Re-Evaluation of the Petroleum Potential of Bornu Basin, Nigeria

Abstract

Bornu Basin is the southwestern Chad Basin in N.E. Nigeria. It has both structural and stratigraphic similarities with the proven petroliferous portions of the Chad Basin in Niger and Chad Republics. Its shales meet the minimum TOC requirement for oil generation. In spite of these favourable oil accumulation factors, only non-commercial gas was found in two out of twenty–three wells drilled in the Bornu Basin. The petroleum potential of the basin is re-evaluated by using the Lopatin method to estimate the shales’ organic maturation level and thereby determine the shales’ petroleum generation status. The highest estimated cumulative Time Temperature Index and Vitrinite reflection values attained by Gongilla are 4.26 and 0.56 % respectively. The corresponding values for Fika Shale are 0.28 and 0.3 %. These values are in the thermal immaturity interval. They reflect that the shales neither witnessed catagenesis nor attained the threshold of intense hydrocarbon generation. The highest paleotemperature experienced by Fika Shale is about 48 ℃, which is much lower than the minimum requirements for entering into the oil window. These results agree with interpretations of earlier workers based on n-alkane fraction of shales, low values of SOM, SHC and SOM/TOC. Oil found in Bima Formation in other portions of the Chad Basin, must have been generated in the underlying Pre-Albian shales. By virtues of their age and stratigraphic position, these Pre-Albian shales must have witnessed catagenesis. Pre-Albian shales may be present in the deeper portion of Bornu Basin. These portions can be delineated by combined analysis of aeromagnetic and aerogravity maps. The non-commercial gas found appears to be biogenically generated by the action of anaerobic bacteria on organic rich Fika Shale.