Kerogen

Evolution of the kerogen

Stage 0 (or pre-stage): died and sedimentation…

On our planet, live and die permanently, of the multitudes of organizations, made up essentially of Carbone, Hydrogène, Azote and Oxygène. They constitute the biomass. A weak part (less than 1%) of this biomass forms a deposit with its death (when it is found included in sedimentary mineral layers in formation). The process of sedimentation is a slow and permanent process at the bottom of the oceans and lakes, which produces certainly few effects on an human life scale, but is of one major importance on a time scale known as “geological” (a few million to a few billion years).

Stage 1: formation of a solid compound called kerogen

All the formed sediments, if they are mineral seemingly, comprise a fraction of organic matter (1% on average), which is found “trapped” in the mineral sediment in formation. This organic fraction undergoes a first transformation by the bacteria at the beginning of sedimentation, and led to the formation of a solid compound called Kérogène, disseminated - considering its small proportion: simple small nets in the mineral part. The latter will be called the “Bed rock”.

Although it is present only in small proportions in the sediments in general, the kerogen represents, on the scale of planet, a total mass of 10.000.000 G T. Only 0,1% of this kerogen (i.e. thousandths of the totality of the formed a deposit organic matter) form coal (but that made 10.000 more WP!), the gas and oil represent each one 0,003% of the total kerogen in order of magnitude (but that still made a few hundreds of billion tons).

The formation itself of the kerogen begins with the formation of the rock known as mother : they are initially muds in which the organic molecules are present.

- Recall: 1 WP = 1 billion tons.

Stage 2: evolution of the kerogen by a beginning of Pyrolysis

From the Plate tectonics , the sediments are inserted slowly in the ground. From the Geothermics, the room temperature increases then gradually (incidentally geothermal energy results from the natural radioactivity of the terrestrial rocks). The speed of hiding being variable, the temperature of the sedimentary unit increases by 0,5 with 20° C per million years. In its turn, each small net of kerogen produced of the water which is sometimes expelled under the effect of the pressure of the layers located above the sediment.

Thus, with this stage, muds of sedimentation solidifies in rocks porous, known as bed rocks , being able to find itself to several hundred meters of depth (up to three kilometers for deepest), while the organic matters change, in several phases, in Eau and Kérogène.

Stage 3: the kerogen undergoes a complete pyrolysis

From 50 to 120 °C, the kerogen undergoes, in anaerobe, a thermal decomposition: pyrolysis. Initially, this decomposition “extracts” water and the CO₂ from the kerogen. Then, the temperatures growing continuously, the kerogen expels liquid hydrocarbons: oil and the “natural” gas. Each small net of kerogen thus starts to produce hydrocarbons. More the sediment is deep (and thus hotter), and more the fraction of gas is important because of a more intense pyrolysis (in time as in temperature), breaking up thus more strongly the kerogen then the liquid hydrocarbons themselves. It “is enough” to a few million years so that the kerogen changes partially, under the effect of heat, out of coal or oil, gas, CO₂ and water.

It is the appearance of gas, as the kerogen is brought up to an increasing temperature (resulting from the hiding), which ends up stopping pyrolysis. The gas pressure in the small pockets which contained the initial kerogen increases indeed in the deep layers (increasingly hot), and when this pressure becomes sufficient to overcome “the impermeability” of the bed rock, the liquid fraction and the gas fraction are gradually expelled of the bed rock.

The age of the bed rock varies from 1 million with 1 billion years at the time of the migration. For oil, the most frequent age being located at the neighborhoods of 100 million years.

Stage 4 of the evolution of the kerogen: fuel formation

Coal

It is due to a particular variety of kerogen, which is formed starting from remains of plants known as “higher” (trees, Fougère S, Prêle S, Lycopode S…). It is a kerogen which shows the characteristic to be dominating in the sediment instead of being minority there. The first stage of sedimentation led to the Peat. During the hiding, pyrolysis leads then to the formation of Lignite, then of Houille, then of Anthracite, which is almost pure Carbone, removed from the essence of sound Hydrogène (and as it is about an ultimate stage of pyrolysis, anthracite is generally deepest of coals). As for the other kerogens, the produced coal of oil and gas during its hiding, although in less quantities with regard to oil. The oil formation starting from coal takes place at the stage coal, and formed methane will be called… the Grisou.

Conversely, one can produce starting from coal:

of the synthetic Gasoline (extremely expensive, its production is valid only in time of war),
of the “Gas for domestic use”, starting from famous “the so expensive gas works” with the former generations,
of the coke (starting from the coking plants), small coal balls usable for the heating and the iron and steel industry.

