Facts, Figures and Explanations The Search for Oil and Gas Search History Click on the thumbnail to view a larger image The formation and accumulation of crude oil and gas has occurred at different times throughout the history of the Earth, so how do we know where to find oil today? In the early days of commercial oil exploration, around 150 years ago, prospectors drilled around the areas where oil seeped to the surface, trying to find the underground source of the oil. They also drilled into domed structures hoping to find oil traps. Sometimes they were lucky but often they found nothing. The first well to be drilled specifically to locate oil was in Titusville, Pennsylvania in 1859. Since then an estimated two million wells have been sunk in the search for oil. Today, drilling for oil is an extremely expensive business and has to be approached scientifically. Through science we have discovered how oil and gas were formed, but how do we know where to start looking for the particular rock formations that are most likely to contain these precious hydrocarbons? How does the search begin? Rock structures that contain oil or gas are hidden from view by the sea or other rock layers but we can't just go around drilling lots of holes and hoping to strike lucky. We have to be more precise and start by doing a full survey of the area. This is where geologists and geophysicists, the scientists who study rocks and their formations, are really valuable. A geologist looks at the makeup of rocks - where they are and the formations they make. A geophysicist, as the name implies, uses the physical characteristics of rocks - their magnetic and gravitational properties, and very importantly, how sound waves travel through different kinds of rocks - to help understand the structures below the Earth's surface. Identifying the Rocks There are clear differences between igneous rocks, which were formed when hot, molten magma from the Earth's interior cooled and solidified and sedimentary rocks. Sedimentary rocks were formed when small worn particles of sediment were compressed together - like the sandstone described earlier or the muddy sediment that sank to the bottom of the sea at the same time as the organic material which went on to form oil and gas. Gravity and magnetic surveys We know that soft, sedimentary rocks that may contain oil or gas are less dense than the heavy igneous rocks which contain a lot of magnetic minerals. These differences can be detected by remote sensing surveys carried out from ships, that make no direct contact with the rock structures beneath the sea. Aerial photography can give useful basic information. Aircraft can also be used to measure accurately the gravitational pull of the Earth over an area. Small differences in the gravitational pull are caused by variations in the density of the underlying rocks. Because some rocks are more magnetic than others, we can also measure variations in the strength of the Earth's magnetic field. So different surveying techniques can be used to locate the particular rock formations that might contain trapped oil. Geologists and geophysicists can build up a detailed picture of the rock structures, even though these are often many hundreds of metres beneath the Earth's surface using the results from these techniques. Surveying Click on the thumbnail to view a larger image General survey techniques cover large areas quite quickly and easily and help to build a broad picture of the underlying rock structures. If the results are encouraging, and a promising area located, a second stage of explorations is undertaken and a more detailed seismic survey is carried out. Seismic survey A seismic survey involves firing pulses of sound energy down through the layers of rock beneath the Earth and recording the energy that is bounced back by the rock layers below the surface. The way in which the sound energy travels back depends on the kinds of rocks and the structures they form. It's a bit like bouncing a ball. As an experiment try bouncing a tennis ball on different surfaces. (Preferably not glass windows!) You will get a different bounce on the playground, grass, sand or against a sloping surface. Seismic surveys at sea In the United Kingdom most of the oil and gas that is produced has been discovered under the sea -offshore. To do a seismic survey at sea, the survey ship fires blasts of compressed air into the water. A long cable with receivers called hydrophones, which are sensitive listening devices, like microphones, is towed behind the ship to detect the echoes from the layers of rocks beneath the seabed. A computer is used to measure and record the echoes, or pulses, that are reflected back to the surface. The geophysicists then have to interpret the data, which is displayed as a series of wavy lines, to build up a detailed picture of the rock formations. Seismic data can now be shown as a three dimensional picture using the latest computer technology. The importance of environmental assessment Before a seismic survey at sea is carried out several things have to be considered. One of these is the effect on birds, fish, and mammals - like whales or dolphins, plant life, and other creatures that live on the seabed. A full environmental assessment is therefore carried out before any seismic survey is started. This involves making a detailed study of the site to see, for example, whether whales migrate through the area and if so, at what time of year. We also need to know the feeding habits of local sea birds and make a record of the types of shellfish that live on the seabed. In fact everything that could possibly be affected by our activities is examined. Marine biologists do this scientific work. Other groups and organizations are also consulted and involved, for example local fishermen and the Royal Society for the Protection of Birds (RSPB). At the end of this process an environmental report is produced. How the environmental report can affect exploration The environmental report shows how damage to a site will be avoided. This might mean that work is not carried out during: certain hours of the day when seabirds are feeding; certain months of the year because fishermen are catching lobsters; or at certain breeding or migrating seasons. Particular areas may be avoided entirely because there is some rare plant or animal life on the seabed. Facts, Figures and Explanations Index Next Page

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