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rock and up into the well. When the well flows, gas, oil and water
is extracted, and the levels will shift as the reservoir is depleted.
The challenge is to plan the drilling so that the reservoir utilization
can be maximized.
Seismic data and advanced visualization 3D models are
used to plan the extraction. Still the average recovery rate is 40%,
leaving 60% of the hydrocarbons trapped in the reservoir. The best
reservoirs with advanced Enhanced Oil Recovery (EOR) allow up
to 70%. Reservoirs can be quite complex, with many folds and
several layers of hydrocarbon bearing rock above each other (in
some areas more than 10). Modern wells are drilled with large
horizontal offsets to reach different parts of the structure and with
multiple completions so that one well can produce from several
locations [1].

Petroleum traps
In order for petroleum to accumulate in commercial
quantities, it must, in its migration process, encounter a subsurface
rock condition that halts further migration and causes the
accumulation to take place. These subsurface conditions are
numerous in type ranging from very simple to extremely complex
forms. Numerous systems of tap classification exists where the
following is an example.
Anyone who has ever seen oil spilled in a water puddle will
notice that it creates colourful patterns in the sunlight. This is
because the oil, which is less dense than water, forms a separate
layer which actually floats on the surface of the water. The
different light refracting properties of the two layers create a prism
effect, and hence, the colour patterns.
Droplets of oil in rocks buried deep underground will also
float above the water that is also present within these rocks. Hence,
these rocks must still contain some water. So, instead of running
downhill as surface water does under the force of gravity, oil
droplets in the subsurface tend to move upward, under the force of
buoyancy, so as to float above the water that shares the same pore
spaces. Driven by buoyancy, these oil droplets migrate upwards
toward the surface through pores and cracks within the layers of

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