Sheila Shipley, age 12, of Victoria, S.C., for her question:
How is oil made in the ground?
The old earth took a long, long time to make her supplies of oil. The recipe took place underground. The precious supplies of oil needed for industry have been hidden under the ground for millions and millions of years. We know the ingredients of the recipe, but we still do not know all the secrets of how it is put together. Crude oil, as it occurs in the ground is mostly hydrogen and carbon. But we cannot put these basic elements together to make oil.
The geologists and the chemists provide us with clues as to how nature makes oil. Oil is a hydrocarbon, which the chemists say was created by living creatures. It is the fossil remains of creatures that lived long ages ago. The geologists say that oil is found where there was once an ancient sea. From these clues we can guess that oil was made from ancient sea creatures.
This does not surprise us, for our seas teem with microscopic forms of life. An acre of warm, shallow sea water may produce a ton of these little animals every year. Most of them are too small for our eyes to see. The microscope shows them as hosts of little one‑celled creatures.
The ancient seas also teemed with such tiny animals. Then as they lived and died in countless numbers, their bodies sank to the sea floor and mingled with the oozing mud. They provided, we believe, the basic ingredients for oil.
The face of our old earth is continually changing. In a few million years, a valley region may become a range of mountains. Lakes appear and disappear. Many times the sea has slopped over on to the continents and then returned to its basins. The old earth shrugs her shoulders, the rocky layers of her crust bond, buckle and break.
These events are earth changes. The ancient seas where the small creatures basked in the sunshine went though many such changes. Their floors of sand, mud and small bodies were covered with a layer of new mud; maybe brought in by silt carrying rivers. The oily sand and mud became a layer below the surface.
Meantime the old earth shrugged her shoulders. The sandwich of rocky layers was heaved up and down in hills and dales. Maybe the sea retreated, leaving a surface of dry land. New layers of soil and rocks were added. The oozy old sea bed, now buried far below, became hard sandstone or limestone. The muddy layer directly above it became a roof of hard shale. While this was going on, the fossil sea creatures were turned into oil. We are not certain how this is done. It may be done by bacteria or some kind of chemistry. Even radioactivity may play a part. This part of oil making is still one of natures secrets.
The so‑called pool of oil is a layer of porous rock. Oil fills the pores and holes as water fills a sponge. Salt water may have been mixed with the ancient oily brew. This is heavier than oil and through the ages it separated and seeped into the rocks below the oil. Gases may have escaped from the oil. They are lighter than the oil and may rest in a layer of rocks above it. Oil is often found sandwiched between a layer of natural gas above and a layer of salt water below.
This sandwich of natural gas, oil and salt water is found where deep layers of rock have been heaved and bent. Certain underground formations form traps which prevent these substances from escaping. Where there is a hard roof of shale, the rich reservoir is safe for millions of years.