Biomarker Fingerprinting of Crude Oils from Niger Delta Depobelts, Nigeria.
Description
Twenty-eight oil samples from four depobelts in the Niger Delta Basin were investigated utilizing gas chromatography and gas chromatography-mass spectrometry to determine their source, paleoenvironment, thermal maturation, and alteration mechanisms utilizing biomarkers. The oils generally have an oleanane/hopane ratio > 0.2, a lower homohopane index, and a predominance of C29 ααα steranes and C30 hopanes, signaling contributions from both marine and terrestrial sources. The substantial concentration of C27 steranes indicates marine algal input, while the prevalence of C29 steranes supports terrigenous organic materials originating from higher plants. Variability in sterane concentrations in the oils reflects complex origins and the dynamic interplay of marine and terrigenous processes within the deltaic-transitional settings. The thermal maturation ratios of sterane biomarkers suggest the crude oils belong to the early-to-peak oil range. Conversely, the hopane result suggests the oils are at peak maturity. Applying the criteria of oleanane, sterane/hopane, and isoprenoids/n-alkane, the oil sets were classified into 2 sub-families, ‘A and B.’ ‘A’ oils are characterized by elevated oleanane/hopane ratios with lower sterane/hopane ratios, sourced from clay-rich rocks in an oxic environment. Family B includes oils from mixed organic matter sources with low oleanane/hopane ratios and low to moderate sterane/hopane ratios. The established oil families are independent of thermal maturities, as the same source rock can generate different thermal maturity fluids based on burial depth and basin evolution. Biodegradations and/or evaporative fractionations were prominent in a few samples, consequence of the depletion of light-end n-alkanes and then the relative enrichment of aromatics in the saturate-aromatic profiles of oils, thereby elevating the isoprenoids/n-alkane ratios. The oils have a low homohopane index, a substantial presence of C27 steranes, which reveals an important contribution from marine algae, and a major presence of C29 steranes, which indicates a significant input from terrestrial organic matter from higher plants.
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Field Sampling Direct field-sample collection of twenty-eight crude oil specimens from various wellheads was carried out, comprising nine active fields throughout four depobelts that span onshore-offshore in the Niger Delta. Analytical methods The oils were separated into saturates, aromatics, resins, and asphaltenes (SARA) components using the Iatroscan Thin Liquid Chromatography-Flame Ionization Detector (TLC-FID). The various stages of techniques were performed by adding 30 ml of a 50:50 (v/v) blend of dichloromethane and methanol, 40 ml of a 90:10 (v/v) blend consisting of n-hexane and dichloromethane, and 20 ml of n-hexane to separate the polar, aromatic, and saturated components, respectively. Gas chromatography The normal and acyclic isoprenoid alkanes used in the gas chromatography-mass spectrometry (GC-MS) investigation were obtained by evaluating both aliphatic and aromatic components using gas chromatography. The GC-FID (Flame Ionization Detector) investigation was conducted using a Hewlett-Packard 5890 Series II GC. A splitless HP6890 autosampler was used to inject the sample (1 μl). FID and injector temperatures were configured to 310 °C and 280 °C respectively. This oven temperature ramp scheme ran for 74 minutes, starting at 50 °C and maintained for a couple of minutes. The amount of heat supply was progressively raised by 5 °C per minute until it reached 300 °C and was kept stable for a total duration of twenty minutes. Meanwhile, a 30-meter fused silica capillary column with an interior diameter of 1/4 millimeters and a one-quarter micrometer-thick covering of 5% phenylmethylpolysiloxane (HP-5 phase) was utilized in the extraction process. Hydrogen gas was used as the collecting agent, which had a split frequency of 30 milliliters per minute, an extraction speed of one milliliter per minute, and an input pressure of 50 kPa. Gas Chromatography-Mass Spectrometry Analysis Gas chromatography-mass spectrometry (GC-MS), operated in the single-ion monitoring technique, was utilized to determine the biomarker concentrations of the saturated and aromatic extracts of the oils. One HP5970 mass selective detector (MSD) and a 30 m x 0.25 mm internal diameter fused silica capillary column, internally lined with DB-5, were utilized in conjunction with the HP 5890 Series II gas chromatograph. The oven's temperature setting was intended to advance from 40 °C to 300 °C at a steady pace of 4 °C per minute, with a 5-minute pause at the target temperature. The mass spectrometer heating system was operated at 250 °C for the ion source, 250 °C for the phase separation, and 70 eV for the electron energy. Consequently, m/z 191 and 217 were used to identify terpanes and steranes, respectively.
Institutions
- Houston Community College System
- Newcastle University
- University of Ibadan Department of Geology