Sedimentological aspects of oil fields in the Niger Delta

Abstract

One of the most conspicuous geological features of the Niger Delta is its growth fault pattern. Almost all the oil reserves are contained in rollover structures which are associated with growth faults. These structural features are thought to have been formed by the force of gravity acting on a thick body of sediments supplied mainly by the Niger River. These sediments form a typical deltaic offlap sequence consisting of a wedge of continental sands grading downwards into marine clayey sediments which, at a greater depth, are in a state of undercompaction. Stratigraphically most of the known oil accumulations occur in the paratic sequence, i.e. the transition zone between the continental and fully marine sediments. The paralic sequence consists of a large number of sedimentary offlap cycles, each cycle starting with a marine clay, generally less than 150 feet thick and changing upwards progressively into proximal fluviomarine interlaminated silt, sand and clay which are usually followed by various types of more sandy barrier-bar and coastal-plain deposits. The cycles are terminated by transgressions which erode away part of the offlap sequence and which are generally represented by a thin, very fossiliferous gravelly sand. In this paper the relationship between sedimentation and growth faults is discussed. Examples are given of depositional cycles and of the associated types of reservoir rock. Attention is given to the influence of the depositional environment on the petrophysical properties of the rock, reservoir continuity and reservoir imhomogeneities.