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Amazing Natural Source of Energy ......Black Gold!!!!!!!!
Most of us use petroleum or byproducts of petroleum but only few of us know how is it formed, how was it discovered by man and how is it located etc.
Many people asked me similar questions that made me think creating such page that will answer questions that may arise in minds of common people.
That's interesting.... isn't it?
So let's learn about it.....
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The name PETROLEUM comes from two Latin words, PETRA means rock and OLEUM means oil. So in other words petroleum means rock oil.
When petroleum first comes out of the ground, the term CRUDE OIL is used. The crude oil comes in different forms. It may be black, green, red or brown in colour. It may be thick and viscous or thin and volatile. When thick and viscous the term "Heavy crude oil" is used and when thin and volatile it is called "Light crude oil".
Light oil float easily on water and have an API gravity of 30 to 40 degrees. The heavy crudes sink easily in water and have an API gravity of less than 12 degrees. Petroleum is found widely all over the world. Now it's available only in the deep-seated reservoirs but may be few decades' back it was found on the surface.
And as a matter of fact this is how man came to know about its existence. It might be interesting to know that in earlier times people had noticed presence of petroleum as a pool on the surface, which had characteristic smell. People referred this black fluid in the pool as ROCK OIL.
It was noticed that such mysterious pools catch fire very easily and this is how people started using it as a fuel for burning purposes.
First large-scale demand for petroleum increased when kerosene lamps were invented in 1854.
An interesting piece of information, which you may like to know, is that first the kerosene was made from coal, but by the late 1880s it was mainly extracted from the crude oil.
That time crude oil was mainly used to produce kerosene and rest of the part was discarded.....AMAZING...isn't it?
In 1859 Edwin.L. Drake drilled first commercial oil well in Pennsylvania at Titusville and this successful well is thought to be the birth of modern petroleum industry. Slowly use of the petroleum increased and by mid of the 19th century it became commercially very important. But till then it was mainly used to extract kerosene, that is why period between 1859 to 1900 is referred to as KEROSENE age.
By the end of the 19th century internal combustion engines were developed and this gave a big boost to the petroleum industry.
With the increased demand search for the petroleum also increased. Most of the source, available on the surface soon exhausted. So need of developing technology to explore sub surface reservoirs was felt badly.
A commonly asked question is about the origin of the petroleum and it will be further interesting to learn about.
Earlier it was considered that the petroleum is a naturally formed mineral and that's how it was called with the names like mineral oil, organic mineral, mineral hydrocarbons etc. But today it is not included under mineral classification and is considered to be formed from the natural organic processes.
Chemically, petroleum is mainly composed of Hydrogen, Carbon with some Nitrogen, sulphur, phosphorous and traces of lead, silver, gold etc. Such chemical composition along with the evidences found in and around the petroleum reservoir, suggests organic origin of petroleum.
A good evidence of its organic origin is the presence of cholesterol in the petroleum which is found only in organic matter.......AMAZING isn't it?
The petroleum has a very charecteristical optical property of changing plane of polarization and this is mainly considered due to the presence of cholesterol in it.
Another evidence that supports the theory of organic origin of petroleum is the presence of Porphyrine and nitrogen in the crude oil. So now it is a widely accepted theory that the petroleum has an organic origin...but now one might think about the nature of the actual source of crude petroleum. Since the process of formation of crude oil from organic matter can not be seen directly, it is difficult to say about the specific primary source.
However, scientists based on their studies made some assumptions that might be interesting for you to know.
Oil is also found in the sedimentary rocks of Cambrian period in geological time scale. In simple words oil is also found in 600 million-year-old rock. From the geological studies it is known that the organisms that existed at this period were simple organism.
So it may be considered that the primary source for petroleum are such simple organisms.
