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is a structure formed when the layers of rock have been buckled
upward.
In oil exploration, it is the job of the geologist and
geophysicist to find these geologic structures which are potential
hydrocarbon traps and thereby recommend drilling locations. We
speak of "potential" hydrocarbon traps because not all structures
contain hydrocarbons. Many structures contain no reservoir rock,
and so no oil or gas can accumulate. Many contain excellent
reservoir rocks, but are full of water. In other structures, the lack of
a good sealing layer prevents oil and gas from accumulating. In
other areas, the lack of a good source rock could be the problem.
The only way to know for sure if oil or gas is present in a
structure is to drill a well.

Porous and Permeable Rocks
We have already established that petroleum is found by
drilling holes into the ground. Our curiosity must now lead us to
wonder what is going on down underground that allows us to drill
a hole and suddenly have this "black gold" come flowing
conveniently to the surface for our use. Are we drilling into
underground rivers or lakes of oil? Caverns filled with natural gas?
We will begin to answer these questions in this chapter. When we
reach an understanding of where the oil comes from, it will
become apparent why oil is found in some areas and not in others.
This information can then be used in the search for new places to
drill for petroleum.

Three Types of Rocks
Before we proceed to discuss where oil is found in the
subsurface, we must become familiar with some new terminology;
namely, the three types of rocks; igneous, sedimentary and
metamorphic.
Igneous rocks are rocks that have solidified from a molten
or liquid state. These rocks can be formed deep in the earth or at
the surface from the cooling of volcanic lava. Igneous rocks do not
normally contain hydrocarbons.

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