© 2014 Society of Exploration Geophysicists. Knowledge of the clay content in clastic reservoirs is important for predicting reservoir quality and properties. We used a microgeometrical model for shaly sand and sandy shale to define the critical-clay-volume fraction and explain the dependence of the bulk modulus on clay content. We found that the concept of the pore-aspect ratio relating to the critical-clay-volume fraction was important to interpret the elastic behavior of shaly sandstone. An abrupt decrease in pore-aspect ratio from about 0.23 to about 0.04 was observed where the clay-volume fraction was greater than the critical value of 32% for the studied data set. At the criticalclay- volume fraction of 32%, an increase in pore compressibility also occurred from about 0.6 to about 11.25 GPa-1. Results revealed that the microgeometrical model compared to other models can better explain the existence of highly scattered compressional velocity-porosity crossplots when the clay content is close to the critical amount. We discovered that the model can be applied in well-logging interpretations of shaly formations for determining shale cut-off and mapping of reservoir pore shape from velocity measurements.