Definition of Petroleum (Crude oil):

Petroleum can be widely stated as the complex mixture of hydrocarbons which occurs in the earth in liquid, gaseous or solid forms. This is a naturally-occurring brown to black flammable liquid. Crude oils are mainly found in oil reservoirs related with the sedimentary rocks under the surface of earth.

Generation of Petroleum (Crude oil):

However, exactly how crude oil originated is not established, it is usually agreed that crude oil is derived from the marine animal and plant debris subjected to high pressures and temperatures. It is as well suspected that the transformation might have been catalyzed via rock constituents. In spite of of their origins, crude oil is mostly constituted of hydrocarbons mixed by variable amounts of sulphur, nitrogen and oxygen compounds. Metals in the forms of inorganic salts or organometallic compounds are present in the crude mixture in trace quantities, the ratio of various constituents in crude oil, though, differs considerably from one reservoir to the other. 

Petroleum generation takes place over long periods of time - millions of years. In order for petroleum generation to take place, organic matter like dead plants or animals should accumulate in big quantities. The organic matter can be deposited all along with sediments and later buried as more sediment accumulates on top. The sediments and organic material that accumulate are known as source rock. After burial, chemical activity in the absence of oxygen lets the organic material in the source rock to transform into petroleum without the organic matter simply rotting. A good petroleum source rock is a sedimentary rock like shale or limestone that includes between 1and 5% organic carbon.

Rocks occur in numerous environments, comprising lakes, deep regions of the oceans and seas, and swamps. The source rocks should be buried deep enough under the surface of the earth to heat up the organic material, however not so deep that the rocks metamorphose or that the organic material transforms to graphite or materials other than hydrocarbons. Temperatures of less than 302°F (150°C) are usual for the petroleum generation.

Geologists frequently refer to the temperature range in which oil forms as an 'oil window' beneath the minimum temperature oil remains trapped in the form of Kerogen and above the maximum temperature the oil is transformed  to  natural  gas  via  the  procedure  of  thermal  cracking. However, this temperature range is found at various depths beneath the surface all through the world, a typical depth for the oil window is 4 to 6 km. At times, oil which is formed at extreme depths might migrate and become trapped at much shallower depths than where it was formed. The Athabasca Oil Sands is an illustration of this.

According to usually accepted theory, petroleum is derived from the ancient biomass. The theory was primarily based on the isolation of molecules from petroleum that closely look like termed as biomolecules (figure shown below). A number of geologists in Russia adhere to the abiogenic petroleum origin hypothesis and maintain that hydrocarbons of purely inorganic origin exist in Earth's interior. Astronomer Thomas Gold championed the theory in the Western world via supporting the work done by Nikolai Kudryavtsev in the year 1950. It is presently supported principally by Kenney and Krayushkin.

Fig: Structure of Vanadium porphyrin compound (left) extracted from petroleum

Biomass, a renewable energy source, is biological material derived from the living, or recently living organisms, like wood, waste and alcohol fuels. Biomass is generally plant matter grown to produce electricity or generate heat. For illustration, forest residues (like dead trees, branches and tree stumps), yard clippings and wood chips might be employed as biomass. Though, biomass as well comprises plant or animal matter employed for production of fibers or chemicals. Biomass might as well comprise biodegradable wastes that can be burnt as fuel. This excludes organic material like fossil fuels that have been converted via geological methods into substances like coal or petroleum. However, fossil fuels have their origin in the ancient  biomass,  they  are  not  considered biomass via the usually  accepted  definition  as  they  have carbon that has been 'out' of the carbon cycle  for a very long time. Their combustion thus disturbs the carbon-dioxide content in the atmosphere.

The abiogenic origin hypothesis lacks scientific support. Extensive research into the chemical structure of Kerogen has recognized algae as the primary source of oil.  The abiogenic origin hypothesis fails to describe the presence of these markers in Kerogen and oil, and also failing to describe how inorganic origin could be accomplished at temperatures and pressures enough to transform Kerogen to graphite. It has not been successfully employed in uncovering oil deposits via geologists, as the hypothesis lacks any method for finding out where the process might take place.

