Geochemical characterization of some biodegraded oils from the Niger Delta Basin, Nigeria
Description
A secondary process like biodegradation typically changes molecular composition and bulk properties (e.g., API gravity) of crude-oils in the reservoirs. This process poses critical challenges to considerable economic production and commercial value of petroleum in the Niger Delta Basin. A deltaic basin, with known complex structural/tectonic settings, coupled with records of secondary and tertiary migrations, along with meteoritic water incursions into shallow reservoirs. Ten oil samples from onshore-offshore fields were analyzed using gas chromatography (GC) and GC-mass spectrometry (GC-MS) methods, aimed at examining the compositional changes in oils resulting from biodegradation, to fathom the extent of degradation, provide clues for interpreting the evolution and the overall effects on the quality of the oils. GC results indicate variable loss of low molecular weight (LMW) alkanes, besides the presence of unresolved complex mixture (UCM). Saturated, aromatic, resins, and asphaltenes (SARA) compositions and low saturate/aromatic ratio confirmed evidence of biodegradation, which infer low API gravity of oils. Biodegradation levels of the considered oils range from light to moderate, based on Peters and Moldowan scale. This influenced shifts in the primary compositions of oils probably from paraffinic or paraffinic-naphthenic oils to aromatic-naphthenic oils. Total ion chromatograms of well 1 and 2 from Northern depobelt, show the presence of LMW alkanes, co-existing side by side with UCM, suggesting multiple charge episodes. However, GC-MS biomarker fingerprints of the oils lack firm evidence of in-reservoir mixing of biodegraded and non-biodegraded oils. Nevertheless, the possibility of oil mixing cannot be excluded, because biodegradation is progressive and is ongoing. This process dramatically impacted the fluid properties, commercial worth, and economic producibility of the investigated oil accumulations in the basin.
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Materials and methods 1 Sampling method: Ten crude-oil samples were carefully collected from wellheads from different producing fields, across four depobelts (onshore and offshore), Niger Delta. 2 Analytical methods: The considered oils (approximately 30mg) were carefully separated, into the saturated and aromatic hydrocarbons, and polar compounds, using n-hexane (30 ml), n-hexane/dichloromethane (60:40, vol/vol/ 40 ml), and dichloromethane/methanol (50:50, 30 ml) respectively. 3 Gas chromatography (GC) Analysis: GC technique was typically used to analyze the saturated fraction, to obtain n-alkanes and acyclic isoprenoid distributions of the investigated crude-oils. The GC-Flame ionization detector (FID) analysis of each oil sample was performed on Hewlett-Packard 5890 Series II GC with injector at 280°C and FID at 310°C. 1 μl of the crude-oil samples were injected by an HP6890 auto-sampler in splitless mode. The oven temperature ramp program used was from 50°C (held for 2 minutes) to 300°C at 5°C per min., and held at the absolute temperature of 300°C for 20 minutes. Hydrogen was used as the carrier gas, and separation was accomplished on a fused silica capillary column (30m x 0.25mm i.d), glazed with 0.25μm 5% phenylmethyl polysiloxane (HP-5 phase). 4 Gas Chromatography-Mass Spectrometry (GC-MS) Analysis: The saturated and aromatic fractions were further analyzed by GC-MS using single ion monitoring (SIM) mode accompanied by an HP 5890 Series II gas chromatograph, fitted with a 30m x 0.25mm internal dimension fused silica capillary column glazed with DB-5, coupled to an HP5970 Mass Selective Detector (MSD). The oven temperature was carefully held at 40 oC for 5min. and programmed, from 40 oC to 300 oC at 4oC/min. the mass spectrometer was typically operated at an electron energy of 70 eV, an ion source temperature of 250oC, and a separator temperature of 250oC. Terpanes together, with steranes, were mainly identified, employing m/z 191 and 217 fragments, respectively.