Application of Post-Stack Seismic Inversion for the Characterization of Porous Reservoir Sand, Gambat Block, Central Indus Basin, Pakistan

Abstract

The Lower Goru C-sand of the Gambat Block, Central Indus Basin, is a proven gas reservoir whose quality varies sharply over short distances. Lateral facies change, diagenetic porosity heterogeneity and fault compartmentalization produce amplitude responses that conventional interpretation cannot resolve unambiguously, and only three wells penetrate the productive interval. This study characterizes the porous reservoir sand by combining four methods: petrophysical evaluation of Tajjal-1, Tajjal-2 and Tajjal-3 in GVERSE Petrophysics; structural and stratigraphic interpretation of a 3D seismic volume in IHS Kingdom; post-stack seismic inversion; and AVO analysis with supporting rock physics, both carried out in CGG Hampson-Russell. Petrophysical evaluation identifies the reservoir intervals as clean quartzose sandstone with low shale volume, developed effective porosity, elevated deep resistivity and reduced water saturation. Neutron-density crossover within the clean sands, corroborated by the density-neutron and M-N cross plots, indicates gas rather than brine. Seismic interpretation traces the Upper Goru marker, the Lower Goru marker and the C, B and A sand intervals across three inlines and three crosslines. The horizons are laterally persistent in both structural directions and gently deformed, cut by a single fault near Tajjal-3, and the gridded picks were converted to depth using velocities from the wells. Model-based inversion weas applied to the same volume. At Tajjal-1 the sparse-spike result reaches a correlation of 0.9888 with a residual error of 0.1968, and the model-based result 0.9986 with an error of 0.0829. Both resolve the porous sands as laterally coherent low-impedance zones at the same positions, and the horizon maps of the top C and top B intervals show corresponding fairway patterns, which indicates that the mapped distributions do not depend on the choice of algorithm. Gassmann fluid substitution and angle-gather modeling identify an AVO class 2 response, in which the reflection is weak at normal incidence and its magnitude increases with offset. The Vp-Vs, acoustic impedance against Poisson's ratio, and lambda-rho against mu-rho cross plots separate gas sands from brine sands and shales, and extended elastic impedance at a chi angle of 32° gives the clearest discrimination obtained in the study. The low-impedance fairways mapped on the top B interval are the most prospective targets in the block. Because the reservoir is weak at near offsets and develops its amplitude only at far offsets, its AVO signature can be overlooked where a class 3 response is assumed