Oil residue in exhumed fractured Precambrian basement rock of the Northern Benue Trough, Nigeria: Implication for oil exploration in the Northern Benue Trough By

This study presents for the first-time field, petrographic and geochemical evidence of oil residue in exhumed fractured Precambrian basement rocks of the Northern Benue Trough. The studied oil residues which are brittle solids, are believed to have been emplaced into fractures of the basement rocks as conventional light oils, but have been degraded. Pyrite crystals are closely associated with the fractured basement reservoir. These pyrite crystals are inferred to have a biogenic origin attributed to microbial activity as part of the biodegradation processes. Total ion current (TIC) fragmentogram of the saturate fractions of the studied oil residue shows progressive depletion of chromatographically resolved hydrocarbons relative to the unresolved hydrocarbon mixture, forming an unresolved complex mixture (UCM) hump consistent with oils that have undergone biodegradation. Pristane (Pr) and Phytane (Ph) are present on the m/z 85 fragmentogram in abundance lower than those of the adjacent n-alkanes, resulting in Pr/nC17, Ph/nC18 and Pr/Ph ratios < 1.0, suggesting suboxic to anoxic depositional environment for the source of the oil. Sterane maturity parameters, ααα C2720S/(20S+20R), ααα C2820S/(20S+20R), and ααα C2920S/(20S+20R), are 0.50, 0.47, and 0.35, respectively, for the studied oil residue, consistent with oils derived from source rocks that have thermal maturities equivalent to early and peak oil generation. The trisnorhopane thermal indicators (Ts:Tm ratio) of 1.08, and 22S/(22R+22S) C32 homohopane isomerization ratio of 0.61 agrees with the sterane maturity parameters of the oil residue which implies that the source rock for the oil residue is within the oil window. Sterane distribution for the oil residue shows strong resemblance to the strerane distribution of extracts of Bima Shales, implying that the oil was sourced from shales of the Bima Formation. The basement rocks occur below a regional plane of unconformity which is inferred to have acted as an important migration pathway for the oil residue. Occurrence of oil residue in fractures of the exhumed Precambrian basement rocks of the Northern Benue Trough implies that the fractured Precambrian basement rocks are important petroleum reservoir rocks in the region.