It is a mineral substance and is produced from the earth. Petroleum exists in the pore spaces of the rock materials. These rocks are generally referred to as reservoir rocks. Petroleum is generally found is sedimentary materials, generally sandstones or limestone. Crude petroleum has been found at various depth of sedimentary basins. Oil and gas are found in localized accumulations under certain conditions of geological environment. Origin of petroleum is generally described by inorganic and organic theories. But organic theories describe its accumulation at business level. The organic theories assume that petroleum evolved from decomposition of vegetables and animal organisms that lived during previous geological ages. The organic theory is supported by large geological evidence. Oil and gas are commonly found in sedimentary basins, and, furthermore, these accumulations are found in the vicinity of beds which contain large amounts of organic matter.
Beds which are rich in organic materials called source beds. For accumulation of petroleum it is necessary to have source beds and a reservoir bed. All porous and permeable rock is called reservoir rocks. Those reservoir rocks which are covered by natural barrier are called TRAPS.
A trap generally contains oil, gas, and / or water. Its distribution is largely controlled by gravitational and capillary forces. Due to density difference lighter fluids to seek the higher position in the trap. But on the contrary capillary forces tend to cause a wetting fluid to rise into pore space containing non wetting fluid. Since water is wetting fluid w.r.t oil and gas, and oil is wetting fluid w.r.t gas. To have accumulation of petroleum there is need of equilibrium between the capillary and gravitational forces. Possible modes of existence of gas are...
In a particular trap there may exists different zone, namely oil, gas and water zone. In between two zones there exit a transition zone. There exists oil – water transition zone in between oil zone and water zone. The pore space of the rock in the oil zone contains a small amount of water (connate water). Saturation of water increases with depth in the transitional zone so that the base of the transitional zone is described by completely water- saturated pore space.