Geochemical characterisation of the Turonian-Maastrichtian bituminous sands in Dahomey Basin, Southwestern Nigeria

Abstract

The Turonian-Maastrichtian bituminous sands resource in the Dahomey (Benin) Basin, Southwestern Nigeria, is a potentially economically viable hydrocarbon reserve, which has recently attracted renewed attention. An integrated study involving the use of elemental composition, and geochemical analyses (gas chromatography flame ionisation detector (GC-FID), Fourier transform infrared (FTIR), and gas chromatography mass spectrometry (GC-MS)) were carried out to provide an improved understanding of this resource using geochemical signatures and evaluate the n-alkane distribution of bituminous sands to determine its aliphatic hydrocarbon distribution and the prevailing depositional conditions. Elemental composition shows the dominance of carbon ( Av. 79.8%). The atomic ratios of H/C and O/C, respectively, range from 0.05 to 0.11, and 0.04 to 0.08. SiO2 is the dominant oxide, with concentrations ranging between 73.15 and 80.50 wt. %, while Zn is the dominant trace element. Twenty-three aliphatic hydrocarbons: C10, C13, C15, C17–C37, were identified on GC-FID, with distribution showing predominance of long-chain n-aliphatics, with a Carbon Preference Index (CPI) value range of 1.01 to 1.02. A bimodal n-alkane fingerprint envelope is visible on GC, with maxima between nC16– nC18 and nC27, indicating varied contributions from both terrigenous and non-terrigenous (possibly marine algae) organic matter. Despite the biodegradation of n-alkanes, GC-MS chromatograms (m/z 256) show evidence of bicyclic sesquiterpenes (BSs) in the bituminous sands investigated. Infrared (IR) spectra show the dominance of the hydroxyl functional group, while 2922.2cm-1 and 2855.1cm−1 are common in all the samples, revealing the presence of residual organics. This study revealed that the bituminous sands are highly waxy in nature and derived from higher plants of both marine and terrestrial source input with more influence from land-dwelling biogenic sources under oxic conditions.