Page 75 - Geology and Geochemistry of Oil and Gas

ABNORMALLY-HIGH FORMATION PRESSURE 51

Fig. 3.10. Vitrinite reflectance R o vs. depth of burial in cores from a well in Texas (after Wescott and
Hood, 1994).

3.2. ABNORMALLY-HIGH FORMATION PRESSURE
Abnormally-high formation (pore) pressure (AHFP) or overpressure is encoun-
tered worldwide in sand–shale or massive carbonate–evaporate sequences from
shallow to great depths, in formations as old as Cambrian. Many problems related
to origin and distribution of abnormal formation pressures were discussed
by Dickinson (1953), Foster and Whalen (1966), Fertl (1976), Fertl and Chilingarian
(1977), Dobrynin and Serebryakov (1978, 1989), Magara (1982), Buryakovsky et al.
(1986b), Aleksandrov (1987), Buryakovsky et al. (2001), Chilingar et al. (2002), etc.
The system of abnormally high formation pressure is presented in Fig. 3.11.
The ability to predict, locate, and evaluate overpressured formations is critical in
drilling and completion operations, and in developing exploration and reservoir
engineering concepts. Although improved during the last decade, the overpressure
prediction methods are still far from being perfect. This has been identified as one of
the challenges of geoscience technologies. For predicting AHFP, the following
geophysical and drilling data are used:

3.2.1. Well-logging data
Well-logging techniques in studying formation pressures were discussed in detail
by Buryakovsky et al. (1986b). Overpressure can be calculated from the resistivity
logs. This method involves the study of shales and sands separately, and then
correcting the shale resistivity for the formation temperature. The temperature
correction is based on an empirical relationship derived for the region or area under