1. Formation of Organic Matter:
* Petroleum originates from ancient marine organisms (plankton, algae, etc.) that died and settled at the bottom of oceans and lakes millions of years ago.
* Over time, these organic remains were buried under layers of sediment, like clay, sand, and silt.
* The weight of the overlying sediment caused pressure and heat, transforming the organic matter into a thick, tar-like substance called kerogen.
2. Transformation into Petroleum:
* With continued burial and increasing pressure and temperature, kerogen undergoes a process called catagenesis.
* This process breaks down the complex molecules in kerogen, releasing hydrocarbons (molecules containing hydrogen and carbon).
* These hydrocarbons form the basis of petroleum, which consists of a complex mixture of various hydrocarbons.
3. Migration and Accumulation:
* As petroleum forms, it becomes less dense than the surrounding rock, allowing it to migrate upwards through porous and permeable rock layers.
* This migration continues until the petroleum encounters an impermeable layer of rock, like shale or salt, that traps it.
* These traps, often called reservoirs, can hold enormous quantities of petroleum.
4. Reservoir Rock:
* The porous and permeable rocks that hold the petroleum are called reservoir rocks.
* These rocks have spaces (pores) and interconnected pathways (permeability) that allow the petroleum to flow and accumulate.
* Examples of reservoir rocks include sandstone, limestone, and fractured shale.
5. Importance of Source Rock:
* The rock that originally contained the organic matter that formed petroleum is called the source rock.
* The source rock must have the right type of organic matter and must have been subjected to the appropriate pressure and temperature conditions for petroleum to form.
In summary, petroleum is found underground because it forms from organic matter buried deep within the Earth, migrates upwards through porous rocks, and gets trapped in reservoir rocks.
