Hydrocarbon Geochemical Characteristics and Depositional Environment of Tertiary Tar Sand from Ondo, SW Nigeria.

Abstract

In the Dahomey Basin, western Nigeria, tar sand occurs in alternation with sand, sand, shale, carbonate, silstone and clays in transgressive deposits of Cretaceous-Tertiary age. The thickness of tar sand seams drilled during the study varies between 10 and 15 m and it even exposed to the surface. Organic geochemical characteristics of the tar sand were evaluated using TOC, pyrolysis and GC analyses. The Ondo tar sand is characterized by very high TOC contents (5.53-74.40 wt%), Hydrogen Index (HI = 519-796 mg HC/g TOC) and very low Oxygen Index (OI = 1-11 mg CO2/g TOC) values. Organic matter in the studied tar sand is of Type II kerogen and shows S2/S3 ratio values between 47.11 and 301.63 and low Tmax (428-439 °C ) and PI (0.18-0.36) values. Gas chromatographic analyses of the bitumen extract show bimodal and unimodal n-alkane distributions. CPI varies from 0.24 to 1.00 in IREN and 0.16 to 4.13 in LODA showing that LODA attended higher level of maturation as compared to IREN. Furthermore, indicated that LODA organic matter was deposited in a deeper environment than IREN or closer to heat source. Pr/Ph ratio (0.78-3.31; 0.87-2.33), Long/Short ratio (0.72-0.77; 1.00-1.60), OEP (0.02-0.68; 0.33-0.42), Pr/n-C17 (0.84-2.25; 0.19-1.61), Ph/n-C18 (0.04-3.73; 1.00-1.67) and C31/(C31+C29) (1.00-3.40; 1.20-3.60) revealed contributions of marine to terrestrially source of organic matter and dysoxic to marine as depositional environment.