Hydrocarbon deposits

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Hydrocarbons (oil and natural gas) were formed millions of years ago and now lie deep below the surface of the earth. They get their name from their molecular composition - hydrogen and carbon - and they are the most important non-renewable energy sources in use today. Hydrocarbons are known to have been used in China as early as the fourth century BC, but commercial exploitation as we know it today only began in the nineteenth century AD.

Origin

Sedimentation

They were formed over millions of years through the decomposition of vast amounts of organic debris (vegetal and animal) which had accumulated on the bottom of lagoons and on sea beds. Successive layers of sediments gradually covered these accumulations of organic matter until they reached great depths.

Migration

Migration The accumulated organic matter was subjected to high temperatures and through the effect of bacteria formed a type of rock capable of generating hydrocarbons. This is known as the "source rock".

The hydrocarbons generated by the source rock migrate to impregnate porous rocks, such as sandstones, in much the same way as water impregnates a sponge. If these formations are sealed off by strata of impermeable rocks, they form a geological trap which stops the oil or natural gas from escaping. This is how hydrocarbon deposits form.

Accumulation of hydrocarbons
In a single deposit it is possible to find gas, oil and water, and different combinations thereof.
Seal
Impermeable rocks such as clay, which prevent the hydrocarbons from escaping.
Reservoir
Porous rock capable of containing hydrocarbons. Hydrocarbons are not stored under the earth's surface in the form of lakes or pockets: instead they are contained in certain porous rocks.
Source rock
Rock with organic matter capable of generating hydrocarbons.

Traps

A distinction is drawn between two general types of hydrocarbon traps:

Structural Traps

In structural traps, the hydrocarbons accumulate as a result of the geometry of the strata: folds and/or faults.

Anticlinal Trap

The pressure pushes the hydrocarbons towards an anticline formation, where they become trapped.
Fault Trap
The hydrocarbons accumulate in a raised structural area, which is a result of faulting in the rock.

Stratigraphic Traps

Stratigraphic Traps Traps created by changes in the porosity of the rock.

Stratigraphic traps are generally caused by lateral and vertical changes in the porosity of the rock, i.e. when the continuity of a porous rock disappears.

Exploration

Two basic types of technique are used to search for hydrocarbons:

Suface exploration:

Geological Method
Amongst other methods, geologists use aerial photography, maps and geological sections to analyse the types of rock and apply this information to the underground geology.

Underground exploration:

Seismic Method
This consists of transmitting and receiving sound waves from the surface and logging the results in terms of frequency, amplitude and waveshape to give information on the structure, physical properties and depth of the underground rocks.

Gravimetric Method
Analyses the differences in gravity due to the density of the rocks.

Well
This consists of drilling vertically from the surface to gain access to underground materials and obtain samples of them, and to record the physical properties of the rocks drilled. It also makes it possible to verify directly whether hydrocarbons exist.

Drilling

Type

Deposits are found off-shore and on-shore. In both cases the drilling system used is the same, although drilling in the sea is very much more costly. The drill depth varies between approximately 900 and 5000 metres.

Once the deposit has been drilled, the hydrocarbons rise to the surface as a result of the internal pressure to which they are subjected. When this pressure falls, pumping systems are used to increase the recovery rate.

In off-shore facilities the derrick is mounted on ships or platforms (fixed or floating). If the sea is not very deep the platform can be secured to the bed.

Equipment

Equipment The rotation system drills through the layers until it reaches the reservoir rock containing the hydrocarbons.

Swivel
Flexible tube through which the drilling mud is pumped.
Pumps
Pumping tools which circulate the mud.
Vibrators
Tools which separate the mud from the fragments of rock extracted during drilling.
Mud box
The area where the mud is prepared before it is pumped into the well.
Safety Valve
Mechanism which prevents gas eruptions.
Drilling mud
As the rock is drilled, a sludge-like fluid is pumped in to remove the drilling residues, lubricate the drill head and shore up the walls of the well.
Drilling tools
Consist of a drill pipe joined to a rotating head. The entire unit is lubricated with mud.
Drilling bit
Rotating cutting tool which is screwed onto the bottom end of the drill pipe. It is made of enormously resistant materials, generally alloys such as steel.