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surrounding beds. Disturbances in the earth shoved great blocks of
land upward, dropped them downward, and moved them sideways.
Wind and water eroded formations, earthquakes buried them, and new
sediments fell onto them. Land blocked a bay’s access to open water,
and the resulting inland sea evaporated. Great rivers carried tons of
sediment; then dried up and became buried by other rocks. In short,
geological forces slowly but constantly altered the very shape of the
earth. These alterations in the layers of rock are important because,
under the right circumstances, they can trap and store hydrocarbons.
Even while the earth changed, the weight of overlying rocks
continued to push downward, forcing hydrocarbons out of their source
rocks. Seeping through subsurface cracks and fissures, oozing through
small connections between rock grains, the hydrocarbons moved
upward. They moved until a subsurface barrier stopped them or until
they reached the earth’s surface, as they did at Oil Creek. Most of the
hydrocarbons, however, did not reach the surface. Instead, they
became trapped and stored in a layer of subsurface rock. Today, the
oil industry seeks petroleum that was formed and trapped millions of
years ago.
Petroleum Traps
A hydrocarbon reservoir has a distinctive shape, or configur-
ation, that prevents the escape of hydrocarbons that migrate into it.
Geologists classify reservoir shapes, or traps, into two types: structural
traps and stratigraphic traps.
Structural Traps
Structural traps form because of a deformation in the rock
layer that contains the hydrocarbons. Two examples of structural traps
arc fault traps and anticlinal traps.
Fault Traps

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