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.