Geological and statistical reservoir characteristics of the Late Carboniferous-Permian Juwayl Member (Wajid Sandstone), southwestern Saudi Arabia

This study was conducted to characterize the sedimentary facies, depositional environments and petrophysical properties (porosity and permeability) for the Late Carboniferous - Permian Juwayl Member of the Wajid Sandstone. The methods of study included field investigations, facies analysis, petrographic analysis (thin section), XRD, SEM, petrophysical analysis and statistical analysis.; The facies analysis of Juwayl Member revealed ten lithofacies and two main depositional environments namely glacio-fluvial and braided river environments. The glacio-fluvial environment is characterized by pebbly to conglomeratic sandstone facies suggesting medium to high energy within a proximal depositional setting. In contrast, the braided river facies are dominated by medium to coarse grained trough- and planner cross-bedded sandstone facies. These lithofacies suggest a distal depositional environment characterized by laterally switching channels within an extensive braid plain system.; Juwayl sandstone is classified as quartz arenite consisting mainly of 93% quartz, feldspar (4%) and micas (<1%) in addition to calcite, clay minerals, rock fragments and iron oxides. Kaolinite is the dominant clay mineral with minor illite and chlorite. The petrophysical analysis revealed good---to very good porosity (17.7 to 32%) and good- to excellent permeability (0.3 to 14.8 Darcy) in Juwayl Member. The Juwayl sandstone is affected by shallow burial conditions during its diagenesis history. The cementing materials, grain size variation, quartz overgrowth and matrix are the main factors controlling porosity and permeability in Juwayl Member.; Characterization of the hydraulic flow units in Juwayl Member indicates that it contains 15 hydraulic flow units. Relatively lower values of the Reservoir Quality Index (RQI) and the Flow Zone Indicator ( FZI) are corresponding to the fluvial facies, whereas the higher values corresponding to the glacial facies.; Statistical analysis indicates normal distribution for porosity and log-normal distribution for permeability in both glacial and fluvial facies. The relationships between porosity and permeability indicate positive correlation with some outliers, The reservoir heterogeneity analysis conducted using three static parameters (coefficient of variation, Dykstra-Parsons coefficient and Lorenz coefficient) indicates that the Juwayl Member can be considered to be heterogeneous to very heterogeneous reservoir.