How petroleum was formed?
It is very difficult to say how petroleum was formed? Petroleum contains nothing from which its origin is easily recognized. It is generally believed that petroleum was formed in a process that begins with decomposing sea organism. Millions of years ago, there were countless small creatures and microscopic plants floating in the sea. These tiny sea creatures, as they died, sank to the bottom of the sea. They were preserved from decay by being covered up with earth sediments and mud brought down by rivers.
The enormous pressure of the layers of sediments together with chemical action and heat, slowly over millions of years, converted the organic matter into oil. The surrounding mud became the rock called shale and are known as source rock or mother formation.
The oil produced filled the pore spaces between the small particles of sediments. The pressure which converted the mud into shale releases much of the petroleum out of the rocks in which it formed. It could not escape downwards, for there the pressure was still greater. Therefore it made its way upwards into any bed that was more porous than the mud in which it has been formed such as sands, sandstones and some limestone. The gas, oil and water rose to a porous part of the earth’s crust where, with an impervious rock layer above called the cap rock and another imperious layer below, they formed an oil reservoir or pool. This movements or migration of petroleum often results in its being discovered far from the rocks in which it formed and in concentration of oil from a great volume of source rock. Such concentrations occur where the rocks have been forced up into the form of a dome by pressures originating in the earth’s crust. A typical oil deposit is built up of three layers. A zone where the pores of the rock are filled with natural gas lies above a zone where the rocks are saturated with oil containing dissolved gas and beneath the oil is water.
Primary and secondary Migration
Usually petroleum is not found in the same rock where it was found.before it can have any commercial value,petroleum must move from the rock where,it originates (source rock),through other rocks(Carrier rock) into the rock where it accumulates (Reservoir rock). The movement of oil is known as migration.
Primary migration is the movement of hydrocarbons from source rock into reservoir rock.
Secondary migration refers to the subsequent movement of the hydrocarbons within reservoir rock,the oil and gas has left the source rock and has entered the reservoir rock.
Source Rock:The rock which generate oil is known as source rock.
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