A model for variation of velocity versus density trends in porous sedimentary rocks

A rock physics model appropriate for porous media in which some of the solid material is “floating” or not involved in load support is developed that explains observed variation in compressional wave velocity versus density trends. This same model predicts no significant change in the shear versus compressional wave velocity trend, as is also observed. These floating grains are correlated with poor sorting of the matrix grains as expected. The presence of the floating grains is found to correlate with a decrease in the permeability of the rock and therefore the viability of potential petroleum reservoirs. Shifts in the velocity versus density relationships can be determined by wireline log but not directly by seismic reflectivity measurements. However, the introduction of an additional concept, the capture fraction of smaller grains, adds another constraint to the model which enables remote sensing of the viability of certain petroleum reservoirs by seismic reflectivity measurements alone. Experimental evidence is described that supports this model and additional experiments are suggested to further confirm it.