The youngest barrel of oil is one million years old; a typical barrel is 40 million years older than that. In spite of the nickname “dino juice”, oil doesn’t come from dead dinosaurs, but rather from tiny, one-celled aquatic plants and animals that take light from the sun and convert it into energy. 600-300 million years ago a series of specific geological events began in the ancient oceans, lakes, and swamps of the world that eventually created crude oil. If any one of these events had failed to occur, oil would never have formed. Given the precise series of geological phenomena that has to occur over millions of years, only about 2% of organic material ever becomes crude oil, and only about 0.5% accumulates in sufficient enough quantities to be recoverable. Although oil is still forming, it is not renewable as we are using it far faster than it is being created. The numbers, labels and descriptions below correspond with the numbers on the illustration above. The descriptions detail the process of how oil forms over millions of years.
1. Energy-Trapping Organisms:
The ancient oceans were full of microscopic life. As these organisms died—especially plankton, diatoms, foraminifera and radiolarian -— they sank to the sand and mud on the ocean floor where the organic material mixed with sediments to form (over thousands of years) a fine-grained shale or source rock.
2. Sediment Layers + 3. Source Rock:
As new layers were deposited on top of the source rock, the heat and pressure they exerted distilled the organic material in the source rock, turning it into natural crude oil.
4. Reservoir Rock:
The oil would then flow from the source rock and accumulate in reservoir rock, a thicker, more porous rock like sandstone or limestone.
5. Oil Traps:
Traps are situations where a non-permeable boundary, the cap rock, causes oil to pool. The most important factors affecting the formation of a trap are the lithology, the geometry, and structural setting of the rocks. With regards to lithology, the grain size and porosity, of the rock are critical to the formation of a trap.
6. Cap Rock:
The oil would continue to migrate out of the reservoir rock until it was stopped by an impermeable rock —- cap rock -– like granite or marble or by a geologic trap, including folds, arches, troughs, domes, and faults.
7. Oil Seeps:
Because of the vast pressure exerted thousands of feet below the earth’s surface, oil tries to move to areas of less pressure. If the oil can find its way around a cap rock or trap, it will seep up to the earth’s surface. Such oil seeps were once common in many places around the world and have been used by humans for thousands of years.
8. Modern Drilling:
On August 27, 1859, Edwin L. Drake found oil under ground and a way that could pump it to the surface. This began the modern era of drilling through the earth to oil deposits and pumping it to the ground.
