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Natural Gas Reservoirs
Natural gas, being less dense than either oil or water, tends
to float above water and oil in the subsurface.
Sometimes, the natural gas is dissolved in the oil, but it
very often forms a separate layer of its own which floats above the
oil layer. In such a case, we get a layer of natural gas floating upon
a layer of oil which is in turn underlain by a layer of water.

Geologic Structures Become Hydrocarbon Traps
Common sense suggests that these sedimentary layers will
tend to be laid down more or less horizontally over large areas. We
also kno w, however, that we need traps to provide areas in which
oil and gas can collect. This means that the layers of rock must
somehow become buckled and bent to provide the geologic
structures that may become traps. Fortunately for oil prospectors,
forces within the earth itself create these geologic structures. These
powerful forces that tend to stretch, squeeze, bend and break the
rock layers are the same forces that cause earthquakes. The
geologic structures that are created (ie. bends and breaks or faults
in the rock layers) can become hydrocarbon traps where certain
conditions are present:
1) There must be hydrocarbon source rock in the area.
Source rock is the fine grained rock in which the organic material
originally present has been converted into hydrocarbons.
2) There must be a porous and permeable rock layer to
provide a reservoir in which the hydrocarbons can accumulate.
3) There must be an impermeable sealing rock overlying
the reservoir rock to trap the hydrocarbons.
Figure 6 shows some examples of geologic structures that
commonly form hydrocarbon traps. Note that there are both
structural traps and stratigraphic traps. Stratigraphic traps are
traps that are formed by changes in the characteristics of the rock
formation such as a loss of permeability or porosity or a break in
continuity of a layer. Structural traps on the other hand are caused
by the bending or breaking of the sedimentary layer. The most
common and simplest type of structural trap is the anticline which

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