5.1 Introduction

Changes of stresses in the subsurface lead to deformation of geological structures. Deformations can be grouped in (1) gradual and continuous, such as folding that creates an anticline, and (2) abrupt and discontinuous, such as faults. The creation of fault discontinuities depends on loading strain rate and rock properties including rock brittleness. Faults are the result of brittle rock failure in shear at the large scale (Fig. 5.1).

Figure 5.1: Simplified schematic of the genesis of a thrust fault.

Faults usually cut through several sedimentary strata (See outcrop fault examples in Fig. 5.2). Faults are important in energy geomechanics because they limit the magnitude of horizontal stresses, may constitute structural traps for fluid flow, favor reservoir compartmentalization, and other times create high permeability conduits for fluid flow. A map of faults and rock units in Texas is available here: https://txpub.usgs.gov/txgeology/.

Figure 5.2: Examples of faults as seen in outcrops.