Page 22 - Origin and Prediction of Abnormal Formation Pressures

INTRODUCTION TO ABNORMALLY PRESSURED FORMATIONS
~z
Z
l

I *

~Y

~z

/

a~z

Fig. 1-2. Stress notation in a cubic argillaceous rock slice. Stress notation of the normal component of stress,
a2, on the plane normal to the z-axis, rzx and rzy refer to the shear stress components in the plane normal to
the z-axis and acting in the x- and y-directions, respectively, cr z + (Oaz)/(Oz)dz is the incremental change in
vertical stress through the free body. (Modified after Rieke and Chilingarian, 1974, fig. 52, p. 93.)

tangential (shear) stresses, ri, that act on each face of the cube normal to the major axes
(Eq. 1-4, and Fig. 1-2). The tensor (S) of the nine stresses may be represented by the
following equation:
I 0" x "Cxy "Cxz

S ~ Ty x Oy "gyz (1-4)
L'Czx 72zy O" z
If compression is produced by tectonic horizontal compressive stresses, such as
folding, the greatest principal stress is horizontal (~), and the least principal stress is
vertical (~z), which is equal to the overburden load per unit area (Pob). The greatest and
least effective stresses may be expressed as follows:

Pex -- Ox -- pp (1-5)

Pez -- ~ -- Pp (1-6)