Page 62 - Geology and Geochemistry of Oil and Gas

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Chapter 3

TEMPERATURE AND PRESSURE IN THE SUBSURFACE

3.1. DEFORMATION OF ROCKS IN DEPTH

3.1.1. Porosity and permeability versus depth of burial
Porosity and permeability of the uncompacted rocks decreases, and density
increases with depth. Compaction of the South Louisiana Tertiary sands (Fig. 3.1) is
a classical example. Sediments experience the total pressure of the overburden (rocks
plus fluids), p t , upon burial. Terzagi determined the overburden pressure (accounting
for lateral compression) as follows:

1 2d
p ¼ Drg 1 þ (3.1)
t
3 ð1 dÞ
where D ¼ the depth, r ¼ the density, g ¼ the gravitational acceleration, and
d ¼ the Poisson’s ratio. If, to take lateral stress into account, water is taken as a bed
model (its Poisson’s ratio is 0.5), Eq. 3.1 is simplified to

p ¼ Drg (3.2)
t
This equation is most common in practical applications.
The rocks under overburden pressure tend to compact, whereas formation
pressure (fluid pressure) resists such compaction. Thus, compaction occurs due to
effective pressure (p e ), which is equal to the total overburden pressure (p t ) minus the
fluid or pore pressure (p p ):
p ¼ p p p (3.3)
t
e

Fig. 3.1. South Louisiana, USA. Compaction of unconsolidated Tertiary sands with admixture of clay
(after P. A. Allen and J. E. Allen, 1990).