Connectivity estimation between turbiditic channels and overbank deposits from the modelling of an outcrop analogue (Pab Formation, Maastrichtian, Pakistan)

Abstract

Abstract The facies associations of the overflow deposits associated with turbidite channels were studied in the outcrops of the Maastrichtian Pab Sandstone, in SW Pakistan. In this area, a basin-floor fan was preserved from the platform to the deep basin setting. In the mid-fan setting, a channel complex crops out in three dimensions, and consists of a dozen channels and their overflow deposits, including levees, crevasse lobes and spill-over lobes. The overbank deposits can be directly in contact with the channel-fill, but, in many cases, matrix-supported debris-flow deposits made lenses close to the channel base. Heterolithic drapes of thin-bedded turbidite deposits were also preserved along the channel margins. A three-dimensional (3D) static model describing the heterogeneity distribution within the turbiditic channel complex was compiled using both stochastic and deterministic approaches in a site where two channels were laterally connected by overbank deposits. Petro-acoustic properties derived from a subsurface database were then assigned to the facies to perform seismic simulations. The synthetic seismic simulations showed that the channel base can easily be misinterpreted compared to the geological model. Dynamic modelling, such as well test and streamline simulations, was also performed using the model to assess the transition between the channel and overbank deposits from a dynamic point of view. As a result of the streamline simulations, the overbank deposits connecting the channel homogenized the pressure regime in the reservoir. However, the sweeping efficiency of water injection can be affected by the heterogeneity distribution along the channel margins. A significant volume of oil could also be by-passed because of the occurrence of early water breakthrough through the spill-over lobes, or because the flow slowed down when it reached the heterogeneity along the channel margin.