Page 59 - Reservoir Geomechanics

43 Pore pressure at depth in sedimentary basins

Pressure/Stress

SHALE
Top

Centroid OVERBURDEN
SAND Depth HYDROSTATIC

Bottom

COMPLETELY SEALED ss
Sand Pres. (Pp )
sh
Shale Pres. (Pp )
Figure 2.12. Illustration of the centroid effect where the tilting of a sand body encased in a low
permeability shale results in a higher pore pressure at the top of the sand than in the shale body at
the equivalent depth (from Finkbeiner, Zoback et al. 2001). AAPG C 2001 reprinted by permission
of the AAPG whose permission is required for futher use.

the sand body is higher than that in the adjacent shale at the same elevation. Pressure in
very weak shales is presumed to increase with a lithostatic gradient (as below 11,000 ft
in Figure 2.2). The depth at which pore pressure is the same in the two bodies is referred
to as the centroid. This concept was first described by Dickinson (1953), and expanded
upon by England, MacKenzie et al.(1987) and Traugott and Heppard (1994). It is
clear that drilling into the top of a sand body with pore pressure significantly higher
than the adjacent shale poses an appreciable drilling hazard. Finkbeiner, Zoback et al.
(2001) discussed centroid effects in the context of pressure-limited column heights in
some reservoirs (in contrast with column heights controlled by structural closure or
sand-to-sand contacts). We revisit this topic in Chapter 11.
Aquathermal pressurization refers to a mechanism of overpressure generation stem-
ming from the fact that as sediments are buried, they are heated. Temperature increases
with depth in the earth due to heat produced by radioactive decay of crystalline base-
ment rocks and heat flowing upward through the crust from the mantle. Because heating
causes expansion of pore fluid at depth, in a confined and relatively incompressible rock
matrix, expanding pore fluid pore would lead in theory to pressure increases. The reason
aquathermal pressure increases are not, in general, thought to be a viable mechanism
of overpressure generation in most places is that the time-scale for appreciable heating
is far longer than the time-scales at which overpressures develop in active sedimentary
systems (Daines 1992; Luo and Vasseur 1992) such that a near-perfect seal would be
required for long periods of geologic time.
Dehydration reactions associated with mineral diagenesis have been proposed as
another mechanism that could lead to overpressure development. Smectite dehydration