## 4 Discussion
### The Mechanism of Petrophysical Differences
#### 4.2.3 The Effect of Rock Structure on Strength  
There have been many studies on the influence of bedding angle on uniaxial compressive strength. The bedding angle of shale has a significant influence on the strength of the sample. The shale strength is the highest when the bedding dip is 90\({}^{\circ}\) and the weakest when the bedding dip is 30-45\({}^{\circ}\)[59]. However, research on the subject has been mostly restricted to comparisons of the same lithology [60; 61], and the effect of beddings angle and density on shale strength can be further studied by comparing marine shale and continental shale because of the more obvious laminar structure differences. In this paper, samples were processed at 90\({}^{\circ}\) and 0\({}^{\circ}\) to analyze the difference in bedding density and angle in the failure mechanism of marine and continental shale. As shown in Figure 11, the marine shale samples have higher uniaxial compressive strength and elastic modulus than the continental shale. However, the strength of the sample at 0\({}^{\circ}\) is only 12% lower than that at 90\({}^{\circ}\) in marine shale, while the strength of the continental shale sample at 0\({}^{\circ}\) is reduced by 22%, which shows the minor strength difference between different directions of marine shale. Furthermore, the fracture failure morphology also presents different types. For the marine shale with homogeneity at the centimeter scale, the fracture propagates through the main axial stress with vertical splitting tensile damage. On the contrary, the continental shale could be regarded as a typically transversely isotropic rock [21; 59], and intensive lithologic stratification can be observed at the centimeter scale (Figure 12c), including shale layer, siltstone layer, carbonatite layer, and lenticular structure. The fracture initially propagates through the principal axial stress, and there will be a cross crack due to the activation of the weak laminas. And the steplike development of continental shale in the stress-strain curves specifically presented the process (Figure 11b). A similar stress-strain curve trend has been observed before [59]. With the increase of the load stress, the microcrack will gradually form and grow, and the stress will temporally decrease when the main crack is extended to the laminas. The crack will then continue to grow and activate the weak laminas until the effective stress exceeds the strength of the lamina's matrix, leading to the destruction of the sample (Figure 12d). This process also explained that the strength of the rocks with high density laminas will be significantly lower than the homogeneous rocks. Since the strength of the marine shale matrix is much greater than the strength of the laminas, the continental shale is easily fractured along the weak surface of the bedding and the strength mostly depends on the weakest laminas strength, while the homogeneity marine shale has a high matrix strength to ensure its high peak strength.  
Figure 11: The uniaxial compressive strength of marine shale (**a**) and continental shale (**b**).  
Furthermore, the initial compacted section of continental shale is longer than that of marine shale (Figure 11). In addition, the marine shale samples presented longer linear elastic deformation, while the continental shale samples showed higher plastic strain, mainly related to the density and mineral composition. The average density of continental shale is only about 80-85% of marine shale, and the porosity of continental shale is about twice that of marine shale. The mineral composition of marine shale is mostly brittle minerals (65.65% on average), while the average brittle mineral composition of continental shale is only 22%. The high content of clay minerals contributes to the plastic strain. Hence the required strain during compacting is more significant.