Page 153 - Origin and Prediction of Abnormal Formation Pressures

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION PRESSURES 129

5000 m
5500 m
2.4 "
ouuu m
2.2 - - f
4500 m

~ej
2.0 - -
4000 m
.=,..
0
E 1.8 --
0
r
<
1.6 -- 3500 m
0
r
3000 m
0
1.4 I
(1) 2000 m
0
0 1.2 ,
I ! I I I I
0
20 40 60 80 1 oo 120 140
Rate of Sedimentation (R), meters per million years
Fig. 5-2. Relationship between the coefficient of abnormally high formation pressure (AHFP) in shales, Ka,
and the rate of sedimentation (m/million years) during Pliocene-Quaternary time at different depths, h (m).
Values are shown on curves. (Modified after Dobrynin and Serebryakov, 1989, fig. 42, p. 93.)

shows the rate of sedimentation, which in many cases determines the creation of
abnormally high pressure.
Using the data from wells drilled in the West Kuban Depression (Azov-Kuban
petroleum basin), the sealing ability of caprocks can be estimated (Eq. 5-2). The
coefficient of abnormally high pressure can be calculated using Fig. 5-1 and Eq. 5-1
as a function of the sedimentation rate, R (Eq. 5-4) and the depth of burial of these
rocks. Examples of calculated coefficients of abnormally high pressures presented as a
function of sedimentation rate, R, during the Pliocene-Quaternary time are shown in
Fig. 5-2. Using this type of calculation, it is possible to predict not only the present-day
values of pressure, but also to explore paleopressure compartmentalization in order to
help understand hydrocarbon migration and accumulation.
This method of calculating abnormal pressure based on the use of Eqs. 5-1 through
5-4, was used in several regions including the West Kuban Depression and Turkmenistan
Basin with good results.