Page 520 - Petrophysics

CHAPTER 8

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NATURALLY

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 an extension 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

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