Page 59 - Origin and Prediction of Abnormal Formation Pressures

42 G.V. CHILINGAR, J.O. ROBERTSON JR. AND H.H. RIEKE III

2000

4000
q
c- Rincon -Dos Cuadros pressure gradient
C~. 6000
a

8000

O0 200 ~90 400 500~ 600 \700 kg/cm 2
10,000 .......
0 2000 4000 6000 8000 10,000
Pressure, psi
Fig. 2-11. Variation in pressure gradient with depth in Rincon-Dos Cuadros, California, USA, abnormally
high formation pressure (AHFP). (Modified after McCulloh, 1965, fig. 11, p. 38.)

Porosity-density variations with depth

A series of curves showing variation in porosity and density of plastic sedimentary
rocks with increasing geostatic loading are presented in Fig. 2-12. Ozerskaya (1965)
presented the following equation for the variation in rock porosity with increasing
geostatic pressure:

~b = ~bmax e-0"45D (2-42)
where ~bmax is the maximum initial porosity of argillaceous sediments, which is
commonly considered to be equal to 60%, and D is the depth of burial. The 60%
value is a good average for deltaic and marine clays; however, 4~max may be significantly
different for continental, lacustrine, deep-ocean, and other types of clays. The formula
relating fractional porosity, 05, bulk density, Pb, and mineralogic density, Ps, can be
presented as follows:

Pb = p~(1 -- 4>) (2-43)
Consequently,

Pb = ps(1 - ~bmaxe -0"45D) (2-44)
In Fig. 2-12 the density of mineral grains was assumed to be equal to 2.7 g/cm 3. It was
assumed that the initial maximum porosity of the clayey sediment was equal to 60%. If
the values read from Fig. 2-12 indicate a lower initial porosity, there are several possible