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It can only be found in the so-called sedimentary basins, land formed by layers of ore and organic material accumulated over millennia. (This organic material, made from the remains of millions of microorganisms, is what makes up petroleum.) There are three different methods used to find such basins. The first is to examine photos taken by satellites, which show general terrain configurations and the boundaries of a basin. The second is called gravimetry. As the sedimentary rocks have less density than the massive ones, in areas with large concentrations of sediments, the Earth’s gravity force is slightly lower. The difference can be detected even from an airplane, by means of extremely sensitive springs and weights. But there is a fundamental detail: the presence of a sedimentary basin alone is not enough. For oil to accumulate, there must be porous rocks capable of absorbing it – and these, in turn, must be surrounded by impermeable rocks so that it does not escape.
Therefore, the most important investigation method is the seismic one: geologists provoke small earthquakes and capture the echoes coming from the depths by means of ultra-sensitive microphones buried in the soil. As each type of rock reflects the vibration in a way, the signals can be analyzed by computers that produce an image of the composition of the subsurface kilometers deep. For this, dynamite explosions were used before. The latest method is to use huge bulldozers that vibrate the ground at a predetermined frequency. In addition to causing less damage to the environment, this method has the advantage of producing echoes that are more easily identifiable by sensors. At sea, instead of explosions, a ship shoots air bubbles to the bottom, with a powerful compressed air cannon – and these vibrations are captured in hundreds of microphones dragged in the water on cables that can be 8 kilometers long.
Even with all these processes, you can only be sure that there is oil in that land after drilling it, making the so-called exploratory wells. “This drilling is very expensive, especially if the well is at the bottom of the sea”, says engineer Giuseppe Bacoccoli, from the Federal University of Rio de Janeiro (UFRJ). A marine well 3 kilometers deep costs around 10 million dollars. And it’s rare to hit on the first shot. “It’s common to have to drill four to ten wells before discovering a new deposit, even in a known basin”, says Giuseppe.