AN ASSESSMENT OF STRUCTURAL TRAPS IN RELATION TO STRESS PATTERNS IN CHAD BASIN

Abstract

The interpretation of 2-D seismic reflection time sections from Doili area of the Chad Basin (N.E. Nigeria) indicates two genetic stratigraphic units separated by an unconformity surface. Post-depositional structural deformation of seismic horizons above the unconformity plane is minimal, while that of seismic horizons below it is intense, resulting in a series of faults (normal and reverse) and folds. In this study, the stress conditions that resulted in the formation of these structural trapping elements are examined. This is to assess their capabilities for petroleum entrapment/retention. The normal faults generally have steep gradient planes, originate within the basement rocks and terminate below the unconformity surface. These faults are due mainly to tensional stress directed in opposing directions of northwest and southeast, away from the main axis of Benue Trough-Chad Basin (northeast - southwest). Generally, fault systems due to tensional stresses leave the fault planes as open fractures, hence imperfect seals. The reverse faults most often are closely associated with the folds (anticlines). They resulted from underthrusting of the Cretaceous strata by igneous intrusives that are randomly distributed within the basin. The resultant effects of the late Cretaceous igneous intrusions are arching/folding of the overlying strata, and reverse faults with steep gradient planes. The associated heat could lead to degradation of accumulated petroleum, while the thrusting could create new avenues for remigration. Structural deformation by compressional stress is very minimal in this area. This is evidenced by a general lack of low angle normal faults, strike-slip faults, and well defined regional patterns of the fold systems. Thus the capabilities of these structural elements when examined in relation to hydrocarbon entrapment leave no one in doubt that there is a high risk of drilling dry holes if solely considered in prospects.


KEY WORDS: Tensional stress, normal fault, magmatic intrusion, fold, reverse fault.


Global Jnl Geological Sciences. Vol.2(1) 2004: 51-59