Summary: Pore structure and grain size play an important role in controlling the complexity of velocity-porosity relationships and permeability heterogeneity in carbonate reservoirs, in addition to mineral composition and pore fluid. A frame flexibility factor (g) has been found to be able to quantify effect of pore structure changes on sonic wave velocity and permeability in carbonate reservoir rocks. Our study of a San Andres carbonate reservoir, Permian Basin, shows that for core samples of given porosity, the lower the frame flexibility factor g, the higher the sonic wave velocity. A frame flexibility factor g value of 3.85 could be used to separate permeability zones of different geological origin for the studied San Andres carbonate reservoir. Samples with frame flexibility factor g <3.85 are either dolograinstone and dolopackstone with visible vuggy pores or tight dolowackstone. On the other hand, samples with frame flexibility factor g >3.85 are dolopackstone with dominant interparticle pores, dolowackstone or dolomudstone with microcrack pores. Using the frame flexibility factor g, different porosity-velocity and porositypermeability trends can be classified with clear geologic interpretation such as carbonate rock and pore types. New porosity-permeability relations with g classification help delineate high-permeability zones in the San Andres reservoir, and may be useful for other similar carbonate reservoirs as well.