do you begin?
wildcat exploration effort, which is
what you are doing when you drill into an area that has not yet been
explored for oil, can be helped by 140 years of geologic experience
with finding oil.
a wildcatter doing oil exploration, you are looking for areas where
oil will be trapped in a permeable layer underneath an inmpermeable
layer. Most rock layers are not flat sheets, and this means that the
oil will be concentrated in the high spots beneath the tilted layers.
Rock layers are bent and tilted by earth forces, including plate tectonic
forces and gravitational forces.
Since the common gasses in
an oil reservoir (methane, CO2) are lighter than oil and lighter than
water, they float on top of the oil which floats on top of the water.
If the hydrocarbons are trapped within a permeable rock layer by an
impermeable seal layer, any gasses will form a cap at the top of the
structure. High concentrations of
gas in a well suggest that you have penetrated the highest part of
the structure. You may find oil by drilling another well a little
to the side of the top of the structure.
There are many types of traps
and seals created by a variety of geologic events. Understanding their
formation helps target their potential as reservoirs.
How does seismic work?
As you explore for oil and gas, you would really like to slice into
the earth and take a look for what is down there.The information you
obtain by drilling is critical because it gives you direct information
about the presence of oil, gas, seals, and traps. However, it is expensive
and really only provides information about that single location. So
geologists have to use several techniques to create a mental slice down
into the earth. If you draw what you visualize, the image is called
a cross section.
Using a seismic survey, we
can generate a similar cross-section such as the one shown below. Waveforms
produced as seismic energy are reflected from rock layers in the subsurface,
and are plotted to approximate the image you would see if you sliced
into the earth.
By interpreting the seismic
information, you can begin to see the important characteristics
which define the location of reservoirs.
Examine the seismic cross-section
examples above and compare them to the geologic examples at the
top of this page. Note the basic structural similarities, then see
if you can figure out where oil or gas would most likely accumulate.
When you know that, you'll know the best places to search for oil