Structural Style of Normal Fault Inversion and Implications for the Hydrocarbon Potential of Rift Basins: Insights From Bornu Basin, Onshore Northeast Nigeria

Abstract

Exploration risk in most inverted rift basins is related to a poor understanding of the structural style and distribution of inversion structures, which are controlled by the presence and orientation of pre-existing structures in addition to the magnitude of shortening strain. Consequently, further studies of the dynamics of the normal fault reverse reactivation during inversion, and its impact on petroleum system development are needed. Here, we use 2D and 3D seismic reflection, geochemical, and borehole data from the frontier Bornu Basin, intra-continental rift basin located in NE Nigeria, to constrain the structural style and evolution of inversion structures. We then undertook basin modelling to assess how intra-plate shortening impacted the development of related petroleum systems. Our data indicate that moderately dipping (45° – 60°), NE-SW striking normal faults occur within and form the margins to the major NE trending rift basin. Thickening of the Bima Formation towards these faults indicates Early Aptian to Cenomanian rifting. The major fault bounding the eastern margin of the basin formed in response to the growth and linkage of isolated segments and displays evidence for reverse reactivation during basin shortening. This fault strikes NW-SE and at its northern end it has a low throw (c.690 ms) and dip (40°), and the shortening magnitude is high (c.45 m). In contrast, at the southern end of the fault its dip (55°) and throw (c.900 ms) are high, but the shortening magnitude is low (c.18 m). We interpret that this along-strike change in shortening magnitude is caused by changes in the fault geometry and orientation with respect to the compression direction. Seismic-stratigraphic observations indicate inversion, which was responsible for the formation of the main hydrocarbon traps, occurred in the Turonian and Campanian. Petroleum systems modelling indicates that peak hydrocarbon generation from lacustrine shales of the Bima Formation occurred from the Maastrichtian to Neogene, post-dating the main phase of trap formation. Our study indicates that the magnitude of hanging wall folding, which by implication determines the size of the inversion-related traps, can vary along fault-strike due to changes in fault geometry and strike. Furthermore, petroleum systems modelling shows hydrocarbon maturation and expulsion in the basin occur after formation of inversion-related trap, making these structures attractive exploration targets in the Bornu Basin.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015