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ORIGIN OF PETROLEUM
Present theory of petroleum origin that most Geologists believe on, is that the crude oil was formed over millions of years from tiny aquatic plants and animals called planktons that lived in ancient water bodies mainly seas. Major contribution of such microorganisms comes from marine environment, as indicated by the study of sedimentary rocks containing crude oil. But some non-marine oil reservoirs are also known that indicates fresh water lake and lagoon environment.
Now read it carefully, I am going to tell you an AMAZING THEORY, which scientist believe. According to this theory energy which we get from petroleum today is originally SUN ENERGY that was stored in plants and animals millions of years ago.
Well that sounds bit odd... but let me explain you how this theory works.
The energy which we are using today i.e. petroleum is mainly composed of mixture of hydrocarbons. In other words the basic composition of petroleum is carbon and hydrogen atoms.
Now, today we know from our study that plant makes its food by the process of photosynthesis. During this process plants use the sun's energy to produce sugar from water and carbon dioxide. This energy is stored in the form of chemicals in the plants. This energy is transferred to the animals when they eat these plants. Over the millions of years of earth history this energy was transformed into fossil fuel as these plants and animals underwent the process of decay, under certain conditions of temperature and pressure.
I bet, for you it will be further interesting to learn about this process of conversion from microorganisms to present day petroleum...so lets continue....
Millions of years back in earth's history, at certain times conditions for oil formation were favourable. During these times the big water bodies on the earth mainly seas and big lakes and lagoons were rich in microorganisms. When these microscopic plants and animals died, they sank to the bottom of the water body in which they lived. This resulted in formation of thick organic layer at the bottom, which was quickly covered with the sedimentary layers, one after another. Due to this cover of sediments, the layer of organic matter was protected from coming in direct contact with the air. So the organic matter could not rot the same way as organic matter rots in a compost pile in the presence of air. It is believed that under such conditions anaerobic bacteria played a big role in decaying the matter in such a way that it resulted in the formation of crude oil. The bacteria broke down the trapped organic matter, molecule by molecule, and converted it into substances that were rich in hydrogen and carbon. This process is further enhanced by the increasing pressure due to the ovelying sediments. This pressure keeps increasing as the sediments continue to deposits. One more factor which plays an important role in this process is the temperature. Temperature, within the earth's crust increases with the depth. So with the piling up sediments on top of the organic layer, temperature and pressure, both keep increasing with time. The organic matter gets slowly cooked due to the increased temperature and slowly changes the organic matter into fluid form.
Increased heat and pressure causes wax, fat and oil from plant and animal remains, to join together. Such fossil organic matter is called KEROGENE.
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Source Rock
Rocks that contain sufficient amount of Kerogen are called SOURCE ROCKS. When a source rock starts generating oil and gas then it is said to be mature.
When the Kerogen is further heated with time, the chains of hydrocarbons break away from the Kerogen and form waxy and viscous crude oil. At greater depth, the temperature is high and the pressure of overlying sediments and water column is also high. This helps the chains of hydrocarbons to become shorter and this results in lighter oil and gas to separate out. So this was the story of OIL GENERATION but ... one more step is the accumulation of oil in a reservoir. Now we learned how oil and gas originated in a source rock.
So now let me tell you about its migration which is an another important factor in oil and gas study.
One might think that the oil is found as underground pools...but in reality it is found trapped in between the particles of sedimentary rocks. The best reservoir that has good pore space and gives economic production is sandstone. The other most commonly found oil and gas reservoir is Limestone. Although limestone becomes more porous and permeable after it undergoes chemical weathering or fracturing due to some tectonic activity.
The oil and gas trapped in the pore spaces is lighter than the surrounding water. When pressure increases due to overlying sediments and water column, the lighter gas molecules starts moving up along with the other light hydrocarbons. This is followed by the water in the pore spaces. The upward movement of gas, oil and water continues till an impermeable layer blocks it. Such layer that restricts the further movement of oil and gas is called Caprock. A caprock can be a sedimentary rock like shale or it can be an igneous rock that may be formed due to lava eruption, covering the sedimentary deposits. This results in the concentration of oil, gas and water in the layer below the caprock and a reservoir is created. Here gas, oil and water are further segregated because of their difference in the densities. Water being the heaviest takes the lower most position. Gas takes the topmost position and the oil is trapped between the two.
