## 4 Field Application and Results
### Pre-Stack Inversion  
The pre-stack seismic inversion technique was used to invert the seismic data into four elastic properties, i.e., P-impedance, S-impedance, Vp/Vs ratio, and density. Figure 12 demonstrates the synthetic seismogram generated from the well log data and composite seismic traces extracted from seismic data at the well location. The correlation of 0.60 was achieved in the seismic-well ties process. Figure 11: SQp vs. SQs cross plot with colour-coding based on resistivity for pay sand delineation. Figure 12: Synthetic seismogram (**blue**) generated from the well log and composite seismic traces (**red**) extracted from seismic data at the well location. A correlation window is displayed in the red box. The inversion analysis at the well location of XD-2 well showed a good fit between inverted P-impedance, S-impedance, Vp/Vs ratio, and density with original well logs as seen in Figures 15 and 16. Figure 15: (**a**) Simultaneous pre-stack inversion analysis plot for P-impedance property for well XD-2 within error calculation time window (yellow colour). The correlation between the original (blue) and inverted (red) P-impedance logs is 0.89. (**b**) Simultaneous pre-stack inversion analysis plot for S-impedance property for well XD-2 within error calculation time window (yellow colour). The correlation between the original (blue) and inverted (red) SI logs is 0.86. The inversion analysis showed a good match with the well data and aided in distinguishing possible reservoirs in the HPHT zone of the Sepat Field. Figure 17a,b illustrate the P-impedance and S-impedance inversion results with visible alternate layers of sand and shale. The low acoustic and shear impedance values ranging from 5600 to 6600 (m/s) \(\times\) (g/cc) between Top H and H160 horizons represent the hydrocarbon-bearing sand. The Vp/Vs and density inversion results in Figure 18a,b also showed low values of Vp/Vs, and density (green intervals) may indicate the delineated hydrocarbon-bearing sand. Figure 16: (**a**) Simultaneous pre-stack inversion analysis plot for density property for well XD-2 within error calculation time window (yellow colour). The correlation between the original (blue) and inverted (red) density logs is 0.70. (**b**) Simultaneous pre-stack inversion analysis plot for Vp/Vs ratio property for well XD-2 within error calculation time window (yellow colour). The correlation between the original (blue) and inverted (red) Vp/Vs ratio logs is 0.85. Figure 17: (**a**) Inverted Zp crossline section containing XD-2 well from pre-stack simultaneous inversion; (**b**) Inverted Zs crossline section containing XD-2 well from pre-stack simultaneous inversion. Figure 18: (**a**) Inverted Vp/Vs ratio crossline section containing XD-2 well from pre-stack simultaneous in-version; (**b**) Inverted density crossline section containing XD-2 well from pre-stack simultaneous inversion. Horizon slices are generated using the data between Top H and H 60 horizons to observe the inversion results spatially. Horizon slices of Zp and Zs exhibited low responses of both impedances in the west of the study area, marked by the dotted line in Figure 19a,b. Low impedance responses in both Zp and Zs imply sand lithology filled by hydrocarbons, possibly gas. Eastwards, the impedance values increase, showing the lithological change of higher impedance, which is shale distribution. Density and Vp/Vs horizon slices in Figure 20a,b displayed the same trend as Zp and Zs horizon slices, which supported the interpretations made on both horizon slices of impedances. Figure 19: (**a**) Averaged Zp data horizon slice within interval between Top H and H60 horizons; (**b**) Averaged Zs data horizon slice within interval between Top H and H60 horizons. The low response of Vp/Vs in the horizon slice (Figure 20b) is due to the increase in Vs and reduction in Vp. A surge in Vs is the reduction in density and the increasing absorption of distortion by free gas in pores.