Organic Geochemical Evaluation and Depositional Environment of Oil Sand from Mbano, SE Nigeria

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In this study, organic geochemical characteristics and depositional environment of the Cretaceous-Tertiary aged oil sand deposits in Mbano Southeast Nigeria have been examined. Oil sands in all the studied areas are typically characterized by high hydrogen index and low oxygen index values. The organic richness of the oil sand, the quality, source and thermal maturity of organic matter discovered at Umuezeala Nsu (MBA) and Umualumaku (MBB) in Mbano were determined on 13 oil sand samples. The results generally suggest relatively high Total organic matter content (TOC) for MBA and MBB oil sand samples ranging from 17.07 to 61.31wt% with a mean of 44.43 wt% for MBA and from 29.55 to 52.94 wt% with a mean of 36.05 wt% for MBB respectively. This serves as a link in determining the quality of source rocks. The free hydrocarbon (S1) versus TOC plot was used to differentiate between allochthonous and autochthonous hydrocarbon. Tmax ranges from 412-431 o C with mean value of 421.8 o C. Hydrogen index (HI) versus Tmax plot confirms that a significant amount of the organic matter is of lacustrine with a mixed marine and terrestrial origin. The HI of MBA and MBB ranges from 771 to 869 mgHCg -1 TOC and 689 to 89 0 mgHCg -1 TOC respectively indicating the presence of type I kerogen. The Tmax versus Production index (PI) shows that MBA and MBB organic matter as immature but the Production index (PI) ranges from 0.16 and 0.29 indicating that the organic matters in MBA and MBB have reached the production stage. The GC-MS results derived by CPI varies from 0.01 to 3.80 in MBA and 0.14 to 0.90 in MBB showing that MBA attended higher level of maturation as compared to MBB. Furthermore, indicated that MBA organic matter was deposited in a deeper environment than MBB or closer to heat source. , Long/Short ratio (0.17-1.28; 0.10-0.66), , Ph/n-C 18 (0.04-3.50; 0.43-1.72) and C 31 /(C 31 +C 29 ) (1.00-3.60) revealed the organic matter depositional environment as dysoxic to marine environment.