Anthony Joseph Chinenyeze MADU
Abstract: The Eastern Niger Delta has unique structural faults mapped as moderate relief East-West trending fourway dip closed anticline at both shallow and deeper levels downthrown to the first major E-W trending structure of IHEOMA structure. The OKWUKWE-1 well crossed a minor fault into shale and penetrated 350ft of the shale before problems of abnormal increase of pressures persisted and led to abandonment. The large thickness represented high rate of sediment supply impaired with low permeability that prevented the sediment from dewatering at the rate quick enough to allow hydrostatic pressure equilibrium hence, overpressure. Heavy gases appeared from the depths of 6760ft, which showed up as indicators of abnormally pressured zone. The Eastern Niger Delta has a trend of deposition of thick sediments with litho-stratigraphic units of Benin Formation, Agbada Formation and the underlying Akata Shale, which originated respectively from paleocontinental, transitional and marine environments composed of shales and marine clays. Some wells in this part of Eastern Niger Delta have pore pressure regime that resulted into overpressure conditions requiring increase of circulating mud weights. Some wells manifested sediments-transition and changes in Agbada Formation sequence of sand and shale transforming to continuous columns of shale. This is the measure of the degree to which pore fluid pressure significantly exceeded the predicted from the normal compaction of sediments with depth. Abnormal gas composition in the shale columns points to highly over-pressured zones.
Keywords: Eastern part of Niger Delta, Overpressured zones, Pore pressure, undercompaction of shales, Agbada Formation, Akatashales and marine clays