Molecular geochemical evaluation of Late Cretaceous sediments from Chad (Bornu) Basin, NE Nigeria: implications for paleodepositional conditions, source input and thermal maturation

The Late Cretaceous Gongila and Fika formation sediments in the Chad (Bornu) Basin, northeastern Nigeria, were analysed to evaluate their paleodepositional conditions and source input as well as to determine their thermal maturity. These were interpreted based on their molecular composition. The composition and distribution of n-alkanes, isoprenoids, and biomarkers indicate a mixture of marine algal/bacterial and land-derived organic matter source input for the Gongila and Fika formation sediments deposited in marine environ- ment. This is indicated by the abundance of short-chain n- alkanes, low CPI and high concentration of tricyclic terpanes, low C24 tetracyclic/C26 tricyclic, low to moderate hopane/ sterane ratios and the relationship between regular sterane compositions. These were probably deposited in environmental conditions which are mainly dysoxic. From the waxiness index and terrigenous/aquatic ratio (TAR), it can be deduced that more terrigenous organic materials were deposited towards the northeastern part of the basin, probably because of their proximity to the Lake Chad. Biomarker maturity parameters such as Ts/(Ts + Tm), C32 22S/(22S + 22R) homohopane, moretane/hopane and 20S/(20S + 20R) and ββ/(ββ + αα) C29 sterane ratios also suggest that the sediments have reached the early to peak stages of hydrocarbon generation.