Quantitative Reservoir Characterization by Integrating Rock Physics Analysis and Seismic Inversion, Offshore Indus Basin of Pakistan

Abstract

The Offshore Indus Basin, along Pakistan's passive continental margin in the Arabian Sea, holds a thick Cenozoic clastic succession with all the essential petroleum system elements, yet remains lightly drilled and commercially unproven. Conventional seismic interpretation maps structure adequately but cannot uniquely resolve porosity, lithology, and fluid saturation at the resolution exploration decisions require. This study addresses that gap by applying an integrated quantitative reservoir characterization workflow — combining petrophysical evaluation, structural seismic interpretation, modelbased acoustic impedance inversion, and rock physics analysis to a Miocene–Pliocene clastic interval within the basin. The dataset comprises eleven 2D seismic lines and wireline logs from three wells, two of which carry the log suite required for petrophysical evaluation. The reported tops place all three wells on a common chronostratigraphic scheme but are too widely spaced to define reservoir units, so four reservoir intervals were established within that framework by gamma ray and deep resistivity correlation, two in the Late Miocene and two in the Middle Miocene, and all subsequent analysis was conducted within them. Petrophysical analysis in GVERSE yielded shale volume (Larionov non-linear correction), average and effective porosity, and water saturation. Seismic interpretation in Kingdom, calibrated by synthetic seismograms, produced fault and horizon picks and time and depth structure maps for four key horizons. Model-based post-stack inversion in Hampson-Russell was applied to two of the strike lines, SPC-86-9007 and SPC-86-9019, and rock physics crossplots of Vp/Vs against P-impedance, bulk density and S-impedance, colour-coded by computed gamma ray, supplied the litho-fluid discrimination template. The results define a vertically stacked sand–shale system in which reservoir quality is governed principally by clay content. All four intervals are shale-rich when averaged over their gross thickness, with shale volumes of 51 to 77 per cent, and the clay correction reduces average porosities of 21 to 32 per cent to effective porosities of 6 to 11 per cent. Water saturations are high throughout, at 80 to 98.5 per cent. The deeper PakCan-01 interval at 2700–2730 m is the cleanest and returns the highest effective porosity and hydrocarbon saturation, at 10.02 and 20.01 per cent; the two Sadaf-01 intervals return 10.64 per cent hydrocarbon saturation; and the shallower PakCan-01 interval at 1710–1770 m is effectively water-bearing at 1.50 per cent. Inversion, validated by well-tie correlations of approximately 0.99, imaged laterally coherent low-impedance anomalies confined to theviii reservoir interval on both inverted lines and spatially coincident with fault-bounded structural closures persistent across all four mapped horizons. Because the computed water saturations are high and the shale volumes substantial, those anomalies are attributed principally to variation in clay content and porosity rather than to fluid fill, and the crossplots separate the data by lithology more clearly than by fluid. The fluid interpretation therefore remains an inference from elastic response rather than a measurement; pre-stack simultaneous inversion, shear velocity control in more than one well, and 3D coverage are recommended as the next steps.