## 4 Field Application and Results  
### Rock Physics Study  
Lithological and fluid characterisation of the HPHT zone is performed at the location of XD-2 well by cross-plotting analysis. It comprises of \(\mathrm{Vp}\)/Vs versus \(\mathrm{Zp}\), \(\mathrm{\mu}\varphi\) versus \(\mathrm{\lambda}\rho\), and SQp versus SQs. The data between Top H and H60 are used in the cross plots by scrutinising the trend of the two rock physics elements plotted on the x- and y-axis. In most cases, the rock physics template consisting of \(\mathrm{Vp}\)/Vs versus \(\mathrm{Zp}\) shows a diagonal separation of fluid and lithology diagnostic zones on the cross plots. Such characterised diagonal zones are due to the low \(\mathrm{Vp}\)/Vs and \(\mathrm{Zp}\) of the hydrocarbon-filled zone, increasing \(\mathrm{Vp}\)/Vs and \(\mathrm{Zp}\) values with fluid change from hydrocarbon to the water-filled zone. \(\mathrm{Vp}\)/Vs is a good fluid indicator as \(\mathrm{Vp}\) has a higher sensitivity to fluid changes, whereas \(\mathrm{Vs}\) is not, except in the special case of very viscous oil. The \(\mathrm{Vp}\)/Vs versus \(\mathrm{Zp}\) cross-plot results in Figure 9 showed three distinctive zone separations, with the lowest \(\mathrm{Vp}\)/Vs and \(\mathrm{Zp}\) zone outlined in red identified as the pay zone. The characterised pay also showed the highest measured resistivity around 8 \(\mathrm{\Omega m}\) in purple-coloured data indicating the presence of hydrocarbons. The medium \(\mathrm{Vp}\)/Vs and \(\mathrm{Zp}\) values outlined in blue are characterised as wet sand. The measured resistivity ranges from 1.8 \(\mathrm{\Omega m}\) to 3 \(\mathrm{\Omega m}\). The highest \(\mathrm{Vp}\)/Vs and \(\mathrm{Zp}\)values in the green outline represented the shale lithology, with the lowest resistivity value zone. The characterised zone of \(\mathrm{Vp}/\mathrm{Vs}\) versus \(\mathrm{Zp}\) cross-plot data based on the measured resistivity provided a good insight into the HPHT reservoir characterisation obtained from the inversion results. The cross-plot of \(\upnu\) versus \(\lambda\uprho\) presented in this study successfully distinguished the pay sand and shale in between Top H and H 60 horizons. From Figure 10, the pay sand is well characterised, occupying the low \(\lambda\uprho\) and high \(\upnu\) values in the red outline. This zone is also presented by the high resistivity data, an indication of hydrocarbon-filled sand. The wet sand is outlined in blue which most of the data within this zone fall within a medium range of \(\lambda\uprho\) and \(\upnu\) values. The distinguished wet sand also showed a good correlation to the lower resistivity values. The shale zone dominates the highest \(\lambda\uprho\) and lowest \(\upnu\) values in a green outline, related to the lowest measured resistivity shown from the resistivity colour scale. Figure 10: Lambda-rho vs. Mu-rho cross plot with colour-coding based on resistivity for pay sand delineation. Figure 9: \(\mathrm{Vp}/\mathrm{Vs}\) ratio vs. P-impedance cross plot with colour-coding based on resistivity for pay sand delineation. The applicability of SQp and SQs attributes cross-plots in the reservoir characterisation is pioneered based on a study in Malay Basin to characterise elastic reservoirs [32]. It can be seen in Figure 11 that the HPHT reservoir in Sepat Field showed a clear distinction between pay sand, wet sand, and shale lithology. The pay sand in red outline, having high SQs and low SQp values, corresponds to the high formation resistivity around 8 \(\Omega\)m of purple data. A decreasing SQs trend with slightly lower resistivity values is categorised as wet sand outlined in the blue circle. The decreasing value trends on SQs and resistivity on wet sand is a convenient observation. The two properties are sensitive to fluid change between hydrocarbon and water-filled zones. The shale zone is outlined in green showed an increasing trend of SQp values and decreasing SQs values.