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 hydrody namics of the depositional environment. There is a relationship betwee n the petrophysical properties of a reservoir rock, such as porosity a nd permeability, and the grain-sie distribution. Therefore, the grain- size distribution is important in interpreting both the depositional e nvironment and the petrophysical properties of a sedimentary rock. Det ermination of the grain-size parameters from gamma-ray and/or other sh ale-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 Sandsto ne 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-grai ned sandstones. Grain-size distribution curves revealed a finer mean, poorer sorting,finer skewness, and a leptokurtic distribution with inc reasing shale content. The shale content associated with the finer gra in sizes promoted a reduction in porosity and permeability, which is a common occurrence in sandstones. In the studied well, a reasonable co rrelation between the grain-size parameters and the shaliness log was observed Vertical variations in the parameters for the sequence indica ted two coarsening-upwards and one fining-upwards depositional cycles.