterally heterogeneous, and has been subdivided into marsh, bay and central basin subenvironments in order of proximality. Arising from recommendation from a previous study, we evaluated its hydrocarbon generating potential using multidisciplinary tools involving visual kerogen analysis, as well as bulk and isotope geochemistry. Seventy-seven sample materials were taken from 3-outcrop sites at Uzebba, Okpekpe and Imiegba locations, Benin flank, SW Anambra Basin, Nigeria. The results show that bulk of the samples have good organic richness. Kerogen quality is dominantly of gas prone Type III kerogen. However, visual kerogen analysis indicates the presence of an oil prone Type II/III kerogen in the central basin subenvironments. An immature thermal maturity is inferred based on spore colour index (SCI) of less than 6 on the SCI chart (thermal alteration index of <2.5). In addition, we hypothesize that the dark mudstone lithofacies possesses biogenic gas potential based on its organic richness, kerogen quality and thermal maturity. Shale gas prospectivity is further enhanced by the low dip of the Mamu Formation, shallow burial as well as high silica content. Worth mentioning is the proximal marsh mudstone (Uzebba location) with suitable microfabric, very high silica as well as >10m of combined (continuous) outcropping and subcropping thickness Keywords: Kerogen, palynofacies, stable isotope geochemistry, shale gas,

Highlights

Geologic Setting: The Anambra Basin (Fig. 1) marks the next to last stage of the Benue Trough’s tectonostratigraphic evolution in Southern Nigeria (Edegbai et al, 2019a)
Thereafter, palynofacies characterization, which involved identification of the kerogen constituents, grouping of palynological organic matter and point counting a minimum of 300 constituents before normalizing to 100%
Total nitrogen, and stable organic carbon isotope measurements: Seventy-four samples were analyzed for total organic carbon (TOC), and total nitrogen (Nt) at the Stable Isotope Mass Spectrometry Laboratory, University of Florida (UF), USA, as well as at the Organic and Isotope Geochemistry Departmental Laboratory at Kiel University (CAU), Germany (Fig 3)

Summary

Introduction

Geologic Setting: The Anambra Basin (Fig. 1) marks the next to last stage of the Benue Trough’s tectonostratigraphic evolution in Southern Nigeria (Edegbai et al, 2019a). At the Imiegba and Uzebba outcrops (Fig. 3), thicknesses of up to 6 m exist It shows poor sorting and varies from fine mudstone to sandy mudstone with fabric characterized by laminations, which may be planar, lenticular, wavy, or curved. It is weakly to moderately bioturbated, which is a function of proximality and water depth (Edegbai, et al, 2019b). It is devoid of calcite, and has a composition that varies between 100% quartz and clay (mainly kaolinite) (Edegbai, et al, 2019b). Geochemical microfabric, paleontological characterization show that the dark mudstone is laterally heterogeneous, which led to the identification of 3-subenvironments: marsh, bay and central basin (Edegbai, et al, 2019b) in order of proximality

Methods

Results

Conclusion