We mapped probabilities of lithology and fluid based on seismic and well observations in a heterogeneous carbonate reservoir of the Sichuan Basin, southwest China, thus characterizing the reservoir complexity and identifying the potential sweet spot. Rock physical analysis showed that the presence of a vuggy-fracture porosity system combined with the fluids’ effect complicate the elastic responses of heterogeneous carbonates. This also gives physical insight into how the elastic properties of different lithofluid classes can be distinguished. With the application of Bayesian linearized amplitude variation with offset inversion, we found that the posterior distribution of P- and S-wave velocities were reliably extracted, whereas the inverted density tended to show high uncertainty due to a lack of wide angle data in the migrated seismic gathers. Finally, a Bayesian approach was implemented to propagate uncertainty from prestack seismic data to lithofluid classes in an integrated framework. The gas-carbonate predictions with high posterior probabilities were consistent with well observations at the well location and the present geological continuity. The probability distributions for the anhydrite, limestone, and dolostone were essential to understand the reservoir architecture and delineate the reservoir heterogeneities. We also ascertained that the uncertainty of lithofacies prediction is determined by the uncertainty of seismic inversion as well as the uncertainty of the link between lithofacies to their corresponding elastic responses.