Acoustic impedance in carbonates is influenced by factors such as porosity, pore structure/fracture, fluid content, and lithology. Occurrence of moldic and vuggy pores, fractures and other pore structures due to diagenesis in carbonate rocks can greatly complicate the relationships between impedance and porosity. Using a frame flexibility factor ( $γ$ $γ$ ) derived from a poroelastic model to characterize pore structure in reservoir rocks, we find that its product with porosity can result in a much better correlation with sonic velocity ( $V_{p}$ $V_{p}$ = $A - B * ϕ * γ$ $A - B * ϕ * γ$ ) and acoustic impedance ( $A I$ $A I$ = $C - D * ϕ * γ)$ $C - D * ϕ * γ)$ , where A, B, C and D is 6.60, 0.03, 18.3 and 0.09, respectively for the deep low-porosity carbonate reservoir studied in this paper. These new relationships can also be useful in improving seismic inversion of ultra-deep hydrocarbon reservoirs in other similar environments.

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