Petroleum (also known as crude oil or petroleum), often referred to as "black gold", is a thick, dark brown or greenish flammable liquid that occurs in the upper layers of certain areas of the earth's crust. Petroleum consists of a complex mixture of various hydrocarbons, mostly the alkane series, but varies in appearance, composition, and purity. Crude oil is taken from oil wells in oil mines. The location of these oil wells was obtained after going through a process of geological studies, analysis of sediments, character and structure of the source, and various other studies.[1][2] After that, petroleum is processed at refineries and separated by boiling point to produce a wide range of fuels, from gasoline and kerosene to asphalt and various chemical reagents needed to make plastics and pharmaceuticals.[3] ] Petroleum is used to produce various kinds of goods and materials needed by humans.[4]
Forming process
Crude oil is the result of the decomposition of plant and animal matter in a slowly subsidence area. The area is usually in the form of sea, lagoon (lake) boundaries along the coast or lakes and swamps on the mainland. Sediment is deposited together with the material and the speed of sediment deposition must be fast enough so that at least part of the organic matter can be stored and properly buried before decomposition occurs. Under certain conditions of circulation and reduction, accumulation of hydrocarbons is found in deep seawater.
Time goes on geologically and depositional areas sink deeper into the earth's surface, due to increased weight by sediments and material that piles up on it, or because of tectonic forces that cause subsidence effects. Organic material is immersed deeper so that it experiences higher pressure and temperature. This process will cause changes in the chemical changes of the organic material. This material change is the forerunner to the formation of a mixture of hydrocarbon materials whose composition is very complex, both liquid and gaseous hydrocarbons.
The increase in temperature with depth on average in this world is about 20-55 degrees Celsius per kilometer. In Sumatra alone it can reach approximately 100 °C/km. Meanwhile, new oil habitats will form at temperatures around 65 - 150 °C which are usually at a depth of 1.5 - 3 km. At a depth of 3 – 6 km the reservoir rock will be dominated by gas rather than oil. For deeper depths the temperature will be higher so that the gas will be higher so that the gas will experience further decomposition.
In general, petroleum is usually deposited in well-porous sedimentary rocks that have a porosity value of 45% (very good reservoir). Because the longer the rock is deposited and material is buried on it, the rock will be compacted and this results in a decrease in the porosity value. Oil, gas and water will collect or be stored in the pore space of the porous rock. Because of the gravitational pressure, the fluid moves in the rock slowly. Rocks that can allow fluids to pass are known as permeable rocks. Permeability of rocks can separate gas, oil, and water physically, namely due to differences in density. Oil and gas with a density lighter than water will move up to the surface as seeps or be trapped in a trap and then stop accumulating until the trap is full.