Abstract

This study examines eight syndepositional faults and syntectonic sediments in five major fault blocks in western Niger Delta, offshore Nigeria, on three-dimensional seismic data. The initiation, lateral growth and retreat, periods of activity and quiescence, and decay of faulting around these blocks can be ascertained by analyzing a series of time-structure and isopach maps. The study area can be subdivided into three structural zones. The first structural zone is a northwestern zone characterized by a major counterregional growth fault in the deep subsurface. This deep-seated structure is superposed by an array of younger regional growth faults displacing a kilometer-thick sedimentary overburden that accumulated on the former footwall. The second structural zone is a central to eastern zone that seems mostly unaffected by young deltaic faulting. This zone is characterized by the thinnest sedimentary record of the study area. The third structural zone is a southeastern zone that is dominated by a large, listric, backstepping master fault zone associated with a kilometer-scale rollover system. Regional structural and stratigraphic analyses document an apparently strong relationship between syntectonic sedimentation and syndepositional fault activity in that phases of significant fault activity, lateral fault growth, and fault migration concur with major depositional phases; in turn, areas and intervals characterized by the least sediment accumulation also record the lowest fault activity. However, one particularity of the studied system is that it underwent at least one period of seaward fault progression that coincided with a backstepping of faulting on the landward side. Although the forward stepping of faulting near the delta front can be interpreted as the consequence of the progressive loading during delta progradation, the contemporaneous backstepping of faulting further inboard likely reflects the sustained lateral growth of mature deltaic faults into previously undeformed proximal parts of the depocenter. The results of this study, thus, document that although an apparent correlation with the superimposed depositional system exists on a regional scale, inboard deltaic faults may persist to grow irrespective of sedimentary loading. The recognition of such fault trends is particularly important for estimating the influence of late-stage fault movement on hydrocarbon migration or the discovery of subtle fault-controlled hanging-wall reservoirs.

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