Page 272 - Applied Petroleum Geomechanics

Abnormal pore pressure mechanisms 263

Figure 7.22 Measured pore pressures (p p ) from drill stem test (DST) and repeat for-
mation tester (RFT) in four wells in the Venture field, Canada (plotted based on the
data presented by Ervine and Bell, 1987). The hydrostatic pressure (p n ), measured leak-
off pressure (LOP), the minimum horizontal stress (S h ), and calculated maximum
horizontal stress (S H ) are also plotted.

hinge zone (Yassir and Bell, 1994). Most of the research on overpressures in
the Scotian Shelf has concentrated on the Venture field, where the
overpressures begin at approximately 4500 m, as shown in Fig. 7.22. Pore
pressure increases in steps, reaching a value close to the lithostatic stress (S V
in the figure) in deep formations. Overpressures occur in high-porosity
sands interbedded with shales. The shales act as seals to overpressured
fluids (Wade, 1991; Drummond, 1992). Disequilibrium compaction for
abnormal pressure generation is ruled out because of the low sedimentation
rates in the region. Gas generation is a key cause of overpressuring because
the overpressured zone is related to a sharp increase in vitrinite reflectance
tested in some wells. Additionally, Wade (1991) noted that the top of

overpressure corresponds with the 130 C isotherm (related to hydrocarbon
maturation).
Fig. 7.22 also shows that there is a consistent relationship between
overpressures and in situ stresses in the basin. The minimum horizontal
stress increases dramatically starting from the top of overpressure at w4500 m.
This implies that contemporary stresses in these sediments are causally
related to overpressuring. The maximum horizontal stress is greater than the