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 critical-clay-volume fraction of 32%, an increase in pore compressibility also occurred from about 0.6 to about 11.25GPa1. 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.

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