Relationships between shale content and grain-size parameters in the Safaniya Sandstone reservoir, NE Saudi Arabia

The grain-size distribution of a sediment is controlled by the hydrodynamics of the depositional environment. There is a relationship between the petrophysical properties of a reservoir rock, such as porosity and permeability, and the grain-size distribution. Therefore, the grain-size distribution is important in interpreting both the depositional environment and the petrophysical properties of a sedimentary rock. Determination of the grain-size parameters from gamma-ray and/or other shale-indicator well logs may be possible if the necessary correlations are established. Relationships between the shale content and the grain-size parameters of samples from the Lower Cretaceous Safaniya Sandstone Member in NE Saudi Arabia were investigated by grain-size analyses of 47 samples using sieving and a Microscan II particle-size analyzer. Analysed samples were "clean" and "shaly" fine- to very fine-grained sandstones. Grain-size distribution curves revealed a finer mean, poorer sorting, finer skewness, and a leptokurtic distribution with increasing shale content. The shale content associated with the finer grain sizes promoted a reduction in porosity and permeability, which is a common occurrence in sandstones. In the studied well, a reasonable correlation between the grain-size parameters and the shaliness log was observed. Vertical variations in the parameters for the sequence indicated two coarsening-upwards and one fining-upwards depositional cycles.