Formation and diagenesis of sedimentary organic matter

1. Sources and types of sedimentary organic matter

Sedimentary organic matter refers to secretions and excretions from various biological relics and their biological metabolism, which directly or indirectly enter the sediments, or enter the sediments through biodegradation and deposition, or generate new organic compounds and their derivatives through condensation.

Organic matter entering the sediment includes: ① in-situ organic matter from lower organisms and higher plants; The main organic components in organisms are lipids, protein, carbohydrates, lignin and so on. When an organism dies, these organic substances will be decomposed to varying degrees under different redox conditions. Some decomposition products will be used as energy by some organisms, thus participating in the recycling of organic carbon in the biosphere. Another part of the decomposition products become simple inorganic small molecules, such as CO2 and H2 O, by physical and chemical action, and the rest, in most cases, only account for a very small part of the original biomass, and enter the sediments to form sedimentary organic matter; (2) Exogenous organic matter formed by rivers, precipitation and wind; ③ The "redeposited" organic matter is transformed by various actions and transferred from ancient sedimentary rocks.

There are two main types of organic matter in geological bodies: one is organic matter with high stability during diagenesis, such as amino acids, fatty acids, porphyrins and pigments; The other is the newly generated organic matter during diagenesis and metamorphism, such as hydrocarbons, humic acid and kerogen. Among them, most of the newly generated organic matter has lost its identity with biology. Biopolymers undergo a series of reactions, such as disappearance of carboxyl, hydroxyl and other functional groups, hydrogenation, isomerization, cracking and so on. The pyrolysis products form a configuration with stable geochemical properties through molecular rearrangement and polymerization.

2. Preservation of organic matter

In most cases, the organic matter in sediments is mainly the tiny residues of phytoplankton, and the factors that affect the preservation of these residues are: the flux of organic matter into sediments, the deposition rate, the particle size of sediments and the oxygen content (Henrich HS, 1993). The flux of organic matter into sediments depends on the biological productivity of surface water and the water depth covered by sediments. A large number of phytoplankton (algae and photosynthetic bacteria) are collectively called phytoplankton, which are the primary producers of organic carbon in almost all marine ecosystems and other freshwater ecosystems. After death, aquatic organisms are rapidly consumed by various bacteria and animals in the process of falling from the photosynthetic zone (generally 200m) on the surface of water. Therefore, the deeper the water body, the less the organic matter content in the sediments reaching the bottom of the water body. In coastal waters, such as deltas, bays, estuaries and adjacent seas, terrestrial higher plants are an important part of organic matter in this region.

The content of organic matter in sediments is inversely proportional to the particle size of sediments. First of all, the density of organic particles is small, and their flow velocity in the water flow is very slow, so that very fine organic particles can gather in the deposition place; Secondly, there is a considerable amount of organic matter attached to the surface of mineral particles in sediments, so the smaller the mineral particles, the larger the specific surface area and the higher the organic matter content.

Mayer (1993) suggested that organic matter adsorbed by mineral particles is more difficult to dissolve than single organic matter particles that are not adsorbed, which means that this part of organic matter can not be easily consumed by heterotrophs in sediments, but can be preserved. In addition, the permeability of fine-grained sediments is lower than that of coarse-grained sediments, and the lower the permeability, the lower the dissolved oxygen content in sediments. Low redox potential, especially low oxygen content, is the most important condition for organic matter preservation. In short, a large amount of organic matter in sediments is preserved, provided that the buried amount of organic matter is greater than the oxidation amount. The amount of oxidation depends on deposition rate, biological disturbance, diffusion and overlying water depth. When the buried amount of organic carbon exceeds the oxidation amount, oxygen will eventually be completely consumed, thus bringing the environment into a reducing state and ending the aerobic respiration of organisms. This may happen in the sediment or the water body itself. In modern oceans, it is very rare for deep water bodies to become anoxic. In fact, most deep-sea areas will not lack oxygen, even in sediments. Hypoxia only occurs in several basins, such as the Black Sea, where deep-water circulation is restricted. However, in some periods of geological history, anoxic conditions seem to be more common, such as Cretaceous, when the ocean circulation was different from today. In many lakes, hypoxia may be more common, and compared with the open marine environment, lakes are richer in nutrients.

It is controversial whether the preservation of a large amount of organic matter in sediments requires an anoxic environment. Calvert et al. (1992) pointed out that the sediments in aerobic and anoxic basins have similar organic carbon content. They also pointed out that the decomposition level of marine organic matter is similar under aerobic and anoxic conditions. Under sulfate reduction, the decomposition degree of terrigenous organic matter is low.

3. Diagenesis of marine sediments

The biological concept of organic diagenesis mainly refers to the change of organic components in newly deposited sediments. In fact, in the process of organic matter crossing the water body, under the action of various animals and bacteria, the change of organic matter composition has begun before it reaches the sediment. Of course, a considerable part of the organic matter reaching the surface of sediments is the excrement particles of all animals such as zooplankton and even whales. After reaching the surface of the sediment, the organic matter continues to decompose, and the subsequent sediments are piled up and buried, and finally the organic matter is separated from the water body. The amount of organic matter deposited here should be enough to consume oxygen, so that organic matter can accumulate.

