Geoscience Reference
In-Depth Information
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Introduction
If you want to find oil and gas accumulations, or produce them efficiently once found,
then you need to understand subsurface geology. At its simplest, this means mapping
subsurface structure to find structures where oil and gas may be trapped, or mapping
faults that may be barriers to oil flow in a producing field. It would be good to have
a map of the quality of the reservoir as well (e.g. its thickness and porosity), partly to
estimate the volume of oil that may be present in a given trap, and partly to plan how
best to get the oil or gas out of the ground. It would be better still to see where oil and
gas are actually present in the subsurface, reducing the risk of drilling an unsuccessful
exploration well, or even following the way that oil flows through the reservoir during
production to make sure we don't leave any more of it than we can help behind in the
ground. Ideally, we would like to get all this information cheaply, which in the offshore
case means using as few boreholes as possible.
One traditional way of understanding the subsurface is from geological mapping at
the surface. In many areas, however, structure and stratigraphy at depths of thousands
of feet cannot be extrapolated from geological observation at the surface. Geological
knowledge then depends on boreholes. They will give very detailed information at the
points on the map where they are drilled. Interpolating between these control points, or
extrapolating away from them into undrilled areas, is where geophysical methods can
be most helpful.
Although some use has been made of gravity and magnetic observations, which re-
spond to changes in rock density and magnetisation respectively, it is the seismic method
that is by far the most widely used geophysical technique for subsurface mapping. The
basic idea is very simple. Low-frequency sound waves are generated at the surface by a
high-energy source (for example a small explosive charge). They travel down through
the earth, and are reflected back from the tops and bases of layers of rock where there
is a change in rock properties. The reflected sound travels back to the surface and is
recorded by receivers resembling microphones. The time taken for the sound to travel
from the source down to the reflecting interface and back to the surface tells us about the
depth of the reflector, and the strength of the reflected signal tells us about the change
of rock properties across the interface. This is similar to the way a ship's echo sounder
can tell us the depth of water and whether the seabed is soft mud or hard rock.
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