Page 161 - Fundamentals of Gas Shale Reservoirs

OVERPRESSuRE-GEnERATInG mEchAnISmS 141
mechanisms (fluid expansion), for example, hydrocarbon is when the rate of sedimentation is faster than the rate at
generation, clay transformation, and aqua‐thermal heating. which the pore fluids are able to escape. Therefore, the
most researchers have observed that the under‐compaction pore fluids are trapped within the pore spaces and the
(compaction disequilibrium) accounts for the majority of porosity would be greater than it should be in normal com-
overpressure situations that were encountered in sedimentary paction circumstances. As a result, the formation becomes
rocks. In under‐compaction situations, overpressure is a overpressured due to the lack of conduits between the pore
result of the rapid loading of sediments with a lack of spaces and the overlaying formations (Eaton, 1975;
communication between the pore fluids and the overlaying Wallace, 1965). The main difference between overpres-
sediments. hence, pore fluids are trapped and become sured formations caused by under‐compaction and nor-
overpressured (Osborne and Swarbrick, 1997). The main mally pressured ones is that, in overpressured formations,
overpressure‐generating mechanisms are discussed in detail the pore fluids no longer have efficient communication
in Sections 7.2.1 and 7.2.2. with the water table.

7.2.1.2 Lateral Tectonic Loading lateral tectonic
7.2.1 Loading Mechanisms
loading causes an increase in lateral stress as a result of com-
loading mechanisms involve increases in compressive paction of the sediments horizontally in addition to the
stresses. loading mechanisms include under‐compaction vertical compaction caused by an increase in overburden
(compaction disequilibrium) where the sediments com- stress. The lateral stress associated with vertical stress causes
pact vertically and also include lateral loading (tectonic overpressure if the pore fluids are not squeezed out by the
compression) where the sediments compact horizontally in compaction (Van Ruth et al., 2003). Another example of
tectonically active areas. overpressure generated by tectonic compression is when a
fault moves; the fault plane separates and lets the high
7.2.1.1 Under-Compaction (Compaction Disequilibrium) pressure zones communicate with the surrounding lower
In normal sedimentary environments, sediments compact pressure sand bodies (Fig. 7.3). however, when the fault
and lose porosity as a result of an increase in the effective closes, the charged sand releases its pressure into surrounding
stress (grain to grain contact). normal compaction creates shales and develops overpressure (Osborne and Swarbrick,
efficient communication between the pore spaces and the 1997). unlike compaction disequilibrium, lateral tectonic
water table; and hence, some of the pore fluids are squeezed loading can generate high magnitude of overpressure that
out as a result of a normal increase in overburden pressure. may cause the vertical effective stress to decrease. This is
Therefore, a normal pore pressure regime is established. attributed to the fact that in tectonically active areas, com-
This pressure trend can be defined by the hydrostatic paction is not controlled only by vertical effective stress
pressure of the water that is contained in the pores (draou (Bowers, 2002).
and Osisanya, 2000). however, in many geological set-
tings, compaction is hindered where many mechanical and 7.2.1.3 Wireline Logs’ Response to Loading Mechanisms
geological variables that preclude the compaction process The responses of wireline logs to overpressure generated by
lead to pore fluids becoming overpressured. The ideal envi- disequilibrium compaction are a constant transit time and a
ronment for overpressure generated by under‐compaction constant density (Ramdhan and Goulty, 2011). The effective stress

8,000 ft
Overpressure Shale 10,000 ft
4,650/8,000 = 0.58 (psi/ft)
Normal pressure
Sand Shale 0.465(psi/ft ×10,000 (ft)
= 4,650 psi

Sealing fault Sand

FIGURE 7.3 Graphic illustration of overpressure generated by lateral tectonic compression.