Petroleum and natural gas are derived from sedimentary rocks. Petroleum is a naturally occurring mixture of liquid hydrocarbons with minor quantities of nitrogen, oxygen and sulphur. Petroleum contains 82 to 87 per cent carbon and 11 to 15 per cent hydrogen, these elements combined in a complex molecular structure.
On distilling off light fractions of crude petroleum we find a residue of light coloured paraffin wax, a dark tarry asphalt or their mixture. Hence, crude oils are at times classified as paraffin-base, asphalt-base or mixed-base oils. The most abundant oils are asphalt-base and mixed-base oils.
It is not well established as to how petroleum is formed. However geologists agree that petroleum is formed from plant and animal remains which accumulated on the sea floor along with materials which formed the sedimentary rocks.
The source beds are rocks where the oil or natural gas originated. Generally thick columns of dark-grey, chocolate-brown blue or blue-black shale darkened by organic matter have served as the sources. In some areas calcareous shales, limestone’s and sandstones have also been sources for petroleum.
Oil is stored underground in permeable rocks called reservoir rocks. Though petroleum may find its origin in shale, it cannot accumulate in large quantity in this sediment as it is not adequately permeable.
For a good reservoir rock the conditions to be satisfied are that there should exist small and large well connected openings to allow not only the considerable storage capacity but also the freedom for discharge. High porosity alone will not be enough.
Generally accumulation of oil takes place in open-textured sandstones or in porous limestone’s or dolomites. Such rocks should have a porosity more than 10 per cent and the rock layers should be some metres thick. Sandstone reservoirs have been extensive sheets. Limestone reefs and solution cavities have become good reservoirs.
There are also places like highly fractured rocks including shale basalt, chert and schist which have become reservoirs to yield oil. When sedimentary rocks are compacted by the weights of subsequent accumulated bed deposits, then petroleum, water, natural gas present are squeezed out of the soft mud sources into the more permeable reservoir rocks.
In the process of compaction of the low level secondary layers the porosity of the shaly beds will get reduced from a high value of 90 per cent to a value of 35 per cent or less and they may become practically impermeable due to the finness of the reduced openings.
As the petroleum is forced out of the source rock into the reservoir rock, the level of petroleum rises to reach the top of the water-filled reservoir since the oil is lighter than water. Where the reservoir is tilted, the oil held under the reservoir roof moves up the slope till it is held in a trap.
Oil Traps:
An oil trap refers to a reservoir rock body surrounded by impervious rock so that the reservoir rock becomes a closed-top container holding the oil. The oil in the oil trap is held under an impermeable cap rock. The cap rock may be clay, shale or dense limestone, thoroughly cemented sand stone. There also exist oil traps formed by structural features like anticlinal folded and domed strata.
Petroleum is mostly associated with natural gas and salt water. Oil is insoluble in water and gas is very slightly soluble. As a consequence, the gas, oil and water get separated into three layers and these separate in the order of increasing density, with the gas as the uppermost layer, the oil as the middle layer and the water as the lowermost layer.
The amount of gas may be small or large. When the gas is at high pressure, some considerable amount of gas is dissolved in the oil. Only when the oil is saturated with gas, free gas appears above the oil. The water below is salty and is believed to be sea water which must have filled the sediment openings while they were laid down in the ocean.
Natural gas may accompany oil in the same rock reservoirs or it may also occur separately. In cases where the gas occurs separately, it is contained in a trap similar to the oil reservoir trap. In places where oil is absent, the gas lies directly above the water. Mostly the natural gas consists of highly inflammable, colourless and odourless methane (CH4).
Other gases like ethane (C2H6), propane (C3H8), butane (C4H10) and pentane (C5H12) along with vapours of the gasoline fraction by compressing the gas, condensing it at low temperatures or passing it through a heavy oil. It is possible to liquefy butane and related gases under pressure and can be stored in tanks. They are also widely used as bottled gas. Natural gas is the highly priced fuel which is piped to all parts of the country.