Page 14 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 9

the hydrocarbons will eventually migrate all the way to the surface.
Where an impermeable barrier is present, hydrocarbons become trapped.
Although the hydrocarbons displace formation water as they migrate,
some residual formation water remains in place. Hydrocarbon traps exist
where permeable reservoir rocks have overlying low permeability forma-
tions. A formation that prevents upward migration of the hydrocarbons
is known as a caprock. These are often compacted shales, evaporites,
or tightly cemented sandstones. There are two main categories of trap,
structural and stratigraphic.
Structural traps hold hydrocarbons because the formation has been
folded or faulted in some way. Structural traps include domes, anticlines,
and sealing faults. The most common is an anticline, accounting for
approximately 75% of all reservoirs traps. Fault traps are rare, making up
only about 1% of reservoirs. They form when faults seal the hydrocarbon
zone. A salt dome or salt diapir forms when salt of a lower density than
the surrounding formation plastically deforms the surrounding formation
as it “flows” upward towards the surface. The deformed structure creates
a trap for hydrocarbons.
Stratigraphic traps are depositional. The hydrocarbon reservoir forms
in place, usually when sandstone or limestone is covered by impermeable
shale. In some, both strata and structure will combine for create a forma-
tion (Fig. 1.4).

1.1.3.3 Porosity and permeability
To form a commercially viable hydrocarbon reservoir, sedimentary rocks
must exhibit two essential characteristics.

Figure 1.4 Hydrocarbon traps.