Counter-regional normal faults in shale-dominated deltas: Origin, mechanism and evolution

摘要

The Niger and Brunei Deltas are two of the major prolific petroleum deltas of the world. As a result, large datasets of 2D and 3D seismic are available. However, some of the structures remain poorly understood because they are poorly imaged and a "shale diapir interpretation" has often prevailed. These two deltas are, in places, driven by particular landward structures described as "counter-regional normal faults" (CRNF). These faults initiated on the apex of early folds, as the shelf-break propagated seaward. During a first stage of deformation, CRNF started as diffuse faults rather than localised single faults. Later, as the sediment load increased in the area affected by the CRNF, a major landward dipping fault developed, controlling the delta-front emplacement. Finally, the fault was choked by the sedimentary system and another fault (landward or seaward) developed. The description of the pre-existing fold structures controlling the development of CRNF allows a reassessment of the structural evolution of shale gravity-driven tectonics. It is agreed that, in many cases, the existence of mud diapirs and ridges may be discarded.