Structural and stratigraphic characteristics on distal parts of the outer fold and thrust belt of southern Niger Delta, Nigeria

Abstract

3-D seismic data obtained from Joint Development Zone (JDZ) Licensing Round, 2004, was used to investigate the structural and stratigraphic characteristics on distal parts of the outer fold and thrust belt of southern Niger Delta, Nigeria. Five main seismic reflectivity units (I, II, III, IV and V) were identified and interpreted based on seismic reflection patterns to represent the prograding cyclic sequence of sedimentation in the study area. These units record the episodic gravity spreading that is responsible for thrusting in the toe of the delta. Faults in the central parts of the study area exhibit hard linkage by the gradational propagation of segments into an overlap that developed into soft-linked relay structures. Thrust faults in the study area can be classified into two groups based on the width of the individual thrust faults and their extension and the geometry of the hanging wall folds. These are E–W south-verging thrusts and NE–SW SE-verging thrusts. In the proximal parts of the JDZ, thrust faults are widely spaced with a large structural closure of the hanging wall folds. Faults in the eastern part are complex and typically verge in the opposite direction. In the distal parts of the study area, thrust faults are regularly spaced with hanging wall folds that occasionally created bathymetric expressions on the seabed. The high concentration of pockmarks in the proximal JDZ and the large structural closure of the hanging wall folds characterized by stacked amplitude anomalies and discernable flat spots imply the possible existence of an active hydrocarbon system in the proximal JDZ. Several lenses of sills observed in unit III may impact on hydrocarbon generation and expulsion in the study area. If hydrocarbon migration into traps is occurring after the intrusion of magma, then the sills could act as a barrier to fluid flow, depending on post-emplacement tectonics. Seismic cross sections show that faults in the study area have variable cross-sectional geometries but possess scaling properties typical of thrust faults.