Production and Accumulation of Organic Matter:

Despite the general occurrence of petroleum, and the huge amount of scientific research on it that has been taken out by many researchers, there remain numerous unresolved questions regarding its origin. However, it is  recognized  that  the  original  source  of  carbon  and  hydrogen in petroleum was in the original materials which made up the primordial earth,  it  is  usually accepted that such two elements have to  pass via an organic stage to  be combined to the varying complex molecules recognized as petroleum. There are many geochemical and geological reasons for this belief, some of which are illustrated below:

a) Petroleum is generally associated by sedimentary rocks, mainly those deposited under marine conditions however as well comprising continental sediments. On the contrary, there is a complete absence of commercial deposits of petroleum where merely igneous or metamorphic rocks are present.

b) The optical activity of petroleum (that is, the capability to rotate the plane of polarized light) is nearly fully confined to compounds of biogenic origin.

c) Most kinds of petroleum have complex hydrocarbon compounds known porphyrins, made either from the green coloring matter of plants (like chlorophyll) or from the red coloring matter of blood (like hemin).

d) Carbon isotope ratios (¹²C/¹³C) point out that petroleum might be derived in large portion from the lipid (that is, fats and waxes) fractions of organisms.

e) Most of the petroleum-like hydrocarbons have been found in recent marine sediments and also in soils in numerous places all through the world. This occurrence makes a link between present living organisms and the petroleum found in the sediments of older geological ages.

Therefore, in order to produce petroleum, organic matter has to be synthesized via living organisms and afterward deposited and preserved in the sediments. Based  on  further  geological  events,  part  of  the sedimentary  organic  matter  might  be  converted  into  petroleum-like compounds known as source rock. It is significant to realize that throughout the history of the earth, the conditions for synthesis, deposition and preservation of the organic matter have changed. 

Organic Source Materials:

The organic material which is the source of most petroleum has probably been  derived  from  the  single-celled  planktonic  (that is, free-floating)  plants like diatoms and  blue green algae,  and single celled planktonic animals like foraminifers, that live in the fresh water.  Such  simple forms were rich in seas long before the starting of the Paleozoic Era (The  Paleozoic  covers  the  period  from  the  first  appearance  of abundant, soft-shelled fossils to when the continents were starting to be dominated by large, comparatively sophisticated reptiles and modern plants) 570,000,000  years  ago,  and  could  have made  the  source organisms  of  the  petroleum  found  in  the  Precambrian  and  early Paleozoic  rocks  they  as well  might  have  contributed  to  much  of  the petroleum found in the younger rocks. Moreover, land plants brought into the lakes and seas via rivers apparently have been the source of a few crude oils. The larger, more complex forms of sea life-like corals, crustaceans, mollusks and shellfish are neither rich adequate nor are there remains sufficiently preserved from the sea scavengers to comprise a source for crude oil.  

The whole organic matter can be categorized into the classes of proteins (amino acids), carbohydrates   (sugars, cellulose), lignin, pigments (comprising porphyrins), and lipids (fats, fatty acids). All however lignin is present in both the living plants and animals the source materials for petroleum are among such five building blocks of the living organisms.

The proteins and their amino acids are comparatively simply decomposed and probably contribute little to the petroleum source material (that is, organic matter). Carbohydrates make up the main part of both plant and animal matter. They are subject to rapid and almost total degradation however might give a logical source material for a few petroleum.

Lignins should definitely be considered as a main contributor to organic deposits of land plants, and however these have contributed largely to lignite and coal deposits, numerous authorities assume that they might comprise a progenitor of petroleum hydrocarbons too. The pigments, specifically the porphyrins, are known to make a minor however recognizable source of crude oil.

It is lipids, though, that might form the main and primary source of petroleum. Biochemically the lipids (that is, fats and fatty acids) are insoluble in water however are soluble in ether, benzene, or chloroform. A few planktonic plants (that is, phytoplanktons) produce and store fatty oils throughout photosynthesis. Furthermore the ¹³C/¹²C ratios in petroleum closely are similar to the ratios found in the lipid fractions of the different phytoplanktons. As the lipid fraction that contains the hydrocarbons most strongly resembling petroleum, it is much more stable as compare to the water-soluble proteins and carbohydrates. It could well be the major building block from which the petroleum is constructed.

