INTRODUCTION

Fractures are displacement discontinuities in rocks, which appear as local breaks in the natural sequence of the rock's properties. Most geological formations in the upper part of the earth's crust are fractured to some extent. The fractures represent mechanical failures of the rock strength to natural geological stresses such as tectonic movement, lithostatic pressure changes, thermal stresses, high fluid pressure, drilling activity, and even fluid withdrawal, since fluid also partially supports the weight of the overburden rock. Although petroleum reservoir rocks can be found at any depth, at the deeper depths pressure of the overburden is sufficient enough to cause plastic deformation of most of the sedimentary rocks. Such rocks are unable to sustain shear stresses over a long period and flow towards an equilibrium condition.

Fractures may appear as microfissures with anextension of only several micrometers, or as continental fractures with an extension of several thousand kilometers. They may be limited to a single rock formation or layer, or propagate through many rock formations or layers. In geological terms, a fracture is any planar or curvi-planar discontinuity that has formed as a result of a process of brittle deformation in the earth's crust. Planes of weakness in rock respond to changing stresses in the earth's crust by fracturing in one or more different ways, depending

on the direction of the maximum stress and the rock type. A fracture may consist of two rock surfaces of irregular shape, being more or less in contact with each other. The volume...