Cosmic and planetary abundance of carbon     
The element carbon (C) is the fourth in order of cosmological abundance, preceded only by hydrogen (H), helium (He) and oxygen (O). The available carbon in the nebula that gave rise to the solar system was built to Earth in the process of planetesimal accretion. The primary geochemical differentiation made heavier elements stay concentrated in the nucleus. Partial melting processes in the continued evolution of the mantle, crust, hydrosphere and atmosphere. Most of the primordial carbon remained in the Earth's mantle.

   Tectonic processes of high-magnitude enable rise of volatiles from the mantle to shallower crustal levels on Earth. Reactivation of the megastructures in sedimentary basins over its geological history may also promote the upwelling and migration of hydrocarbons.

 

Cosmic abundance of the elements

According to studies performed by Massachusetts Institute of Technology (MIT) to estimate the distribution of carbon on Earth is:

Biosphere, oceans, atmosphere ....... 3.7 x 10e+18 moles

Crust

Organic carbon .............................. 1100 x 10e+18 moles

Carbonates .................................... 5200 x 10e+18 moles

Mantle ....................................... 100000 x 10e+18 moles

Earth's Carbon Budget (MIT)

   Earth's mantle contains according that estimating about 20 times more carbon than in the superficial layers of the planet. This carbon within the mantle is in the oxidized form such as carbon dioxide, carbonates; and not oxidized asdiamonds, hydrocarbons (oil and natural gas) and possibly metal carbides.   There is a serious problem when we use the word "organic carbon" or organic chemistry. Dr. Thomas Gold reminds us that we can read a whole book of organic chemistry without mentioning any organism (biology). A rock thatcontains carbon does not mean that all or part of this carbon is of biological origin, i.e. carbon of real organic biological origin, fossil. This carbon may have migrated in the form as hydrocarbons and inorganic interacted with the rock atlow pressure, including reworking by deep biosphere, by microorganisms that feed on hydrocarbons (archaea) whichalso leave their fingerprints as (biomarkers). Therefore, also the so-called geochemical analyzes of total organic carbon (TOC) in rocks such as shales, actually, do not refer to the organic carbon content of biological origin (as the traditional view would hold), but the analysis of carbon originating from primordial and allocthonous hydrocarbons that migrated from deep sources and are present in these laminates shales. This then leads to a wrong reasoning for suggesting that hydrocarbons would be formed miraculously inside the so-called "source rocks". Therefore unconventional hydrocarbon accumulations in United States such as shale gas (e.g. in Marcellus, Barnett, Bakken, Eagle Ford, Fayetteville, Woodford, Niobrara and all others) and also oil shale as Green River are simply microporous reservoirs and not source-rocks. Other high-order nonsense is to imagine that heat by intrusions of magmas that form igneous rocks such as diabase sills would form hydrocarbons in contact with carbonaceous shales.

   In the process of migration from greater depths hydrocarbons rise to shallower crustal levels carried by helium (He) and Nitrogen (N₂), through cracks in the basement, where subtle decompression occurs. They can stay in porous rocks, fractures and accumulation occurs also trapped in rocks as laminated shales with high microporosity, since the initial migration stages are mostly gas and high pressure systems aided by presence of helium can fix hydrocarbons within these shales.

    It should be noted that the material of biological origin the Earth's surface has a low rate of preservation, initially due decomposition by microorganisms and mainly by oxidation processes. Also in biological detritus dominate biological molecules and other carbohydrate oxidized and no properly hydrocarbon compounds, such as molecules that are dominated in oil and natural gas. Hydrocarbons present in shales are very rich in hydrogen and incompatible with intrinsic biological derivation.

Source: 

http://origeminorganicadopetroleo.blogspot.com.uy/2011/02/normal-0-21-false-false-false-pt-br-x.html