84- www. world- petroleum. org 6.1- Understanding oil and gas True, the first step in the exploration proc-ess is political and commercial, not techni-cal. Development permits must be secured. And companies must carefully weigh up ge-ological potential, political risk and the in-vestment terms on offer before embarking on an expensive exploration campaign. However, once those hurdles have been cleared, the scientists move in. Technology cannot change geology, but it can - and has - improved the chances of finding oil and gas and, equally importantly, finding ways of producing it profitably. When oil exploration began in the US 150 years ago, drilling was mostly done around visible oil seeps at the earth's sur-face. Now E& P operations are common in waters far offshore in depths of over 2,000 metres and technology continues to push the boundaries of what can be commer-cially produced. In that century and half, millions of wells have been drilled and ex-ploration techniques have been gradually refined, reducing the risk of drilling a dry well - the explorer's nightmare - to as low as one in three or four. The kit has got better too. In the mid- 19th century, wells were drilled by hammering steel pipe into the rock. Now, a rotary drilling bit - a revolving steel bit at the bottom of a string of pipe - grinds its way through the rock layers, lubricated by special drilling fluid. But first earth scientists must identify the right rocks. An oil field is like a sponge, not some vast underground lake of oil: oil and gas accumulate within porous rock forma-tions in the earth's crust over millions of years. A layer of impermeable rock on top stops the oil and gas from escaping. Geophysicists start to identify suitable rocks by measuring their gravitational and magnetic properties. Soft, sedimentary rocks, such as limestone, which are capable of holding hydrocarbons, are less dense than heavy, igneous rocks. Aeroplanes measure the earth's gravitational pull; small differ-ences caused by variations in the density of the underlying rocks provide vital clues about the geology. Variations in the earth's magnetic field can provide useful data too. The less magnetic the better - sedimentary rocks are virtually non- magnetic. This sort of evidence is enough for the pe-troleum industry's earth scientists - geolo-gists, geophysicists, geochemists and pal-aeontologists - to begin to build a picture of what the subsurface is likely to hold. But it's nowhere near enough to bet $ 100 million on. The next step is seismic - where explo-ration starts to get really serious ( see p49). Seismic shocks Seismic identifies the best point at which to drill, but the evidence at this stage remains circumstantial. " When you do a seismic sur-vey, you have no idea whether the rock you've mapped contains oil or not," says a field engi-neer. " Drilling is the only way of getting hard proof that hydrocarbons are present." Lengths of drillpipe, tipped with the drill bit, are lowered into the hole from the drill-ing rig - or derrick - and new sections of drillpipe are added as the hole becomes deeper, telescoping down in ever decreas-ing sizes. When the total depth of a well can amount to many kilometres, that requires precision engineering. As one engineer puts it, oil exploration is " a brutally heavy industry with amazing finesse". Get drilling When drilling starts, rock fragments flushed out by the drilling fluid - known as mud - are regularly sampled and examined by geo-chemists for traces of oil. As the well is drilled deeper, a more detailed picture is built up of Oil and gas accumulate within porous rock formations in the earth's crust over millions of years. A layer of impermeable rock on top stops them from escaping

85- www. energy- future. com 6.1- Understanding oil and gas the stratigraphic sequence outlined by seis-mic, through a process called well logging. Derived from the word log in the sense of a record, logging involves lowering a tool down the well on an electrical wire to measure the properties of the rock around the borehole. The core measurement is resistivity - essentially the same as the breakthrough innovation made by Conrad and Marcel Schlumberger in 1927. The brothers Schlumberger measured the electricity resistivity of rocks in oil wells to determine the nature of that rock and whether it could, theoretically, hold oil. The measurement of the speed of sound along the borehole wall and radioactivity logs also yield data on the thickness and depth of res-ervoirs and their probable content. After a discovery has been made, ap-praisal wells are drilled to determine the size and composition of the reservoir, which will consist of water and either oil or gas - of-ten both. The all- important question that the exploration company needs answered is whether the reservoir can be produced prof-itably. There are lots of reasons why a dis-covery might not be economic even if oil and gas are present. The field might consist of multiple reservoirs and faults, which is tech-nically more difficult - and expensive - to produce. If it's offshore, it may not be prac-tical to drill the necessary number of wells from one platform. Perhaps the oil is too thick and viscous to pump to the surface without special - and expensive - equipment. Heavy duty After drilling, steel pipe called casing is set in the hole and is cemented into place ( see diagram). A heavy- duty system of valves called a Christmas tree is positioned at the wellhead to control the flow of the oil, gas TheDiscoverer Deep Seas drill ship. Drilling is the only way of getting hard proof that hydrocarbons are present