tions in terms of geological regimes and rock lithology are briefly discussed. These fracture network patterns are compared with those commonly used in reservoir simulations.

Fractures are mechanical breaks in rocks involving discontinuities in displacement across surfaces or narrow zones. Fracture is a term used for all types of generic discontinuities. This usage is common among scientists inside and outside the earth sciences and is used in other chapters of this report. However, different kinds of fractures exist, with different geometries, mechanical effects, and flow properties. Based on the nature of the displacement discontinuity, commonly encountered fractures can be classified into three geologically based major groups: (1) dilating fractures/joints, (2) shearing fractures/faults, and (3) closing fractures/pressure solution surfaces. (Pressure solution surfaces are fractures in sedimentary rock that are welded together by solution that occurs at the contact surfaces of grains [Bates and Jackson, 1980].) This chapter is concerned with the first two of these groups, joints and faults, illustrated schematically in Figure 2.1. Dilating fractures, which are also referred to as joints, can be idealized as two rough surfaces with normal displacement discontinuity; that is, the surfaces have moved away from each other in a direction perpendicular to the surfaces (Figure 2.1b). (They are also called mode I fractures in engineering fracture mechanics [Lawn and Wilshaw, 1975].) Shear fractures, which are also referred to as faults, are shear displacement discontinuities where the fracture surfaces move predominantly parallel to each other. This relative movement is either perpendicular (mode II) or parallel (mode III) to the fracture front (Figure 2.1b). Pressure solution surfaces, also referred to as stylolites, are known as anticracks in which the sense of the displacement discontinuity is opposite that of dilating fractures or mode I fractures (Fletcher and Pollard, 1981). For a review of pressure solution surfaces and their hydraulic properties, the reader is referred to Nelson (1985).

Fractures with a combination of these modes (mixed-mode fractures) also are possible. Rock masses with complex deformational histories have fractures produced by two or more of these modes in a sequential manner (Figure 2.2 a–d). This yields fractures with overprinted displacement discontinuities (Barton, 1983; Dyer, 1983; Segall and Pollard, 1983a,b; Zhao and Johnson, 1992). All combinations of displacement discontinuities may occur in nature, but the most common ones are faulted joints and jointed faults. Although these two types of fractures are kinematically similar to faults and joints, which were defined previously, their total displacement discontinuities, geometries, and internal structures may be very different because of the overprinting of two different modes of deformation.