Outcrop analog for sandy braided stream reservoirs; permeability patterns in the Triassic Hawkesbury Sandstone, Sydney Basin, Australia

Permeability and porosity measurements on 1228 core plugs taken from the Triassic Hawkesbury Sandstone in the Sydney basin, Australia, were analyzed by both sedimentological and geostatistical approaches to assess horizontal and vertical permeability and porosity variations in deposits of braided river systems. The samples are variably cemented with porosities ranging from 5 to 20%, and permeabilities range from 0.1 to 1200 md. The permeability of the Hawkesbury Sandstone does not show any obvious relationship to its porosity. The permeability variations are closely related to sedimentary facies type, while the porosity distribution primarily reflects postdepositional diagenesis of the sandstone. Statistical analysis of all plug data shows a strong azimuth-directional permeability in the horizontal plane. The horizontal semivariogram correlation length ranges for two adjacent orthogonal outcrops are 4.5 and 2.5 m, respectively. The permeability correlation length ranges in the vertical direction for the two outcrops are similar, ranging from 1.3 to 1.6 m. Permeability is much more variable in the vertical direction than in the horizontal direction. The high degree of spatial variability observed in the permeability data is not consistent with fractal models based on the Gaussian-normal probability distribution. Subdivision of the sandstone into genetically related sedimentary facies significantly improves the predictability of the permeability distribution. Alternatively, the high degree of the permeability heterogeneity can be described using a new model based on the Levy-stable family of probability distributions.