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

摘要: Well Nasara-1, one of three exploration wells recently drilled in the Gongola Basin Upper Benue Trough (onshore Nigeria), was tested and found to be dry. The well penetrated an entirely Cretaceous succession comprising Pindiga, Yolde ?Bima Formations, standard organic geochemical analyses were carried out assess source-rock potential selected samples. Total carbon (TOC) contents generally very low, with no values exceeding 1.0wt%, about one-half them ranging between 0.50 0.87wt%. Hydrogen indices (HIs) correlated against Tmax indicate some gas-generative potential. However, depth interval 4,710ft 4,770ft, TOC 52.1 55.2wt% recorded; these are characteristic coals. This is first report a coal within or Bima Formations. HIs 564 589 mgHC/gTOC 423–428°C. Although hydrogen can misleading assessing oil-generative coal, as high those recorded Nasara-1 permit capabilities inferred. ion chromatograms saturated hydrocarbon fractions coaly samples show ramping unresolved complex mixtures attributable biodegradation. Further biomarker data dominance low molecular weight n-alkanes (C15–C25), pristane/phytane ratios 0.8 1.3, C28 regular steranes. These attributes, together HIs, that oils generated from probably deeper-seated laterally-located (and yet identified) lacustrine source rock must have migrated been adsorbed into facies, which later intermittently subjected anoxic suboxic biodegradation processes.