Geostatistical modeling and spatial distribution analysis of porosity, permeability and volume of shale in the Upper Angel Formation, Dampier Sub-basin, offshore N.W. Australia

The objectives of this study were to analyze the spatial behaviors of the porosity, permeability and volume of shale and establish models that explain the variability and heterogeneity of these three variables in the Upper Angel Formation of the Dampier Sub-basin, Offshore N. W. Australia. A deterministic geological model of the study area was generated based on the measured values of these variables together with a tectono-stratigraphic framework based on published reports and papers, well-log data and seismic interpretations of the Northern Carnarvon Basin. This framework was divided into 12 cycles that formed the stratigraphic and structural elements of the pre, syn and post rift stages of the Dampier Sub-basin. Univariate and bivariate statistical analyses of the three variables then provided the framework to the geostatistical modeling that followed. Spatial analysis of the three variables was then carried out using the semivariogram to calculate the geostatistical parameters. Based on that kriged and conditionally simulated models of the three variables for 67 levels (each 0.5 meters in thickness) were produced. The generated data has the same statistical and spatial characteristics as the original data. The results of this study introduced a comprehensive and understandable model for the tectono-stratigraphic framework of the Dampier sub-basin starting from the Early Permian to Recent. It introduced an unconventional method to interpret large-scale sedimentological processes from the univariate and bivariate statistics of the three variables. It showed that in general, the best quality reservoir units are located in the upper to middle levels (Level-12 to Level-48) where the high porosity, high permeability and low volume of shale exist. The study also showed that the variability of both porosity and permeability increase in the middle levels where the volume of shale records low values. This implies that the variability of the porosity and permeability is largely controlled by diagenetic processes rather than depositional.