How was oil trapped beneath the Earth's surface?

By: Contributors

Oil didn't just happen; it took quite a long time for oil to develop below the Earth's surface. The first step in the process happened somewhere between 10 million and 600 million years ago (give or take a few years). At this time, tiny plants and animals (plankton) lived and died in the vast, ancient seas. As these organisms died, they sank to the bottom of the sea, mixing with the sand and mud there. Since these plants and animals were dead, they began to decay. As their organic material decayed, it settled among different sedimentary layers. None of these sedimentary layers had any oxygen, so the microorganisms that broke down the remains transformed them into carbon-rich compounds.

These carbon-rich compounds remained organic material, which continued to mix with sediment to create fine-grained shale, or source rock. This process continued for millions of years, with more and more layers of sediment and source rock piling on top of each other. These continuous layers in turn exerted incredible pressure and heat on the layers and rocks beneath them. This heat and pressure is what ultimately distilled the carbon-rich compounds stuck in the source rocks into crude oil and natural gas.


Once in the form of crude oil or natural gas, the substance would ooze away from the source rock where it would gather in the thicker and more porous rocks, like limestone and sandstone. Also called reservoir rocks because they hold the crude oil, such rocks were able to hold on to the crude oil or natural gas in them because the Earth's movements pushed layers of impermeable rocks, like marble or granite, above them. The Earth's movements, which helped keep the crude oil and natural gas trapped, called folding, where inward movement pushes rock upwards into a fold; faults, where rocks crack and one side moves up or down; and pinching out, where impermeable rock pushes up into reservoir rock.

ScienceEnergy ProductionHow Oil Drilling WorksScienceEnergy ProductionWhat's oil shale?ScienceEngines & EquipmentHow Oil Tankers WorkScienceEnergy ProductionWho owns Arctic oil?ScienceEnergy ProductionEverglades Oil: What's to drill?ScienceEnergy ProductionHow Oil Refining WorksScienceEnergy ProductionOil Field PicturesScienceEnergy ProductionWhat is enhanced oil recovery?ScienceEnergy ProductionHow the Crude Oil Market WorksScienceEnergy ProductionHow far underground are oil deposits?ScienceEnergy ProductionThe Oil Drilling Process ExplainedScienceGreen ScienceHow do you clean up an oil spill?ScienceEnergy ProductionHow does drilling for oil in the Arctic work?ScienceEnergy ProductionAre we drilling for oil in the U.S.?ScienceEnergy ProductionWhat is primary oil recovery?ScienceEnergy ProductionHow long will the U.S. oil reserves last?ScienceEnergy ProductionTop 5 Innovations in Oil DrillingScienceEnergy ProductionWill we ever cut our dependence on foreign oil?ScienceEnergy ProductionHow have we improved oil rig technology?ScienceEnergy ProductionWhen will we run out of oil, and what happens then? ScienceEnergy ProductionWhat's the environmental impact of the Deepwater Horizon oil spill?ScienceGreen ScienceWhat would happen to the environment if U.S. federal lands were open to oil drilling?ScienceEnergy Production5 Improvements in Offshore Oil DrillingScienceEnergy ProductionHow do they prepare land for an oil rig?ScienceEnergy ProductionHow was oil trapped beneath the Earth's surface?ScienceEnergy ProductionHow much oil does the U.S. produce and import?ScienceConservation IssuesDeepwater Horizon Oil Found in Land-based Birds for First TimeScienceEnergy ProductionFrom Crude Oil - How Oil Refining WorksScienceEnergy ProductionChemical Processing - How Oil Refining WorksScienceEnergy ProductionFractional Distillation - How Oil Refining WorksScienceEnergy ProductionCrude Oil - How Oil Refining Works