Page 235 - Geology and Geochemistry of Oil and Gas

VERTICAL ZONATION OF HYDROCARBON ACCUMULATIONS 203

Fig. 10.14. Three major patterns of variation of formation pressure with depth. 1 – Hydrostatic pressure
3
2
gradient (r w ¼ 1 g=cm ; 62.4 lb/cu. ft.; 0.433 psi/ft). 2 – total overburden pressure (1.0 psi/ft; 0.23 kg/cm /
3
m). I – Hydrostatic pressure gradient (r w ¼ 1:25 g=cm ); II – pressure change in communicating accu-
mulation at overpressure; and III – abnormally high formation pressure. (After Eremenko and Chilingar,
1996, p.126.)
In the intercommunicating gas accumulations (e.g., massive accumulation of the
Shebelinka Field, Ukraine), at r 0 the Line I will be vertical. An increase in the
AHFP along the Line III will result in the formation of fault-block reservoirs with
different AHFP values.
From the locations of points in the graph, the connectivity of the productive
horizons can be determined.
There are some examples (e.g., East Siberia) of abnormally low formation pres-
sure (ALFP). These pressures are lower than the hydrostatic (Chilingar et al., 2002).
Water in some parts of such formations is found in solid state (due to the perma-
frost), whereas in some other areas the water is not present at all. In the absence of
water (ice instead of water), there is no hydrostatic piezometric surface.