Preliminary geochemical investigation of some crude oils from the Niger Delta Basin Nigeria
Description
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.
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Materials and methods: 1. Sampling method: Twenty-eight dead oil samples retrieved from 9 producing oilfields from four depobelts (i.e., onshore and offshore depobelts) across the Niger Delta were collected for the present study. 2. Iatroscan thin- layer chromatography-flame ionization detector (TLC-FID) analysis: The relative composition of saturates, aromatics, resins, and asphaltenes (SARA) of the oil samples was determined using the Iatroscan TLC-FID analyzer. 3µl aliquot of each diluted oil sample was used. Each sample was analyzed in duplicate to estimate the repeatability of the process. Blanks were additionally analyzed on all the racks to assess any contamination. The bottom of the rack of each rod was immersed in a tank comprising 150 ml of n-hexane as a mobile phase until the applied solvent moved a conspicuous mark of ~95%. The rods were then left to dry for 3 minutes, later the rods were immersed in a tank containing 150 ml of toluene, which was allowed to pass up the rods until it reached the 60% mark. All the rods were later removed from the tank and left to dry normally for 6 minutes, before being immersed in the last container containing 150 ml of a mixture of Dichloromethane/Methanol (93:7, v/v), and the solvent front was allowed to get to the 30% mark before removing the rods and then analyzed on the TLC-FID analyzer. 3. Gas chromatography-flame ionization (GC-FID) analysis: Gas chromatographic (GC) method was carefully applied to analyze the saturated hydrocarbon fraction, to obtain n-alkanes and acyclic isoprenoid distributions of the considered crude oils. The GC-Flame ionization detector (FID) analysis of each oil sample was performed on the Buck Scientific Gas Chromatograph (BSGC) model 910, fitted to FID, and a Restek 30m MXT-1 columns. 1 μl of each oil sample was injected by an HP6890 auto-sampler in splitless mode. The injector and detection temperature was programmed to 250oC and 280oC, respectively, with helium used as the carrier gas.