Page 57 - Reservoir Geomechanics

41 Pore pressure at depth in sedimentary basins

since the Pleistocene (the past ∼15,000 years) due to locally rapid erosion caused by
post-glacial uplift (Riis 1992; Dore and Jensen 1996).
It is relatively straightforward to understand the conditions under which compaction
disequilibrium will result in overpressure development. The characteristic time, τ, for
linear diffusion is given by

l 2 (φβ f + β r )ηl 2
τ = = (2.2)
κ k
where l is a characteristic length-scale of the process, κ ≈ k/(φβ f + β r )is the hydraulic
diffusivity, β f and β r are the fluid and rock compressibilities, respectively, φ is the rock
2
2
porosity, k is the permeability in m (10 −12 m = 1 Darcy), and η is the fluid viscosity.
For relatively compliant sedimentary rocks, equation (2.2)gives
log τ = 2log l − log k − 16 (2.3)

where τ and l are in years and kilometers, respectively. In low-permeability sands with
2
a permeability of about 10 −15 m (∼1 md), the characteristic time for fluid transport
over length-scales of 0.1 km is of the order of years, a relatively short amount of time in
2
geological terms. However, in low-permeability shales where k ∼ 10 −20 m (∼10 nd)
(Kwon, Kronenberg et al. 2001), the diffusion time for a distance of 0.1 km is ∼10 5
years, which is clearly sufficient time for increases in compressive stresses due to
sediment loading, or tectonic compression, to enable compaction-driven pressure to
build up faster than it can diffuse away.
Tectonic compression is a mechanism for pore pressure generation that is analogous
to compaction disequilibrium if large-scale tectonic stress changes occur over geolog-
ically short periods of time. Reservoirs located in areas under tectonic compression
are the most likely places for this process to be important, such as the coast ranges of
California (Berry 1973), or the Cooper basin in central Australia although changes in
intraplate stress due to plate tectonic processes can also lead to pore pressure changes
(Van Balen and Cloetingh 1993). In the northern North Sea offshore Norway, as well
as along the mid-Norwegian margin, Grollimund and Zoback (2001)have shown that
compressivestressesassociatedwithlithosphericflexureresultingdeglaciationbetween
15,000 and 10,000 years ago appear capable of explaining some of the pore pressure
variations in Figure 2.3, with higher pore pressures in areas of induced compression
and lower pore pressures in the areas of induced extension. Thus, along the Norwegian
margin there appear to be three mechanisms for generating excess pore pressure, two
mechanisms related to deglaciation – changes in the vertical stress due to recently rapid
sedimentation and increases in horizontal compression due to lithospheric flexure –
and hydrocarbon generation (discussed below). Interestingly, the two mechanisms asso-
ciated with deglaciation have resulted in spatially variable pore pressure changes over
the past few thousand years.