A large amount of organic matter in sediments exists in solid form, and only a small amount of dissolved components become the main source of microbial nutrients. Usually, complex organic macromolecules cannot be completely decomposed by an organism. Because it is impossible for any organism to secrete all the enzymes needed for the decomposition of organic macromolecules, the decomposition of organic macromolecules requires bacterial flora, and in the further decomposition, part of energy is consumed, garbage is produced, and smaller organic molecules are formed. In this way, protein, carbohydrates and lipids are decomposed into amino acids, monosaccharides and long-chain fatty acids, and these small molecules are decomposed into acetic acid and other short-chain carboxylic acids, ethanol, hydrogen and CO2 by fermentation bacteria. Finally, the above products were converted into methane by methanogenic bacteria. In this process, the remains of bacteria themselves become an important part of sedimentary organic matter. In biological communities, every step of organic matter oxidation will lead to interdependence among different bacterial populations. For example, many bacteria depend on the excrement "garbage" of other bacteria to survive. In sediments, there are also interdependent relationships among different biological communities. For example, anaerobic bacteria depend on the anoxic environment produced by aerobic bacteria. Reducing substances, such as sulfide, ammonia and methane, are all products excreted by deep anaerobic bacteria, which diffuse upward into the oxidation zone and are oxidized by various photosynthetic bacteria, chemosynthetic bacteria and methane-oxidizing bacteria.

Therefore, simple organic molecules, such as amino acids, sugars and short-chain carboxylic acids, can be rapidly decomposed by bacteria (in the range of days to weeks); However, more complex molecules, such as polysaccharides and fatty acids, take months to years to decompose (Henrich HS, 1993). Some compounds, especially organic compounds with cell structure, such as algae, have the ability to resist bacterial decomposition, so they can be preserved. Organic matter from higher plants in different places is also an important part of organic matter rich in aromatic compounds in coastal environmental sediments. However, a small amount of volatile compounds can be preserved, even in relatively old sediments, although bacteria decompose organic matter to a great extent, there can still be a small amount of volatile organic matter. These molecules can be preserved because they are in a small environment protected by bacterial enzymes, and these volatile molecules are preserved by being wrapped in structures with strong decomposition resistance (such as spores and pollen). Organic matter is adsorbed on the surface of inorganic mineral particles and also has certain anti-decomposition ability. Organic molecules adsorbed on the surface of inorganic mineral particles are not easily affected by enzymes, and organic molecules partially or completely wrapped in micropores on the solid surface can be better protected. Similarly, protein organic matter in carbonate shells can be protected from bacterial enzymes to some extent, thus being preserved.

4. Diagenesis of continental sediments

The diagenesis of continental sediments is generally similar to that of marine sediments. Most of the organic debris in the fresh water environment comes from plants, and the organic matter from animals is less than 10%. The main difference between lake diagenesis and marine diagenesis is the low sulfate content in lake environment. Sulfate is an important oxidant. In the early diagenetic process, sulfur can combine with organic molecules (mainly lipids), and this process becomes "natural vulcanization". Because of the low sulfur content in fresh water, the sulfate reduction zone is very limited, and the vulcanization process is difficult to occur. In a huge lake, a large amount of organic matter that reaches the sediments comes from the original source (that is, it is all produced by the lake itself, mainly phytoplankton), but there are often organic matter from different places in land plants, which becomes an important part of organic matter in freshwater sediments. Higher plants growing in water are also the main source of organic matter, such as higher plants with in-situ origin in swamp and wetland environments. Higher plants are richer in aromatic compounds than algae. These aromatic compounds are very stable, such as lignin, tannin, resin and cork. They all come from higher plants and have strong resistance to bacterial decomposition, so they are easier to be preserved in sediments.

Coal is formed by compaction and diagenesis of sediments in peat, a swamp wetland environment. Unlike petroleum, oil source rocks contain only a few percent of organic matter, while the organic matter content of source rocks that form coal is very high. In the modern sedimentary environment rich in organic matter, peat content is the result of the comprehensive action of many factors. The first is biological productivity. Wetlands usually have the characteristics of high biological productivity, so the flux of organic matter transferred to sediments is high. The second is hydrological conditions. Peat is formed in water-saturated soil environment, which prevents oxygen from entering the sediment, thus making the sediment below the water interface become anoxic environment soon. The third is a large number of dissolved organic acids, which can be decomposed and produced, and the rest are secreted by moss and bacteria. These organic acids reduce the environmental pH value and inhibit the ability of bacterial decomposition. The fourth type is biotype. In the above environment, the main producers are bryophytes and various plants. These organisms contain relatively more aromatic compounds. Compared with the dominant aliphatic compounds in algae and bacteria, aromatic compounds have stronger anti-decomposition ability.

