Geochemical characterization of some biodegraded oils from the Niger Delta Basin, Nigeria

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.