4.2.2. Mechanical Stratigraphy

Mechanical stratigraphy subdivides stratified rock into discrete mechanical units consisting of one or more stratified rock units with consistent or similar rock mechanical properties such as brittleness, tensile strength, elastic stiffness, and fracture mechanics properties [49,67,68]. These mechanical units are generally, but not always, one layer with uniform lithology, which is not exactly the same as the lithologic layer. Further analysis in these basements reveals that mechanical stratigraphy which controls tectonic fractures can work in two ways: (1) the interface of the mechanical unit controls the

growth and termination of tectonic fractures, and (2) the thickness of the mechanical unit controls the development degree and height of tectonic fractures.

Tectonic fractures in the outcrops mainly are developed inside the mechanical unit, and most of them cut through the entire mechanical unit and end at the interface of two separate mechanical units (Figure 14). These fractures are nearly perpendicular to or are inclined at a high angle to the interface of mechanical units. As the mechanical unit thickness increases, the height of tectonic fractures increases as well. Based on core observations, tectonic fractures only are developed in the same mechanical unit and terminated at the interface when lithological variations dictate a different mechanical unit (Figure 15). Moreover, tectonic fractures of the same set are developed at approximately equal intervals in the same mechanical unit. The outcrop observations show that in the mechanical units that are limited in thickness, the mean spacing of tectonic fractures displays a strong linear relationship with the thickness of mechanical units, which means the mean spacing of tectonic fractures increases as the thickness of mechanical units increases (Figure 16) [45,69,70]. Consequently, the stress regime has caused tectonic fractures in the thinner mechanical unit to be more frequent and with smaller heights, unlike thicker mechanical units.

**Figure 14.** The outcrop section shows tectonic fractures in different mechanical units. I to V refer to the number of mechanical units. The linear density of tectonic fractures is 4.3 m<sup>−</sup><sup>1</sup> in I, 10.6 m<sup>−</sup><sup>1</sup> in II, 9.3 m<sup>−</sup><sup>1</sup> in III, 3.6 m<sup>−</sup><sup>1</sup> in IV, and 6.7 m<sup>−</sup><sup>1</sup> in V.

**Figure 15.** Tectonic fractures in the cores of Well R15, depth 2598.08 m (8523.88 ft). The fractures are mainly developed in layer A and end at the interface between separate layers A and B.

**Figure 16.** The relationship between the mean fracture spacing and the thickness of mechanical units from 52 data points on outcrops.
