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13 Chapter One: How Oil Formed

Kerogens contain mainly the chemical substances known as hydrocarbons . These are chemicals composed of only the elements (pure substances) carbon (symbol C) and hydrogen (symbol H). Millions more years passed. In some places, the settled layers were buried even deeper. At temperatures of around 120 to 300˚F (50 to 150˚C), usually a depth of 1 to 2 miles (1.6 to 3.2 kilometers) below the surface, the kerogens broke down further. They gradually changed into what we call crude oil or petroleum . Like kerogens, the oil—and natural gas, formed in much the same way—contained mostly hydrocarbons. This is why now they are sometimes known as hydrocarbon fuels or hydrocarbon materials. Another name for oil or the fuels made from it is

The “Oil Window” What happens to kerogens depends on pressure and temperature. In the broad range of 120–300°F (50–150°C), they become oil. This range is called the “oil window,” and most oil forms within it, at 140–250°F (60–120°C). The temperature rises even more with greater depth, up to 212–390°F (100–200°C) at 4 miles (6.4 kilometers) down. This is the “gas window,” when kerogens form natural gas (see Natural Gas in this series).

fossil fuels , since they formed by the process known as fossilization. Yet another term is organic materials , or organic fuels , which means they are connected to living things and nature, rather than being man-made or artificial. This process took immense lengths of time and occurred only in certain places around the world. It depended on the right conditions coming together, from the layers of dead sea-life forming in the first place, to deeper burial, and high pressures and temperatures. Also needed were suitable kinds of rocks. Some were porous , with tiny holes and channels like a sponge. This allowed oil to collect in them, in the same way that a sponge holds water. The oil might seep or travel for hundreds of miles when doing this. The main kinds of oil-bearing porous rocks were sandstone and a type of limestone known as “coarse grained,” due to its relatively large particles, or grains. These rocks were hard, not soft like a sponge, but they did have very tiny holes and cavities, which the oil filled. The oil was kept here by other kinds of rocks around them, which were nonporous and prevented the oil from oozing away.