RELATIONSHIPS BETWEEN SHALE CONTENT AND GRAIN‐SIZE PARAMETERS IN THE SAFANIYA SANDSTONE RESERVOIR, NE SAUDI ARABIA

Abstract

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.