Petroleum Maturation, Accumulation, and Transformation
Petroleum Maturation
In the first few meters of burial, veins are remnants of bacteria. Due to the anaerobic attack, they provoke the release of methane. As we descend, the combined action of pressure and temperature gives rise to a series of reactions that convert kerogen (organic remains) into an organic polymer, a solid carbon and hydrogen-rich layer formed hundreds of meters deep. With increasing pressure and temperature conditions, the kerogen starts to decompose, distilling oil that gets trapped in the pores of the rock. If the depth of burial keeps increasing, kerogen and oil droplets start to transform into gas.
Sites of Petroleum Accumulation
For oil to be formed and accumulated, four geological circumstances need to take place:
- The kerogen present in a sedimentary rock can produce oil if the rock is buried deep enough and at the appropriate temperature. This fine-grained, dark rock, now impregnated with microscopic droplets of dispersed oil, is called the source rock. During burial, the pressure tends to squeeze the source rock, which causes the expulsion of some of the oil. This oil leaves the rock in a movement called primary migration.
- Sedimentary rocks with large and interconnected pores allow easy passage of fluids. These rocks, such as sandstone, are called reservoir rock. If a reservoir rock is in the vicinity of a source rock, oil will accumulate in it.
- The caprock prevents the oil from reaching the surface. Due to its low density, the oil present in the reservoir rock tends to move through it in an upward movement called secondary migration. Therefore, to find an oil deposit, a caprock is needed to prevent the rise of oil.
- In petroleum geology, the oil present in the roof of the reservoir rock may continue its secondary migration, rising until it reaches a structure called an oil trap. Within the trap, the fluids are arranged from bottom to top as water, oil, and gas. The most common trap is the anticline.
Petroleum Transformation
From the wells, crude oil is extracted as a mixture of hydrocarbons that has no direct application. For its utilization, it must pass through a series of fractional distillation processes. As the temperature rises progressively, it separates liquids from gases. These hydrocarbons are still not suitable and have to undergo further procedures. Among the main uses of oil, we could cite gasoline and kerosene. The principal use of fuel is for transportation.
Natural Gas
Natural gas comes from the fermentation of organic matter within accumulated sediments. It is composed of a mixture of methane, butane, propane, hydrogen, and other gases in variable proportions. Its extraction is very simple because, due to the pressure exerted by the overlying sediments, gas flows without problems. Its extraction is very economical and it is transported through pipelines, although it requires a strong investment, they are very simple and low risk. Natural gas is used directly in homes and industry, and in power stations, it is beginning to replace coal. Both emit the same amount of CO2, but gas produces no sulfur pollutants, unlike coal.