Sandstone reservoir zonation using conventional core data: A case study of lower cretaceous sandstones, Orange Basin, South Africa

摘要

This study outlines the different methodologies used to identify and classify different rock units in the sandstone reservoirs of the Orange Basin in South Africa into flow units. The flow units are defined on the basis of conventional core porosity and the permeability of the reservoir occurring in the fluvial and shallow marine depositional environments within the lower Cretaceous period. A total of six facies were identified from the core data, and they were corroborated with gamma ray log data. Fades A1, A2, A3, and A4 are the main reservoir units with generally fine-to medium-grained sands, which dominantly comprise quartz grains. Fades A5 mainly comprises silts with lenses of fine-grained sands, while Fades A6 is claystone laminated with minor burrows filled with shale and occasional siltstone presence. The porosity and permeability were used to compute the permeability/porosity relationship, the rock quality index, pore spaces, storage and flow capacity values, and to indicate the flow zone. The highest porosity and permeability values correspond to fades Al and A2, which are represented by mega pore spaces, and they displayed high flow capacity values and moderate to high storage capacities. The lowest porosity and permeability values belonged to core facies A6 and AS, which mostly comprise siltstone and clay with Nano pore spaces. The flow zone indicator (FZI) values determined ranges from 0.01 to 13.9 mu m, which were grouped into four different hydraulic flow units (HFUs) within the cored intervals. The defined hydraulic units do not always coincide with the facies. The Windland pore throat radius (r35) calculated that pore spaces correlated better with the Lorenz flow zone plot and fades. It is recommended that the statistically derived reservoir quality index (RQI) equation of fades A1 and A2 be used to extrapolate the RQI and FZI values for other wells in the field. It is thus recommended that dynamic data be used to validate flow units.