Onocerane and other triterpenoids in Late Cretaceous sediments from the Upper Benue Trough, Nigeria: tectonic and palaeoenvironmental implications

The aliphatic hydrocarbon bitumen fraction has been investigated in Late Cretaceous outcrop samples from the Upper Benue Trough, inundated by the sea only during the maximum mid-Cretaceous transgression. Thermal maturity in nearly all formations is pre-oil window, indicating only limited, mainly Tertiary burial. The Santonian deformation, a significant thermal influence to the south-west, appears to have had little effect here. Onocerane I is the dominant triterpenoid in the Pindiga Formation but is absent elsewhere. Its origin is uncertain but it appears to be associated with deposition in a restricted basin, possibly a brackish lagoon, receiving dominantly terrestrial organic input. A review of possible natural product precursor compounds suggests α-onoceradiene from a fern-like plant was a likely source. Even carbon number predominance of n-alkanes in the range C16 to C22 in some formations is tentatively interpreted to indicate hypersalinity.