Page 157 - Origin and Prediction of Abnormal Formation Pressures

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION PRESSURES 133
Pore Pressure (Pc,), MPa Pore Pressure (p,:,), Mpa
(AI (B)

0 ~ 40 80 0 40 80
E ,,, I 1 I

.,c 4000
0.. i ,;oo2.oo oo.. k \\\ oo oo ..
L3
Pore Pressure (Pc, l, MPa
(c)
0 40 80

I[.. -1 21"-..I 31~

4000 ' \\
a ,,,,,

Fig. 5-4. Theoretical dependence of anomalous pressures in rocks with hydraulically closed pores on
magnitude (amplitude) of downthrust (subsidence) and upthrust (uplift) of blocks. Figures on the curves
for (A) and (B) represent amplitude of downthrust of blocks in m; for (C), amplitude of upthrust blocks
in m. 1 = Normal hydrostatic pressure; 2 = geostatic (total overburden) pressure; and 3 = theoretical
curves for pressure in hydraulically closed pores. Geothermal gradients: (A) and (B): 3 x 10-2~ (C):
4 x 10-2~ (Modified after Dobrynin and Serebryakov, 1989, fig. 26, p. 61.)

Cross-plots for estimating the abnormally high pressure in regions with downthrown
blocks (subsidence) are presented in Fig. 5-4A,B, whereas for the estimation of
abnormally low pressure in regions with uplift and erosion, Fig. 5-4C can be used.
An estimate of the pressure in the Dniepr-Donetz (Ukraine) and Middle Kura
(Georgia) basins can be made. In Georgia, an abnormally low pressure was estimated in
Eocene deposits at depths of 2000-3000 m. The amplitude of the compressional thrust
is 800 m. According to Fig. 5-4, coefficients of abnormal pressure range from 0.7 to 0.9.
In the Shebelin gas field (Ukraine), abnormally high pressures are related to strike-slip
faulting with displacements of 1000 m. Using Fig. 5-4, at a depth below 5400 m, the
coefficient of abnormal pressure is 1.5.
It should be noted that the methods described lack precision and should only be
used as a preliminary pressure prediction prior to drilling. More precise methods of
calculating abnormal pore pressures are utilized during drilling, using well log and
drilling data.