Figure 10: Indiana limestone sample cluster analysis at different threshold values: (A) 0.60, and (B) 0.80 that correspond to the valleys found in Figure 9.
Figure 11 shows the PCA analysis of the non-fractured Madison Limestone sample. The valleys are observed at threshold values of 0.35 and 0.55. The gravimetric porosity for this sample is close to the MRI porosity calculated at a threshold value of 0.55 (Table 3). As seen for the non-fractured Indiana Limestone, the first cluster valley (Figure 11) at threshold 0.35, is indicative of small pores and vugs. These vugs connect the small pore systems as shown in Figure 12A. The second valley at threshold of 0.55 (Figure 11) is a depiction of the vug system. When the small pore systems are ignored at a higher threshold value of 0.55, the vugs can be seen distinctly as indicated in Figure 12B. The histograms in Figure 12 show that there are fewer clusters when compared to the Berea sandstone, meaning that this is a more highly connected pore system. There are a greater number of higher proton density distributions in the Madison limestone when compared to the Indiana limestone, which can be correlated to a higher number of larger pores being observed in the Madison limestone when compared to the Indiana limestone. We attribute this result to the structural differences between Indiana and Madison limestone.