THE PRINCIPAL SOURCE ROCKS FOR PETROLEUM GENERATION IN THE DAHOMEY BASIN, SOUTHWESTERN NIGERIA
Authors/Creators
- 1. Department of Geology and Mineral Sciences, University of Ilorin, P. M. B. 1515, Ilorin, Nigeria.
Description
The Upper Cretaceous (Maastrichtian) Araromi Shale formation in the Nigeria sector of the Dahomey basin has been investigated for its petroleum generation potential. From three exploratory wells, Araromi, Bode Ashe and Gbekebo in the eastern end (west of Niger Delta), source rock potential has been evaluated for over one hundred (100) drill core and ditch cutting samples. The investigated shallow marine shale facies have Total Organic Carbon (TOC) value range of 0.50-4.78wt% and Hydrogen Index (HI) value range of 1-327mgHC/gTOC with the maceral composition of liptinite (av. 26.0%) and abundance of vitrinite (av. 38.1%) plus inertinite (av. 35.9%) in all the samples investigated. Vitrinite reflectance values vary from 0.51-0.68%Ro. The Tmax values vary from 398oC-437oC and the kerogen Types include type II, II/III, III, and IV in all the samples. The Source Potential (SP) values range from 0.01-14.56kgHC/ton of rock. Biomarker analysis of the shale samples generally reveal a bimodal n-alkane envelope with maxima between (nC16 and nC18) and (nC27 and nC29) suggesting that the source organic matter were derived from a variable mixture of algae and higher plant materials with q relative higher input from marine algae as reveal by the presence of the C30 24-n-propyl cholestane (%C30 sterane range from 0.45 to as high as 5.23%). The presence of organic matter of marine algae (%C27 sterane av. 52% and positive detection of C30 sedimentary n-propyl cholestane av. 3.7%) suggest that the control on the HI as an indication of source rock quality in varying level of organic matter preservation. The source rocks have predominantly gas prone with lesser oil prone organic matter ranging from immature to marginally mature at shallow levels but reaching proven mature levels in the subsurface. Hydrocarbons in the basin would be predominantly gas dominated, generated from deeper Cretaceous source rock (marine shale) in the subsurface levels with greater chances of heavy oils and tars at shallow levels.