Predicting Lithology and Reservoir Quality from Acoustic Velocities Based on Petrophysical Classification of Siliciclastics

Lev Vernik

Based on the recently developed petrophysical classification of siliciclastics, which takes into account amount C and textural position of clay, it is shown that acoustic velocities can be a fairly accurate tool in predicting lithology, porosity, and permeability of these rocks. Four major petrophysical groups of carbonate- and organic-poor siliciclastics are distinguished: 1) clean arenites (C < 2%), 2) arenites and arkoses (C=2-15%), 3) wackes (C=15-35%), 4) shales (C > 35%).

Compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients. This allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average (Wyllie) equation, its improved modification (Raymer equations), both of which neglect textural factors. The processes of incipient consolidation of clastic sediments are characterized by a steeper slope of the velocity-porosity transform due to a more pronounced velocity increase compared to the porosity reduction at this stage. Finally, the potential for improved permeability prediction via porosity, which can be estimated from the velocity data and ligthology, is shown.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995