Preliminary geochemical investigation of some crude oils from the Niger Delta Basin Nigeria

Geochemical studies on twenty-eight crude-oil samples from nine oilfields, onshore-offshore parts of Niger Delta Basin, were performed to classify the oils into various classes, determine the bulk compositions of oils, organic matter inputs, source depositional environment, thermal maturation, and in-reservoir processes, for petroleum system prediction. The oils were fractionated into individual bulk compositions of saturates, aromatics, resins, and asphaltenes (SARA), using the Iatroscan thin-layer chromatography-flame ionization detector (FID) technique. Column chromatography was utilized to separate the saturates, and were further analyzed with gas chromatography-FID for n-alkane and acyclic isoprenoid distributions. SARA results, classified the oils into: paraffinic (>72%Hc, sat/arom: 2.61-3.68); paraffinic-naphthenic (63-70%Hc, sat/arom:1.77-2.31) and aromatic-naphthenic/aromatic-intermediate (<60%Hc, sat/arom: <1.44) oils, generated from source rocks of terrigenous-deltaic origin. Pristane/phytane and n-alkane/isoprenoid ratios delineated the oils into terrigenous (Type III kerogen) and mixed (terrigenous-marine) Type II/III kerogen organic matter source, deposited in oxidizing to transitional environment conditions. Thermal maturity parameters suggest the oil originated from source rocks of early to peak oil window maturity. Additionally, n-alkane/isoprenoid and saturate/aromatic ratios established evidence of biodegradation in a few samples across the depobelts. However, SARA compositions, n-alkane, and isoprenoid distributions of the oils critically permit preliminary assessment study, consequently allows a first-hand geochemical interpretation of the Niger Delta petroleum systems.