Lithostratigraphy and tectonic evolution of the north-eastern Bornu basin, from integrated surface and subsurface interpretation
Student thesis: Doctoral Thesis
Surface and subsurface structural lineaments are important in understanding tectonic movements and oil and gas trapping architecture in sedimentary basins. However, good exposures of these geological features are always lacking in semi-arid regions with extensive surficial sediment cover. An integrated multisource data analysis is applied to constrain the tectonic setting, structure and lithostratigraphy of the north-eastern Bornu basin, which were previously poorly understood. Detailed geological analyses are based on integration and geospatial correlations of surface datasets, including optical, radar and DEM Earth Observation imageries with subsurface datasets, including, seismic, well log, gravity and aeromagnetic. The Bornu basin, situated in the West African Rift System (WARS), overlain by Quaternary - Recent sediments is found herein to contain > 5000 m thick of Cretaceous rocks overlying the Precambrian basement migmatite-gneiss complex. New insights into the basin evolution model from pre-rift to post-rift tectonic settings are developed using deductions from the integrated studies herein. This study outlines additional tectonic regimes that were not identified in the previous tectonic model for the basin including: (1) Barremian (120 Ma), (2) Late Aptian – Early Albian (101 Ma), Late Santonian (84 Ma) and (4) Quaternary – Recent. Furthermore, the research identified that basement control, pre-existing basement lineaments, transfer faulting and upward fault propagation have all influenced the tectonic evolution of the basin. Lithology and stratigraphy of the subsurface formations are mapped using Combined Log Pattern (CLP) method adapted herein involving Gamma Ray, Resistivity, Bulk Density and Sonic logs. Four subsurface stratigraphic formations including Bima, Gongila, Fika and Chad Formations are mapped from validated seismic and well log stratigraphy in the north-eastern Bornu basin. Predictive Spectral Lithological (PSL) units mapped represent the various surface Quaternary-Recent deposits in the area using improved spectral mapping approach involving combined band combination, band ratio and Spectral Angle Mapper (SAM) supervised classification. Geospatial correlations and interpretations of lineaments indicate that the north-eastern Bornu basin is controlled by two predominant lineament systems trending NE-SW and NW-SE. The geospatial correlation method visually illustrates linkages between subsurface tectonic lineaments and lithostratigraphy with the surface structures. Accordingly, the geospatial correlation has established the relationship between the palaeoshoreline and palaeodrainage systems with the subsurface structural setting of the basin. Relationships of the main structural systems in the north eastern Bornu basin may have developed potential oil and gas trapping systems associated with the regional pattern. Two potential petroleum systems from pre-rift to post rift tectonic regimes derived from the tectonic evolution model of the basin developed herein are presented. The Tertiary non-deposition in the north-eastern Bornu basin is determined to be due to a pinch out of the Tertiary Kerri-Kerri Formation that extends from the Benue trough and terminated near the Maiduguri area.
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