Photosynthesis: Basis for Mass Production of Organic Matter:

Photosynthesis is the base for the mass production of organic matter. Around two billion years ago in the Precambrian time, photosynthesis appeared as a universal phenomenon. The emergence of photosynthesis as a universal phenomenon is a remarkable historical event with respect to the formation of potential source rocks.

Photosynthetic method transforms light energy to chemical energy. This is principally a transfer of hydrogen from the water to carbon-dioxide to produce organic matter in the form of glucose and oxygen.

The oxygen generated in this reaction is from the water molecule and not from the carbon-dioxide. Autotrophic organisms (are organisms which produce their own organic compounds by using carbon-dioxide from the air or water in which they live) can then synthesize polysaccharide, like cellulose and starch, and all other essential constituents from the glucose produced throughout photosynthesis. Primitive autotrophic organisms, like photosynthetic bacteria and blue-green algae were the first organisms responsible for this mass production. The fundamental need for photosynthesis is the light absorbing green pigment known as chlorophyll.

Fig: Photosynthesis-Basis for Mass Production of Organic Matter

Equation 1: Equation of Photosynthesis.  Glucose comparatively rich in energy is made up by green plants by the help of sunlight (h.v). Oxygen is the by-product of this procedure.

We might be wondering that of what importance is photosynthesis to the production of organic matter that ultimately leads to the production of source rocks. Though, don't lose sight of the fact that devoid of the production of glucose as an outcome of photosynthesis there will be nothing for autotrophic organisms to synthesize polysaccharide.

Carbon Cycle:

A nearby look at the equation of photosynthesis exhibits that carbon-dioxide is used up in the reaction. As photosynthesis is a continuous procedure it will come to a time that almost all the carbon-dioxide in the atmosphere will be used up and photosynthesis will come to a stop and consequently deposition of organic matter will as well stop. Though, this doesn't happen, carbon dioxide is reintroduced to the atmosphere. Therefore, reintroduction and mass balance of carbon employed in photosynthesis is what is termed as carbon cycle.

The carbon cycle is the biogeochemical cycle via which carbon is replaced among the biosphere, pedosphere, geosphere, hydrosphere and atmosphere of the Earth. The carbon cycle is generally thought of as four main reservoirs of carbon interconnected via pathways of exchange. Such reservoirs are:

• The plants
• The terrestrial biosphere that is generally defined to comprise fresh water systems and non-living organic material, like soil and carbon
• The oceans, comprising dissolved inorganic carbon and living and non-living marine biota.
• The sediments comprising fossil fuels.

The annual movements of carbon, the carbon exchanges between the reservoirs, take place because of different chemical, physical, geological and biological methods. The ocean comprises the biggest active pool of carbon close to the surface of the Earth, however the deep ocean part of this pool doesn't rapidly exchange with the atmosphere. The global carbon budget is the balance of the exchanges (that is, incomes and losses) of carbon between the carbon reservoirs or between one particular loop (example: atmosphere ↔ biosphere) of the carbon cycle. The examination of the carbon budget of a pool or reservoir can give information regarding whether the pool or reservoir is functioning as the source or sink for carbon-dioxide.

Most of the carbon on earth is concentrated in the sedimentary rocks of earth's crust. Part of it is fixed as organic carbon, and a greater portion as carbonate carbon that is, compounds having carbonate anion. A relationship obviously exists between the organic carbon and carbonate carbon. The atmospheric carbon-dioxide reservoir is in the constant exchange by the hydrospheric carbon-dioxide reservoir. From aquatic atmospheres, carbonates might be precipitated or deposited via organisms (shells, skeleton and so on) to form carbonate sediments. On the contrary, carbonate rocks might be dissolved to contribute to the equilibrium reaction between CO₃^2-, HCO₃^- and CO₂ in waters.            

The main organic matter is made directly from the atmospheric reservoir via terrestrial plants, or via photosynthesis of marine plants from dissolved CO₂ in the hydrosphere.

Terrestrial and marine organic matter, in turn, is mainly destroyed via oxidation. Therefore, CO₂    is returned for re-circulation in the system. A simplified sketch exhibiting the major processes and pathways regarding the element carbon in the earth's crust is represented in the figure shown below. Only an almost negligible part of the organic carbon in the earth's crust, comprising the hydrosphere, is found in the living organisms and in dissolved state.

Fig: Diagram of the carbon cycle

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