Abstract

This study presents petrographic and geochemical evidences of hydrocarbon occurring in pores of sandstone of the Cretaceous Yolde Formation. The studied sample was observed to be poorly sorted, with particles ranging in size from coarse to very fine-grained sand. Detrital grains in the sample include quartz and feldspar (K-feldspar), while clay minerals are the cements. Oil extracted from sandstone of the Yolde Formation is enriched in the polar fractions (asphaltene + resins) and depleted in saturates and aromatics, which is consistent with biodegraded oils. Total ion current (TIC) fragmentogram of the saturate fraction shows unresolved complex mixture (UCM) hump consistent with oils that have undergone biodegradation. Sterane maturity parameter (C29 20S/20S + 20R) and the trisnorhopane thermal indicator (Ts/Tm) of 0.42 and 0.72 respectively indicate thermal maturities equivalent to early-peak oil generation for the studied oils. Stratigraphic position of the studied sample of the Yolde Formation in relation to the mid-Santonian unconformity trap suggests that the oil was generated and emplaced into sandstones of the Yolde Formation after the mid-Santonian invasion, thermotectonic sagging/subsidence and deposition of the Campanian Fika Shales and possibly other younger sediments. The Fika Shale provided the seal cover, while the mid-Santonian plane of unconformity acted as a migration pathway. Multiple fracture networks and other planes of unconformities in the Northern Benue Trough also provided both vertical and horizontal migration routes and played important role of distributing generated hydrocarbons to various play sands in the Northern Benue Trough.

Key words: Yolde Formation; Northern Benue Trough; Petroleum Exploration; Biodegradation; Unresolved complex mixture UCM.