## 3 Materials and Methods
### Pore Pressure Prediction Methods
#### 3.1.2 [PERSON]'s Sonic Velocity Method with Depth-Dependent Normal Compaction Trend (NCT) Line  
[PERSON]'s method for estimating pore pressure sonic log can be presented as follows:  
\[\text{PP = OBG - (OBG - Prg) \times (\Delta\text{tn}/\Delta\text{t})} \tag{4}\]  
where  
\(\Delta\text{tn}\) = sonic transit travel time/the slowness in shale at normal pressure (\(\mu\)s/ft).  
[PERSON] expanded on [PERSON]'s formula as follows [51; 52]:  
\[\text{V}=\text{V0 + KZ} \tag{5}\]  
where  
V = seismic velocity at depth Z;  
V0 = ground surface velocity;  
K = a constant.  
An exponential relationship between the mean transit time from 17 wells that have normal pressure conditions and a Normal Compaction Trend for shale transit time with depth is given as follows [53]:  
\[\Delta\text{tn = 225 + 391e}^{-0.00103Z} \tag{6}\]  
where  
\(\Delta tn=(\mu\text{s/m})\);  
Z = the depth in meters.  
Ref. [38] used a similar relationship in their study and presented it as follows:  
\[\Delta\text{tn = 176.5 + 461.5e}^{-0.0007Z} \tag{7}\]The regular compression tendency of the transit time in certain petroleum basins, like Brunei and the deep-water Sabah fold belt, subsequently gave rise to the following relationship:  
\[\Delta\text{tn}\ =\ \Delta\text{tm}\ +\ (\Delta\text{tml}\ -\ \Delta\text{tm}) \text{e}^{-\text{cz}} \tag{8}\]  
where  
\(\Delta\text{tm}\) = compressional transit time in the porosity-free shale matrix;  
\(\Delta\text{tml}\) = mudline transit time;  
\(c\) = is the constant.  
The following is the [PERSON]'s sonic velocity adjustment method when Equation (7) is substituted into Equation (9):  
\[\text{Ppg}\ -\ \text{OBG}\ -\ (\text{OBG}\ -\ \text{Png})\ \big{(}\Delta \text{tm}\ +\ (\Delta\text{tml}\ -\ \Delta\text{tm})\ \text{e}^{-\text{cz}}/\Delta\text{t} \tag{9}\]  
1.3 Application of [PERSON]'s Method by Developing Normal Compaction Trend (NCT) in Resistivity and Sonic Velocity Log Plots  
The normal compact trend (NCT) is the best-fitting linear trend for identifying overpressure and normal pressure formations [4]. To obtain the correct NCT, several factors, such as the regional geology of the study area, pressure data, and comparisons of data from multiple wells, should be taken into account. The geology should reflect the rock types and the nature of compaction. The NCT should be consistent with the pressure data to avoid incorrect pressure zones. Pore pressure was predicted in the studied reservoirs using [PERSON]'s method by developing a depth-dependent Normal Compaction Trend (Equations (3) and (9)), using resistivity and sonic wireline logs for the required pore pressure attributes such as pore pressure, pore pressure gradient, fracture pressure, fracture gradients, effective stress (ES), and the overburden gradient.  
Based on [PERSON]'s method, the shale resistivity and sonic shale logs can either increase or decrease from the established NCT. This divergence is used to identify the overpressured and normal pressure zones. An overpressured formation is detected when the shale resistivity decreases from the NCT [54; 55]. On the other hand, an increase in shale resistivity, as determined by the NCT, indicates a normal pressure formation. However, sonic shale logs show a reverse pattern to shale resistivity logs. A divergence of sonic shale logs (DT) from the established NCT towards the low values scale indicates a normal pressure zone, and a deviation toward the higher values suggests an overpressured zone. The proper selection of the Normal Compaction Trend (NCT) accounts for a sizable amount of uncertainty in predicting pore pressure. As a result, the NCT developed for this study is mostly based on velocity logs obtained from sonic data and resistivity logs [56].