Who finds petroleum and where?

Finding petroleum was once a job for prospectors who dug wells in places that they thought would be suitable - perhaps a line of hills or a swamp. So they drilled a hole there and it was known as a “wild-cat” well. But the demand for petroleum became too great to rely on guesswork. Wild­cat prospectors had to give way to specialist scientists. These scientists are geologists and geophysicists.

Geologists study rocks and soils in the layers of the earth’s crust. Many geologists are specialists. Some are interested in sedimentary rocks. These are called sedimen­tologists. Others study fossils. They are palaeontologists.

Others specialise in the chemistry of rocks and the fluids they con­tain. These are geochemists. They work closely with geophysicists in the search for petroleum. Geophysicists study and measure the physical phenomena of the earth. They measure temperature, magnetism, earth movements, water flow, assess earthquakes. Their measurements are used for three purposes:

• First, to improve our knowledge and understanding of the earth (what it is and what makes it tick).
• Second, to find out what is the nature of the earth for the construction of tunnels, buildings, power stations and other structures.
• And third, to explore the earth for petroleum, natural gas and minerals, and to establish whether these are suitable for commercial exploitation.

To make their measurements, they “read” gravity, electricity, light, shock waves, sound, heat and radioactivity. They put their instru­ments on the surface, underground, in water, in the air - or all four. They also produce vibrations in the earth using vibrat­ing machines or compressed air “guns” in the ocean. Recordings of the transmitted and reflected vibrations will “tell” them what sort of rocks there are. These are called seismic surveys. (“Seis­mology” is the study of earthquakes).

We know that almost all of the world’s oil has been found in organic sedimentary rocks. For more information , go to:
www.windows.ucar.edu

Geophysicists can tell petroleum explorers where these large areas of sedimentary rocks (sedimentary basins) are located. They do this by measuring the gravitational force and the earth’s magnetic field at the earth’s sur­face. Areas of lower magnetic field strength and lower gravitational force are areas underlain by sedimentary rocks. There are a number of Australian sedimentary basins.

There are two seismic methods to detect sedimentary rocks: reflection and refraction. In both methods, shock waves pass through the rock structure and are recorded on a device called a geophone. When their underground maps are ready, geophysicists hand them over to petroleum engineers, because the only sure way to tell if the petroleum is actually there is to drill. Tests wells are drilled, to bring up samples of rock. These are studied for evidence of petroleum.

Petroleum geophysicists look for sediments. But they know that petroleum often has migrated from the place where it was formed. So they look for the non-porous rocks that have “trapped” petroleum. When oil exploration wells are drilled, a range of “down hole” measurement tools can detect more information from the well, such as rock density, radioactivity, magnetic strength and porosity. Geophysicists can “read” the differences because their instru­ments “behave” in different ways for each. Similarly, differences be­tween coal, water and petroleum can be detected. So geophysicists look for “con­trasts” and these differences tell them about ore bodies and reefs; or ancient earth movements; or strange rocks that earthquakes have shoved into other rocks.

If petroleum is there, how much? Can it be recovered? How much can be recovered? If oil or gas is produced from a well, its pressure and flow rates are accurately measured. Most of the wells that are sunk are dry. Either no petroleum or gas is found, or the amount is so small it is not worth getting out. The search never stops!

For more information about petroleum drilling and production, go to:
www.doir.wa.gov.au
http://www.spe.org

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