When plants and animals died, their bodies sank to the bottom, mixing with sediments. Crude oil is less dense than the rock formations surrounding it, so it begins to migrate upward through porous rock layers.
How Crude Oil Moves and Gets Trapped in Reservoirs
Unlike material that decomposes on the surface, this organic matter was buried quickly under layers of mud and silt. Higher temperatures generally produce lighter hydrocarbons, while lower temperatures yield heavier oils.
This prolonged exposure to heat between 90°C and 160°C "cooks" the kerogen, a process geologists call diagenesis and catagenesis. Oil moves until it encounters a non-porous cap rock, such as shale or salt, which blocks its further ascent.
How Crude Oil Moves and Gets Trapped in Reservoirs
However, the earth’s geology acts like a filter. Here, the increasing weight created intense pressure, while the Earth’s internal heat raised the temperature.
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