Oil

Each small net of kerogen produced about all the hydrocarbons which it could produce (there does not remain almost any more hydrogen in the sediment). Under the pressure of gas, “the primary migration” starts.
After being expelled of the Bed rock, the hydrocarbons, the gas and water start a “secondary migration then”: they “ooze” along the permeable layers which are next to the layers of bed rock (which is generally not very permeable, as he is explained above), while moving towards surface under the effect of the pressure of the layers of sediment located above.
These oil escapes are frequent, and as they can come either from bed rocks, or of already formed tanks whose sealing is broken, they were used a long time as markers to find layers, at the beginning of oil exploration.

So that exists an exploitable liquid hydrocarbon layer, it is necessary that they “concentrate” some share before arriving at the ground, which, practically, requires that they are stopped in their increase towards surface by a “trap”. In practice, this trap is a new impermeable layer generally forming a species “of circumflex accent” above the porous rock in which oil circulates. It can be a question of a layer of salt, marl, etc Because of their respective density, the expelled water of the bed rock comes to be placed in lower part of oil, and gas above. At this stage, oil is known as “conventional”. The rock which contains oil calls a tank.

When the kerogen produced all the hydrocarbons which it could produce, that means that it lost any hydrogen sound. There remains a compound close to coal, but not necessarily exploitable for as much because he is always disseminated in the bed rock with contents lower than 1% on average.

But the history of this oil tank does not stop there: taken in the movement of plate tectonics, it is unrelentingly involved towards the increasingly hot deep layers. So oil can undergo a new pyrolysis, (a little the equivalent a distillation in refinery) which will produce gas and a particular variety of Bitume (the pyrobitume) in increasing quantities with time and the temperature.

The natural gas

If the tank, described above, is quite tight (of Argile, of Glaise,…), this new diving involves the formation of a primarily gas layer. If the tank is insufficiently tight, the gas escapes and there remain only the bitumens (or asphalts) in porosities of the rock tank. This explains why, in the sedimentary basins, the gas tanks are generally deeper than the oil layers (in fact pétro-gas).

to summarize the overall diagram of the suitable products for migrations (oil and gas):

1 - primary migration;
2 - secondary migration, along the porous rocks, of the faults;
3 - Dysmigration: oil and gas, escape from the tank where it had accumulated.

Other possible evolutions of the kerogen

the bituminous Sands and the Oils extra-doors, which correspond to the pockets where formed oil lost its volatile elements. If nothing stops the migration to the top of hydrocarbons, they end up arriving close to the ground, where they are degraded by the action of the bacteria and lead to the bitumen formation. The tar sands bituminous (or) of the Athabasca, to the Canada, which constitute the greatest known accumulation of bitumens of this nature in the world, correspond at this stage of the evolution of “oil”. In a certain manner, we have there business with a compound “older than oil”. It is thus about “older” oil than the conventional oil, and which deteriorated close to terrestrial surface while growing rich in heavy molecules;

the oil shales, which indicate - wrongly since it do not contain any bitumen - a mixture of rocks and kerogen not pyrolyzed. They are thus fossile fuels which stopped at the stage “before oil” in the chain of transformation, and these resources should rather be counted in the category of coals, following the example peat or lignite. They can be transformed into oil by undergoing a pyrolysis (with 500° C not to await a million years) in a factory, but the energy assessment is very bad (in general the output is negative, i.e. one spends more energy than one will obtain some then by burning fuel obtained).

Exploitation

At the beginning of the oil exploitation, oil that we could extract was “conventional” oil, i.e. a liquid produced by the pyrolysis of the kerogen, having been expelled of the bed rock, then having had the good idea to concentrate in a tank. To exploit this oil is relatively easy: a part leaves all alone under the pressure of generally associated gas, and another part “is pumped” with various techniques, which do not cease becoming more sophisticated. With this “conventional” oil, the extraction consumes on average, only one any small portion of the supplied energy in extracted oil.

But today, the layers becoming exhausted, the operators are interested more and more in the “nonconventional” oil, which corresponds to “pasty” products, even solids, often very minority within a rock part. These layers are thus much more difficult to exploit…

if they are oils extra-doors, or of bituminous sands, he is necessary for example to inject vapor there under pressure (to flux “oil” by heating it, and to allow him to leave under the steam pressure), which requires to devote to the extraction a few tens of percent of the energy which will be provided by extracted “oil”;

if they are oil shales, the extraction of fuel is connected with a mining activity, and the fuel can represent only a few percent, in weight, rock which he impregnates. Certain geologists even refuse to hold of it account in the inventory of oil reserves.

Conclusions

The formation of the fossil combustiles (coal, hydrocarbons and gas) is thus a natural consequence, with very long run, sedimentation since there is a notable organic fraction in the starting matter. This sedimentation produces a particular body: the kerogen, able itself to produce (coal or oil and/or gas) according to the evolution of the various environmental conditions. But without also long processes (several million years) that private individuals (plate tectonics) making it possible “to concentrate” the formed products, in the beginning, in a very diffuse way, no fossil layer of combustile would exist today.

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