Page 232 - Geology and Geochemistry of Oil and Gas

VERTICAL ZONATION OF HYDROCARBON ACCUMULATIONS 201

Fig. 10.13. Cross-sectional view of an anticlinal reservoir sandwiched between two impervious shale
bodies, showing abnormal pressures in hydrocarbon accumulation in hydrostatic water environment.
3
3
g 0 ¼ specific weight of oil (e.g., in lb/ft ); g w ¼ specific weight of water (e.g., in lb/ft ); z ¼ elevation (e.g.,
2
2
ft); p ¼ pressure (e.g., in lb/ft ); overpressure ½Dp ¼ ðp o1 p w1 Þ ¼ ðg w g o Þðz 2 z 1 ފ in lb/ft (modified
after Hubbert and Rubey, 1959, p. 150; and Gretener, 1969, p. 267; in: Rieke and Chiligarian, 1974, Fig.
264, p. 310).
90–1001C (South Caspian Basin). Rough estimates, however, indicate that the tem-
perature within Zone 3 is above 100–1201C and may increase to 160–1801C within
Zone 4, and higher in Zone 5. The vertical zonation controls the possible depth of
occurrence of crude oils with a certain phase composition and corresponding pres-
sures (normal and abnormal) and the temperature, hence the oil and gas properties
and the patterns of their change up the section.
Relationship between the pressure and depth is presented in Fig. 10.14. There are
three different scenarios of the reservoir pressure change with depth:
(a) Reservoir pressure changes according to the hydrostatic pressure gradient
(Line I).
(b) Within the intercommunicating accumulations, according to the surplus pres-
sure gradient (Line II, which is always steeper than Line I).
(c) Abnormally-high formation pressure (AHFP) (Line III, which is always flatter
than Line I).