© 2012 SEG. Occurrence of moldic and vuggy pores, fractures and other pore structures due to diagenesis in carbonate rocks much complicates the relationships between impedance and porosity. Acoustic impedance in carbonates is influenced by factors such as porosity, pore structure/fracture, fluid content, and lithology. Using a frame flexibility factor (γ) derived from a poroelastic model to characterize pore structure in reservoir rocks, we found that its product with porosity could result in a much better correlation with sonic velocity (Vp = A-B*γφ) and acoustic impedance (AI = CD*γφ), where A, B, C and D is 6.60, 3.14, 18.21, and 9.88 respectively for a deep low-porosity carbonate reservoir studied in this paper. These new relationships could also be useful to improve seismic inversion of ultra-deep hydrocarbon reservoirs in other similar environments.