Introduction
Petroleum is essentially a complex mixture of alkane hydrocarbons (from C₁-C40), which are present in it right from methane up to the higher members containing as many as forty carbon atoms. Its elemental composition is primarily carbon and hydrogen with variable quantities of oxygen and sulfur and trace amounts of nitrogen, metals and other elements. It also contains varying amounts of aromatic hydrocarbons and cycloalkanes. However, the oil’s actual composition varies with the place of origin. Besides the various types of hydrocarbons, there are present in Petroleum organic compounds containing nitrogen and sulphur. Most natural oils are found to be optically active. Since Petroleum has an entirely different composition from vegetable and animal oils, it is commonly called mineral oil to differentiate it from them. This name points to its origin in mines. Petroleum fractions containing up to carbon four are usually gases, carbon five to sixteen are liquids, and those containing carbon seventeen or more are usually solids.
Crude oil is classified by the carbon-hydrogen ratio, lower for the more desirable crudes containing smaller molecules (light crude) and higher for the less desirable crudes containing primarily larger molecules (heavy crude). Crude oil low in sulfur is called sweet crude, while the less desirable crude oil with higher sulfur content is called sour crude. Crude oil is found in many parts of the world. Major producing areas include the Southern United States, Western Canada, Mexico, Venezuela, the Middle East, the Eastern Soviet Union, Rumania, Nigeria, and Indonesia. Crude oil can be transported economically long distances overland by pipeline and overseas in enormous tanker vessels.
Prospecting for Oil
Before the systematic search of oil-bearing deposits, an explorer had to depend upon certain visible signs on the earth’s surface. Sometimes the petroleum gas seeped through the ground and burnt on the surface resulting in ‘natural fire,’ as we have at Jwalamukhi, Punjab. The rainbow smears of oil in water form springs were often suggestive and gave a clear indication of the prospects for finding hidden oil deposits; such clues to the existence of oil are available in Jammu and Kashmir State and several other regions in India.
For locating the prospective areas which are likely to be petroleum-bearing, various scientific methods are now employed. The more important ones are stated below:
Gravimetric method
It depends on the fact that the pull of gravity in a given area on the earth’s surface varies with the density of the rocks beneath. The sedimentary rocks below the oil reserves are less dense compared to igneous rock elsewhere and show a relatively lower gravitational attraction. Thus gravity meters show a fall in the value of gravity near the petroleum fields. A geologist can adequately guess the location of oil fields by a gravity survey’ of a region.
Magnetic method
Oil fields are often found in the vicinity of the salt domes within the crust of the earth. These salt domes are diamagnetic and weaken the intensity of the earth’s magnetic field. By determining the magnetic gradient with the help of magnetometers at various places in the area, petroleum-rich rocks can be found.
Seismic method
This is by far the most reliable and effective method for locating oil fields. In this method, a heavy charge of dynamite buried under the earth’s surface is exploded. It creates a miniature earthquake and sends shock waves through the ground. These waves are reflected from the petroleum surface, and the dense underlying strata and are thus returned to the ground surface. Here, the geophones receive the shock waves or vibrations (a special type of microphone) connected to an automatic recording device called a seismometer. The time between the dynamite blast and the reception of vibrations on the surface reveals the nature of the rock beneath and the oil reserve’s existence and depth.
Despite all the evidence collected by the above methods, the explorer cannot be sure about the presence of oil below. The only real proof is to bore down into the rock and find out. Nearly twenty percent of these trial borings strike oil.
Drilling for Oil
The oil, having been located, has to be pumped to the surface. Therefore, we have first to bore down into the rock to obtain an ‘oil well. An efficient rotary drill bites into the rock with a circular motion kept in order by an engine on the surface. The cutting part of the drill called a bit, is kept cool by a current of lubricating mud, which flushes out the rock pieces and helps prevent oil blowout. The pipes are fed into the bored hole with the help of a ‘derrick’ constructed at the surface. Oil wells as deep as five miles have been drilled with such a device.
After the drill strikes oil, the underground pressure of the gas above the oil usually is sufficient to force the oil to the surface. The oil flow from such a well called ‘gushers’ may continue unabated for years.
As the gas pressure subsides or when there is no gas, air pressure is applied to lift the oil from the well. Nuclear explosions have been recently employed to rejuvenate the dried well when an oil well is entirely run out. Such explosions hit the unexplored rocks in the vicinity, causing a system of fissures to restore the well’s productivity.
The oil obtained from the mine is conveyed by pipelines to a distant place and distilled. The distillation is not carried out on the spot, so the inflammable natural gas or other dissolved gases may not catch fire and thus damage the whole mine.
