Estimation of subsurface material properties and their spatial variation is important in hydrocarbon exploration and detection, and crucial to subsequent reservoir characterization and management. I conduct a pilot study to test my prestack impedance inversion theory and algorithms on a marine data set from the Gulf of Mexico, acquired over a known producing gas reservoir. First, I perform a high-resolution migration velocity analysis at every adjacent CMP location, and estimate a migration/inversion velocity model which is optimal in a Monte Carlo automatic velocity fitting technique. Next, I perform a least-squares Kirchhoff migration to estimate the angle-dependent reflectivity at each subsurface point, followed by a linearized Zoeppritz elastic parameter inversion for relative contrasts in compressional and shear wave impedance. Finally, I combine the P and S impedance contrast maps into a single image indicative of anomalous gas saturation levels. This hydrocarbon indicator correlates very well with the known gas reservoir interval, and suggests lateral heterogeneity of gas saturation along the reservoir unit.