Seismic attributes and impedance simulations image lower Cretaceous hydrocarbon-bearing sedimentary systems, Indus Basin

Abstract The upper shoreface depositional systems (UDSE) of deltaic depositional sequences form excellent stratigraphic traps for petroleum exploration that are developed during falling seas and slow rise. They have thin-bedded (hydrocarbon-bearing) sandstone lenses, which complete a depositional sequence. However, they are constrained by the full-spectrum seismic amplitude profiles (FAP) to quantify them for average porosity (Pavg), hydrocarbon saturation (SH), water saturation (Sw), effective porosity (Peff), acoustic impedances AI g/cm3*m/s for lithology contrasts, controls on the fluid migrations (both vertical and lateral), and hence, to discriminate the types of reservoir facies inside the diversified parasequences. Therefore, this study is intended to execute the petrophysical modelling (PM) and MBBI tools on a gas field in Central Indus, Pakistan. The conventional seismic amplitude could not predict the presence of any fractured zones, which eventually failed to implicate the controls on the vertical/ lateral facies and fluid. The PM predicts 15% Pavg, 14% Peff, 37% Sw, and 63%. The MBBI using a broadband frequency bandwidth spectrum of 12-43-Hz and a ~ 36-Hz tuned-frequency have improved quantification and enhancement of S/R for UDSE. The MBBI has accurately predicted the AI of 7828 g/cm3*m/s for thin-bedded (gas-bearing) aggradational parasequences with parallel seismic reflection configurations of fluvial-dominated point bars-sourced sedimentary influxes for UDSE, ~ 10550 g/cm3*m/s for transgressive top sealing, progradational, ~ 12520 g/ cm3*m/s for bottom sealing parasequences, and 9300–12220 g/cm3*m/s for lateral retrogradational parasequences, which implicates the standstill and moderate-to-high rising sea-levels. The east-west fluctuating AI from ~ 7800–9800 g/cm3*m/s implies the lateral facies-controlled gas migrations.