Assessment of Permeability Distribution and Vertical Communication Between Stacked Sands in the Distal Lobe Facies of OML 78, Rivers State, South South Nigeria

The stacked sand reservoirs of the distal lobe facies in the central Niger Delta contain significant hydrocarbon resources, but their development efficiency depends critically on understanding the permeability distribution within each sand body and the degree of vertical hydraulic communication between adjacent reservoir intervals. This study assesses permeability distribution and vertical communication characteristics in stacked sand reservoirs of OML 78, Rivers State. Core plug permeability measurements from six wells were integrated with Nuclear Magnetic Resonance log-derived permeability profiles to construct high-resolution permeability models for three target reservoir intervals. Pressure transient data from twenty-three well tests were analysed to estimate the degree of vertical cross-flow between adjacent sands. Semivariogram analysis of log-derived permeability sequences revealed that horizontal correlation lengths range from 350 to 900 metres, while vertical communication is confirmed between the upper and middle reservoir intervals based on consistent pressure equilibration rates observed during extended build-up tests. The lower reservoir interval appears hydraulically isolated from the upper intervals, as supported by independent hydrocarbon contact levels and distinctly different produced water salinity profiles. These findings have direct implications for well completion strategy, supporting a selective perforation approach that avoids commingling the isolated lower interval with the communicating upper sands to preserve reservoir management flexibility. Keywords: permeability distribution, stacked sands, vertical communication, OML 78, Niger Delta.