Page 35 - Drilling Technology in Nontechnical Language

26 Drilling Technology in Nontechnical Language Second Edition

Fig. 2–2. Types of structures that can form a trap

A trap may be formed by movements within the earth’s crust that
deform the rock. One type of structural trap is an anticline, which has
a structure like an inverted cup. Thick salt deposits can flow under high

pressures, and sometimes a bubble of salt rises from the salt bed to form a
salt dome. Salt domes distort the rocks above them and can provide large
structural traps suitable for a reservoir. Reservoirs may also be formed by
permeable beds that are tilted by crust movements, eroded after exposure
to the surface, and buried by new layers of impermeable rock deposits. This
is called an angular unconformity. Figure 2–2 illustrates these structures.

Reservoir Rock

In a reservoir, the gas, oil, and water are found in pore spaces or
fractures within the rock matrix. Most reservoirs worldwide are contained
in sandstone structures that have sufficient porosity to give a good volume of

reserves and a high enough permeability to be able to produce it. However,
limestones (carbonates) with fractures and/or pits (due to water dissolving
some of the material) can give extremely high porosity and permeability.
Some limestone reservoirs originate from coral reefs, and these also will
tend to be prolific reservoirs, with high porosity and permeability. In the

United States, around 80% of reserves are held in sandstone reservoirs and
20% in carbonate reservoirs. In the Middle East, almost 100% of reserves
are in carbonate reservoirs.

Reservoir permeability ranges from a couple of hundred millidarcies
to 15 darcies or more. The more permeable a reservoir, the faster the
hydrocarbons within it can be produced.

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