Page 36 - Hydrocarbon Exploration and Production Second Edition

Exploration 23

in pressure, temperature or acidity of pore fluids. Sandstone reservoirs form after the
sand grains have been transported over large distances and have deposited in
particular environments of deposition.
Carbonate reservoir rock is usually found at the place of formation (in situ).
Carbonate rocks are susceptible to alteration by the processes of diagenesis.
The pores between the rock components, for example the sand grains in a
sandstone reservoir, will initially be filled with the pore water. The migrating
hydrocarbons will displace the water and thus gradually fill the reservoir. For a
reservoir to be effective, the pores need to be in communication to allow migration,
and also need to allow flow towards the borehole once a well is drilled into the
structure. The pore space is referred to as porosity in oil field terms. Permeability
measures the ability of a rock to allow fluid flow through its pore system. A reservoir
rock which has some porosity but too low a permeability to allow fluid flow
is termed ‘tight’. In Section 6.1, Chapter 6, we will examine the properties and
lateral distribution of reservoir rocks in detail.

3.1.7. Traps
Hydrocarbons are normally of a lower density than formation water. Thus, if no
mechanism is in place to stop their upward migration they will eventually seep to
the surface. On seabed surveys in some offshore areas we can detect crater-like
features (pock marks) which also bear witness to the escape of oil and gas to the
surface. It is assumed that throughout the geologic past, vast quantities of
hydrocarbons have been lost in this manner from sedimentary basins.
There are three basic forms of trap as shown in Figure 3.5. These are

Anticlinal traps which are the result of ductile crustal deformations.
Fault traps which are the result of brittle crustal deformations.
Stratigraphic traps where impermeable strata seal the reservoir.
In many oil and gas fields throughout the world hydrocarbons are found in fault
bound anticlinal structures. This type of trapping mechanism is called a combination trap.
Even if all of the elements described so far have been present within a
sedimentary basin an accumulation will not necessarily be formed. One of the

HCWC

Anticline Fault Stratigraphic

Figure 3.5 Main trapping mechanisms.