# Source Rock Prediction Using Well Log and Seismic Data:
## 5 Prediction of Source Rock Distribution  
### Total Organic Carbon Method  
[PERSON] et al. (2018) overlapped the resistivity curve with the acoustic wave time difference curve and found that the curves in the non-source-rock segment basically overlapped, while the curves in the source rock segment formed obvious intervals. The value at the time of overlap is taken as the baseline value and the amplitude difference at the time of the interval is expressed using the \(\Delta\)logR method, an empirical formula for the conversion between \(\Delta\)logR and TOC, which is currently a commonly used method for predicting the distribution of source rocks with well logging at present. However, there are many parameters involved in this method that need to be determined manually, which can easily makes it easy to cause errors due to subjective judgments ([PERSON] et al., 2018). According to the existing logging data in the study area, combined with the improved method developed by [PERSON] et al. (2018), we propose a relationship suitable for the study area:  
\[\text{TOC}=\text{a}\times\text{logR}+\text{b}\times\text{DT}+\text{c} \tag{1}\]  
where R is the measured resistivity (\(\Omega\cdot\)m); DT is the sonic time difference (\(\mu\)s/m); and a, b, and c are the coefficients, which are obtained via the multiple regression analysis of the measured value of the sample.  
The 37 measured TOC values of the nine wells in the basin, the corresponding acoustic time difference, and the resistivity value are analyzed via multiple regression according to Formula (1), and the values of a, b, and c are obtained. Thus, the logging prediction model for the Lower-Cretaceous-Albian-stage source rocks in the Cote d'Ivoire Basin is obtained:  
\[\text{TOC}=0.728\times\text{logR}+0.033\times\text{DT}-1.432\]  
This method was used to predict the source rocks in the area, and the measured TOC value was compared with the predicted log TOC value. We calculated the correlation coefficient R\({}^{2}\) = 0.8083, indicating that the prediction accuracy of TOC was high (Figure 7), and this method was adopted for the whole area. In order to verify the universality of the model, Well X was selected for verification. The predicted total organic carbon content value was compared with the measured total organic carbon content value, and the results showed a high level of agreement between them, indicating that the logging interpretation model made using this method was more accurate (Figure 8).  
Figure 7: Correlation between measured the TOC (green circles) and predicted TOC (dotted line) of Lower-Cretaceous-Albian-stage source rocks in the Côte d’Ivoire Basin.  
The method was applied to each single well in the study area to predict the TOC value of the source rock, and the plane distribution of the TOC in the study area was obtained after taking the average value (Figure 9). In the deep basin near Well R, the TOC values are generally high, with an average value greater than 4 wt.%, indicating the good quality of the source rocks. But the TOC value is low in Wells M, N, L, Y, and other wells at the edge of the basin, with a maximum value of about 1 wt.%. From the center of the basin to the edge of the transition area, wells such P and X have TOC values of about 2 wt.%-3 wt.%. The source rocks in the central area of the basin are obviously better than those in the peripheral area.  
Figure 8: Comparison between the predicted TOC and measured TOC of a single well of Lower-Cretaceous-Albian-stage source rocks in the Côte d’Ivoire Basin. GR = natural gamma, SP = spontaneous potential, ILD = induction resistivity, DT = sonic differential time.