Subsurface oil and surface seep samples from the Niger Delta Nigeria, Tano and Cape Three Points Basins in Ghana, and the Orcadian Basin in Scotland were analysed using an organic geochemistry approach. Source organofacies, oil thermal maturity and the effect of alteration processes were evaluated, with the aim of developing a better understanding of the origin, nature and type of petroleum present. Biomarker analyses suggested that undegraded oils are found with biodegraded oils with a range of levels of biodegradation in the Niger Delta. On the commonly used Peters and Moldowan scale of biodegradation, the samples rank between PM level 0 and 7. The Ghanaian and Scottish oils and seeps showed a range of biodegradation levels between PM level 2 and 7. Biodegradation appeared to be the major control on the composition and physical properties of the oils and seeps from Niger Delta and Ghana while there is evidence that a stage of water washing was important for the Scottish seeps. Key aspects of the biodegradation process were reflected in the variable occurrence of hopanoids and other biomarkers. 25-Norhopanes were detected in all samples biodegraded in subsurface reservoirs but absent in seeps degraded at surface conditions. This was observed for both the Ghanaian and Nigeria samples. The inverse relationship of hopane to 25- norhopanes in the Niger Delta is presented for the first time. This evidence supports the hypothesis that 25-norhopanes are a product of hopane demethylation. Furthermore a pathway for the formation of 25-norhopane from 25-norhopanoic acid by decarboxylation is evidence by nature of the covariation in relative concentration of these compounds. Seeps from Scotland show that only moderate levels of biodegradation (PM 3) have significantly reduced monoaromatic and triaromatic steroids abundances. The most likely cause for this is by water washing at surface conditions. Reservoir temperature and oil charge histories appeared to have had a significant influence on the extent of biodegradation in the Niger Delta. The late migration of oil from deeper hotter subsurface regions to shallow reservoirs accounts for the presence of undegraded oils amidst the biodegraded oils in the Niger Delta.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:558641 |
| Date | January 2012 |
| Creators | Lamorde, Umar Abdullahi |
| Publisher | University of Aberdeen |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://digitool.abdn.ac.uk:80/webclient/DeliveryManager?pid=186760 |
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