NASARA‐I WELL, GONGOLA BASIN (UPPER BENUE TROUGH, NIGERIA): SOURCE‐ROCK EVALUATION

Abstract

Well Nasara‐I, one of three exploration wells recently drilled in the Gongola Basin in the Upper Benue Trough (onshore Nigeria), was tested and found to be dry. The well penetrated an entirely Cretaceous succession comprising the Pindiga, Yolde and? Bima Formations, and standard organic geochemical analyses were carried out to assess the source‐rock potential of selected samples. Total organic carbon (TOC) contents were found generally to be very low, with no values exceeding 1.0wt%, and about one‐half of them ranging between 0.50 and 0.87wt%. Hydrogen indices (Hls) correlated against Tmax indicate some gas‐generative potential.However, in the depth interval between 4,710ft and 4,770ft, TOC values of between 52.1 and 55.2 wt% were recorded; these are characteristic of coals. This is the first report of a coal within the Pindiga, Yolde or Bima Formations. Hls were between 564 and 589 mgHC/gTOC and Tmax was 423–428°C. Although hydrogen indices can be misleading in assessing the oil‐generative potential of a coal, values as high as those recorded in Nasara‐I permit oil‐generative capabilities to be inferred.Total ion chromatograms of the saturated hydrocarbon fractions of the coaly samples show some ramping of unresolved complex mixtures attributable to biodegradation. Further biomarker data indicate a dominance of low molecular weight n‐alkanes (C15–C25), pristane/phytane ratios of 0.8 to 1.3, and very high contents of C28 regular steranes. These attributes, together with the very high Hls, indicate that some oils generated from a probably deeper‐seated or laterally‐located (and yet to be identified) lacustrine source rock must have migrated and been adsorbed into the coaly facies, which were later intermittently subjected to anoxic to suboxic biodegradation processes.