5. Several important chemical reactions in thermal evolution of sedimentary organic matter

Organic matter in geological bodies is a complex organic mixture composed of various organic compounds. Under the action of various geological forces, a series of chemical reactions will occur, mainly including the following aspects.

(1) decay reaction

In the early stage of diagenesis, after the death of animals and plants, their remains began to decompose under the action of autolytic enzymes existing in tissues, and then microorganisms such as bacteria participated in and completed the process of decomposition, destruction and mineralization. Microbial respiration is the main reason for the decomposition of sedimentary organic matter. Decomposition can occur in aerobic and anaerobic environments. Aerobic decomposition is the biological oxidation with molecular oxygen as the final acceptor of hydrogen, and the final products are CO2 and H2 O. Aerobic microorganisms such as Bacillus, rhizobia, nitrogen-fixing bacteria, actinomycetes and molds all obtain energy through aerobic decomposition. Anaerobic decomposition is a biological oxidation process in an environment without atmospheric oxygen. As the acceptor of hydrogen and electrons, it is not free oxygen, but inorganic substances such as, and so on. Anaerobic microorganisms, such as methanogens and desulfovibrio, all obtain energy through anaerobic decomposition. Fermentation is an oxidation without external electron acceptor. In the process of glycolysis, different parts of the same organic molecule are used as donors and acceptors of electrons and hydrogen respectively, so the oxidation is incomplete and the energy produced is low.

(2) Redox reaction

Redox reaction is a kind of important chemical reaction commonly existing in the process of organic matter formation and decomposition. Photosynthesis is a redox reaction. Usually, adding oxygen or dehydrogenation to organic molecules is called oxidation reaction, and hydrogenation or deoxidation reaction is called reduction reaction. For example:

CH3CHOHCOOH (lactic acid) → ch3cooh (pyruvate) +2h+2e

(3) Addition reaction

Organic molecules contain unsaturated double bonds, which break in the reaction, and new atoms or groups are added to the carbon atoms at both ends of the original unsaturated bonds to generate saturated organic compounds. For example:

RCH=CH2+H2 →RCH2—CH3

(4) condensation reaction and polymerization reaction

Generally, in the process of combining organic compounds with the same or different molecules, there is a reaction to remove small molecular compounds such as H2 O and HX, which is called condensation reaction. The reaction in which low molecular compounds (monomers) combine to form high molecular compounds (polymers) is called polymerization. Generally speaking, polymerization is a reaction in which one or more organic monomers with two or more functional groups are condensed with each other to form polymers, and at the same time, small molecular compounds such as water, nitrogen and alcohol are precipitated. Polymerization is carried out through organic active groups (-COOH, >; C=O, -OH, -NH2, etc. ) (Figure 8- 18).

Figure 8- 18 Polymerization and depolymerization of sucrose

(5) depolymerization reaction

The process of splitting macromolecules into small molecules is called depolymerization, which is the reverse process of polymerization. For example, starch breaks down into monosaccharides, and protein breaks down into amino acids. For example, the dehydration and condensation of glucose and fructose form sucrose, while the hydrolysis of sucrose into glucose and fructose is a depolymerization reaction (Figure 8- 18). The depolymerization of long-chain hydrocarbons by heating or catalysts is often called cracking.

6. Several main changes of organic matter during diagenesis.

In the process of diagenesis in which bacteria participate, the changes of organic matter can be summarized as follows:

1) functional groups such as carboxyl and hydroxyl groups are preferentially separated from the parent molecule.

2) The abundance of organic matter prone to metabolic changes decreases. The contents of unstable components such as nucleic acids and amino acids decrease rapidly, followed by carbohydrates, especially simple carbohydrates and carbohydrates (such as starch), which are more easily destroyed than carbohydrates with special structures (cellulose).

3) Due to the hydrogenation of double bond carbon, the content of unsaturated hydrocarbon is lower than that of saturated hydrocarbon.

4) Compared with aromatic compounds, the content of aliphatic compounds is reduced. On the one hand, it is caused by aromatization of unsaturated fatty acids, on the other hand, it is caused by the stability of aromatic compounds.

5) Compared with long-chain organic molecules, the content of short-chain molecules (alkanes and fatty acids) is reduced.

6) Hydrolysis of complex molecules produces a series of molecular fragments, which combine with other molecules to produce some new organic molecules.

7) In the environment of high-sulfur marine sediments, H2 S (produced by sulfur-reducing bacteria) combines with long-chain double-bond organic compounds such as isoprenoid hydrocarbons to produce thiol functional groups. These functional groups then form organic molecules with cyclic structures, and finally form aromatic phenylthio groups. This process is natural vulcanization.

8) Many molecules and molecular fragments condense to form more complex macromolecules.

The main product of these processes is kerogen, which is a mixture of complex organic compounds dominated by organic debris in sediments.