EXPLORATION TECHNIQUES
We need to know the broad geographical areas in the world where these sedimentary rocks may be found. This gives us a first clue where should we drill.
Sedimentary Basins
Sedimentary basins or geographical areas, where large quantities of sedimentary rock are found, will be located where old river systems have deposited large quantities of sediment into ancient seas. The location of these seas is well known by geologists, based on a wide range of other evidence.
Since oil companies want to know where to drill the first well precisely. Therefore we require other techniques which will pinpoint more accurately the most favorable location for drilling.
Exploration Techniques – Large Scale
Land
Geologists have found that they can sometimes identify subsurface structures like faults and domes by viewing ground contours at the surface. Aerial photographic surveys are a most effective means of gaining this broad impression.
The use of 3-dimesional photography increases the ease with which surface characteristics can be picked out from aerial photographs.
Nowadays, in addition photograph from orbiting satellites are increasingly used for this type of exploration work.
Aerial surveys are often accompanied by a field surveys. Geologists on the ground Investigate in more detail.
Exploration Techniques - Small Scale
At this stage, explorers for oil would want some confirmation that what they saw on the surface actually indicated a trap type structure underground.
We will look at three common methods for obtaining this information:
• Gravity surveys
• Magnetic surveys
• Seismic surveys
Gravity surveys
Gravity surveys, as the name suggests, measure three differences in the pull of gravity from one underground rock layer to another. Dense rocks in Figure 1.4 will have a higher gravitational pull. A cap rock will be present in some petroleum reservoirs and, as this may well be denser than the underlying reservoir rock, a gravity survey could be a useful tool to assess the results.
Magnetic Surveys on Land
In this type of survey, a magnetometer is used to measure the strength and direction of the local ground magnetic force. The presence of underground rocks containing, particularly, iron will distort the normal pattern of the earth’s magnetic field in that area.
This distortion can be measured and recorded to give a picture of structures containing magnetic materials.
Seismic Surveys
The word seismic means relating to earthquakes and this gives a clue to the principle of this technique. A shock wave is set off at the surface and travels down through successive rock layers. At the boundary of each one, part of the wave energy is reflected back to the surface where it is picked up by sensitive instruments called geophones and or hydrophones (on water).
The shock waves themselves can be generated in a variety of ways. For example:
On land:
• By explosive charge
• By thumper truck, this drops a heavy weight onto the ground
• By vibrator, where a vibrating surface is kept in contact with the ground
Over water:
By compressed charge, which offers less threat to marine life than explosives in both cases, the shock waves travel through different types of rock at different speeds and therefore will arrive back at the surface at different times. It obviously takes a computer to analyze the large number of signals being received at the surface, but a remarkably accurate picture can be built up of subsurface formations in terms of both shape and physical characteristics.
Conclusion:
You will notice that all of the techniques described are carried out on or above the surface, although they do give us some good indications of what lies below.
When the drilling site is selected and the well is actually being drilled, however, we find ourselves directly in contact with deeper and deeper subsurface rock formations.
This gives us new opportunities for looking at, and getting information from, these rock layers.
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