Exploration

Oil is found in porous rock forming large sedimentary basins where the oil and gas has been trapped by some kind of barrier. This forms an oil reservoir. Locating an oil 'trap' - as it is known - and extracting the oil in it is a highly technical and expensive operation.

Earth scientists in the petroleum industry - including geologists, geophysicists, geochemists and palaeontologists - study what has happened to rocks that may be buried thousands of metres below the surface. They examine how those rocks were formed and affected by events stretching back millions of years, and how to identify traps where oil and gas have accumulated within rock formations.

Geophysicists use a seismic survey to lay out a line, or several lines, of sensitive receivers, called geophones or jugs, on the ground. Then explosions or mechanical vibrations are created on the surface. The geophones record the energy reflected back as seismic waves from rock layers at various depths.

To reduce environmental impact, many geophysicists today use the mechanical vibroseis method to send energy waves from a heavy, vibrating vehicle into the earth. Like the rest of the petroleum industry, explorers have adopted ever higher environmental standards for seismic surveys. For example, improved methods for clearing and using narrower cutlines have reduced the impacts on soil, water, plants and wildlife in forested areas. In other sensitive areas such as mountainous terrain, seismic crews use helicopters or even packhorses to limit surface disturbance.

In offshore exploration "air guns" using compressed air have replaced dynamite as a better, safer energy source that also minimises the impact on marine life. A marine vessel records the reflected energy from a towed array of hydrophones. In shallow waters, the hydrophones may be laid out on the seabed.

Drillers then turn theory into hard economic reality. Even when a development well is located right between two producing wells, there is still a risk that nothing will be found - and also the possibility of greater than expected success. The stakes are much higher when the well is a wildcat in unexplored territory. The basic drilling process is simple. A revolving steel bit at the bottom of a string of pipe grinds a hole through the rock layers.

There are many different types of rigs. The smallest are service rigs mounted on trucks, while the largest are installed on ships or offshore platforms. Some are specially equipped for sour gas exploration, Arctic operations, slanted holes, or horizontal drilling. As a general rule, the bigger the rig, the deeper it can drill.