Application of continuous porosity-dependent decomposed density and velocity geophysical modelling for 4-D Seismic-based Quantification of hydrocarbon-bearing upper-Cretaceous stratigraphic systems, Southern Onshore, Pakistan

Abstract

4D seismic was frequently used for assessing the migration of hydrocarbon-bearing fluids within the fluvial sedimentary systems. The current study focused on enhancing a detailed stratigraphic reservoir characterization of a transgressive system tract (TST) within a gas field in Pakistan. This system is dominated by ultra-thin-bedded mixed shale-sand layers with low-impedance contrast, which adversely attenuate the true amplitudes of seismic data. The continuous wavelet transforms (CWT)-based quantitative porosity-dependent decomposed density and velocity modelling (CPDV) is applied to predict the thickness and porosity based on a 4D seismic approach. A combination of 2D and 3D seismic surveys (A & B) were used in this research work. A 16.85 m thick fluvial point bar (PB) was resolved by 35 Hz CWT amplitude. The 34 Hz CWT amplitude revealed an 11.85 m thick shale-dominated channel sand-fills (CS). The CPDV assigned a porosity range of 21 to 27% for PBs. Meanwhile, the band-limited seismic attributes remained poor in predicting the actual stratigraphic prospect. The CPDV has predicted the exact orientation of the coarse-grained sandstone lens of PB, which pinch out to the east. The zone of erosion is predicted where the porosity decreases from 17 to 8%, the density increases from 2.3 to 2.5 g/c.c, and the velocity increases from 3500 to 4300 m/s. The depositional zone have experienced a velocity range of ∼2200 to 3000 m/s, density range of 2 to 2.2 g/c.c, and porosity range of 17 to 27 %, which can serve to ascertain the depleting stratigraphic plays for Worldwide Basins.