Petrographic and geochemical evidence of hydrocarbon in pore spaces of sandstone of the Yolde Formation, Northern Benue Trough, Nigeria

Abstract

This study presents petrographic and geochemical evidence 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 fragmentogram of the saturate fraction
shows the unresolved complex mixture hump consistent with oils that have undergone bio-
degradation. Sterane maturity parameter (C29 20S/20S + 20R) and the trisnorhopane ther-
mal 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 an important role of dis-
tributing generated hydrocarbons to various play sands in the Northern Benue Trough.