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Entrapment
After migration the next most important stage is the entrapment of oil and gas. A reservoir becomes more economically valuable when oil and gas becomes concentrated in one part of a reservoir. This is mainly caused by tilting of sedimentary beds. Causes of tilting can be various; for example a volcanic eruption as shown in the figure to the left, may result in the upliftment of a reservoir beds. This results in the concentration of oil and gas below the cap rock, at the highest part of the reservoir rock, as oil and gas tend to move upwards.
The other causes of sedimentary bed inclination are earthquakes, folding, faulting, erosion etc.
So, wasn't that interesting to learn about the origin of oil and gas reservoir. Let me summarize the whole process in few steps:
1. Microorganisms once lived in a large water body, died and sank to the bottom.
2. They were quickly covered with the sediments creating an anaerobic environment
3. Increased temperature and pressure along with the anaerobic environment resulted in decay of microorganisms in a very special manner.
4. Decayed matter became Kerogen, which released Hydrocarbons with further increase in temperature and pressure
5. These Hydrocarbons then migrated from SOURCE ROCK to a more porous and permeable RESERVOIR ROCK, topped by a CAPROCK.
6. Irregular geologic structures created due to volcanic activities, earthquakes, erosion or some other tectonic activities, results in entrapment of oil and gas in topmost part of an inclined reservoir rock.
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RESERVOIR TRAPS
As mentioned above, after oil generation, its migration and entrapment is a very important step in making the reservoir an economic source. So let's learn about the different types of reservoir traps.
Mainly there are three types of reservoir traps:
1. Structural Traps
2. Stratigraphic Traps
3. Combination Traps
Structural Trap:
In this type of trap the confining element is the structural positions of the rocks, for example domes, folds, faults.
Geosynclines are the best sites for the structural traps. This is mainly because such places are normally very active and mobile resulting in folding and faulting of the rocks. At such places sediment deposition followed by subsidence, causes rocks to get folded and faulted and these are the most favorable requirements for an oil and gas reservoir formation. This is the reason, today worlds most oil fields are located in Geosynclines. The most commonly seen structural trap is an anticline. The anticlines are very important traps as the vertical thickness of the structure extends over several layers of sedimentary rocks, which may result is entrapment of oil in different layers, making more than one economic reservoir in a sedimentary sequence.
The other most commonly seen structural trap is by faulting such as normal and reverse faults. The fault itself may be the soul cause of entrapment but normally in nature they are found in combination with other structures such as folding, tilting and arching. A major fault may result in breaking up of a big reservoir into separate oil reserves.
There are other structural traps for oil and gas accumulation such as fractures, fissures, salt domes, igneous intrusion etc.
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Stratigraphic Traps:
Here the principal confining element is lateral startigraphic or lithological variation. These lateral changes in lithology causes a permeable rock to become impermeable and thus prevent further flow of the oil. Typical examples of such traps are reef deposits, channel deposits, sandstone lenses, unconformities etc.
Stratigraphic traps are of two types:
A. Primary stratigraphic traps: The traps which are formed during deposition or diagenesis of the rocks icluding those formed by lenses, facies changes, shoe string sand and coral reefs.
B. Secondary stratigraphic traps: These traps are formed due to later cause such as unconformity, solution and cementation. In this unconformity makes the very important type of trap. For example a tilted old layers of sandstone may be blocked at the top by an unconformity and the overlying beds.
Rocks below the unconformity are generally highly weathered making the rocks more porous and permeable and thus forming a good reservoir.
Combination Traps:
The combination trap reservoir is one in which structural features and lithology changes combine to form a trap.
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