NECESSARY CONDITIONS FOR PETROLEUM GENERATIONS

The presence of organic-rich sediments in sedimentary basins forms the basis for petroleum generation. The remains of decayed plants and animals within the soil layers are termed organic matter, in which phytoplankton; zooplankton, higher plants, and bacteria are the most important contributors. The production, accumulation, and preservation of ungraded organic matter are pre-requisite for the existence of petroleum source rocks. Petroleum source rock is fine grain sediments that, in its natural setting, has generated and released enough hydrocarbons to form a commercial accumulation of oil, gas, or both. Source rocks are usually shales and limestone. The organic matter in the source rocks is disseminated and insoluble in normal petroleum solvents such as carbon bisulfide. This is known as “kerogen”. Chemical, kerogen consists of carbon, hydrogen, and oxygen, with minor amounts of nitrogen and sulphur. The three basic kerogen types are type I-algal, type II liptinitic, and type III-humic. Temperature plays an important role in kerogen maturation, in which the rate of increase in temperature in the earth with depth is termed as “geothermal gradient”; at this gradient, petroleum is released from kerogen during catagenesis, with the first oil and later gas being released.