Where the Oil Is: Four Things That Must Happen Before There Is an Oil Field

Where the Oil Is

[1]In 1948 a drilling crew in the Saudi Arabian desert struck oil, and the Ghawar field turned out to be one of the largest ever found. The oil they tapped did not begin in a desert. It began as countless microscopic organisms drifting in a warm, shallow sea well over 100 million years ago. Understanding how those organisms became oil explains something a world map makes obvious: a few regions hold enormous amounts of petroleum, and most regions hold none at all.

[2]The first requirement is a sea full of tiny life and a seafloor that preserves it. Plankton and algae live, die and sink in every ocean. On most seafloors their remains are eaten by scavengers or broken down by bacteria that use oxygen, and nothing is left. But in some sheltered basins the bottom water is still and nearly free of oxygen. There the remains pile up with fine mud faster than they can be destroyed. Over thousands of years, this organic-rich mud becomes a dark rock called source rock.

[3]The second requirement is burial and heat. New layers of sediment keep arriving, and the source rock is pushed deeper. Earth grows warmer with depth, about 25 degrees Celsius for every kilometer on average, because heat flows up from the hot interior. At roughly 2 to 4 kilometers down, where the rock reaches about 60 to 120 degrees Celsius, the buried organic matter slowly breaks apart into the molecules of crude oil. Pressure also rises with depth, but it is the heat, acting over millions of years, that does the chemical work. Bury the source rock too deep and too hot, and the oil breaks down further into natural gas, or is destroyed.

[4]The third requirement is a place for the oil to collect. Oil is less dense than the water that fills the pores of most rock, so once it forms it moves upward through any rock with connected openings, such as sandstone or cracked limestone. These porous rocks are called reservoir rocks. The oil keeps rising until it meets a layer it cannot pass through, such as a thick shale or a bed of salt, called a cap rock. Where the cap rock has been folded into an arch, oil gathers beneath the crest like air trapped under an upturned bowl. That arrangement is a trap, and it is what drillers aim for. Without a trap, oil keeps rising and leaks out at the surface. The sticky asphalt that still oozes up at the La Brea tar pits in Los Angeles is petroleum that reached the surface because nothing held it underground.

[5]Put the requirements together and the uneven map explains itself. An oil field needs a source rock that formed in an ancient oxygen-poor sea, burial to the right depth and temperature, a porous reservoir above it, and a cap rock folded into a trap. Regions that were once covered by warm shallow seas and were later gently folded, such as the land around the Persian Gulf, the Gulf of Mexico and the North Sea, have all four. Regions made of ancient granite, which never collected organic mud, are missing the first requirement and can never have the rest. Geologists do not drill at random: the whole chain tells them where oil cannot be as clearly as where it can.

[6]The map is also changing, and this time people are the process. Oil that took millions of years to form is pumped out in decades, and nothing replaces it on a human timescale. As the easiest fields are drained, companies drill deeper, farther offshore and in colder places. Today's distribution of petroleum is therefore the product of two histories: the geologic one that put the oil in a few places, and the human one that has been removing it for more than 160 years. Most of what is pumped fuels transportation and is made into plastics and other materials; only a small share is burned to make electricity. Every barrel burned also releases carbon dioxide, which is one more reason that the ancient sea's gift is treated as a limited, nonrenewable resource.