Cause and identification of carbon dioxide

According to the genesis, carbon dioxide can be divided into two types: organic and inorganic, and the main indexes of its discrimination method are carbon dioxide content, stable carbon isotope and 3He/4He ratio. Dai Jinxing et al. proposed that the δ 13CCO2 of mantle-derived inorganic carbon dioxide is mainly between-8 ‰ and-4 ‰, 3He/4He ≥1.10-5, and CO2/3He is (1~ 65435).

1. Organic carbon dioxide

Carbon dioxide is produced during the diagenetic evolution of organic matter. For example, a large amount of carbon dioxide can be formed when organic matter is biodegraded, and a certain amount of carbon dioxide can also be formed when kerogen evolves. In addition, the oxidation of crude oil and hydrocarbon gas can also generate carbon dioxide, which is mainly associated with hydrocarbon natural gas, with the content less than 10% (and mostly less than 1% or even 2%) and δ 13co2.

2. Inorganic carbon dioxide

Inorganic carbon dioxide mainly includes mantle-derived carbon dioxide formed by mantle degassing, followed by carbon dioxide produced by petrochemical reaction of crust and thermal decomposition of carbonate rocks, and these two kinds of carbon dioxide are often associated, such as Huangqiao carbon dioxide gas field, Wanjinta carbon dioxide gas reservoir and Fangwang carbon dioxide gas reservoir.

(1) mantle-derived carbon dioxide

Mantle-derived carbon dioxide refers to the direct discharge of mantle-derived gas to the shallow part along the open deep fault. Under the condition that deep, medium and shallow faults are connected, mantle flow gas migrates upward along gas source faults to favorable structural positions, and enriches oil and gas reservoirs under suitable sealing, sealing and preservation conditions, such as gas reservoirs in East Africa Rift Valley and Baikal Rift Valley.

(2) Carbon dioxide produced by petrochemical reaction.

The magma formed by the chemical reaction of crustal rocks also contains a certain amount of volatile gases. Although the abundance of carbon dioxide in this genesis is lower than that of ungassed mantle magma, the main component of degassed gas is carbon dioxide [33]. The carbon dioxide produced by this kind of magma degassing shows changeable geochemical characteristics because of the different sources of its magma source carbon. It is generally believed that the δ 13CCO2 of this kind of carbon dioxide is between-10 ‰ and -6 ‰, and the 3he/4He ratio of the associated He is between N×10-7 and N×10-6.

(3) Carbon dioxide formed by thermal decomposition of carbonate rocks.

That is, carbonate rocks in sedimentary strata are decomposed into carbon dioxide under the high temperature and heat of magma or deep fluid, and the reaction process is as follows:

CaCO3 (calcite) →CaO+CO2

FeCO3 (siderite) →FeO+CO2

CaMg(CO3)2 (dolomite) →CaO+MgO+2CO2

CaCO3 (calcite) +SiO2→CO2+CaSiO3

CaMg(CO3)2 (dolomite) +SiO2→2CO2+CaMgSiO4

5FeCO3 (siderite) +SiO2+Al2Si2O5(OH)4 (kaolinite)

+2H2O→Fe5Al2Si3O 10(OH)8 (chlorite) +5CO2

Under the same pressure, the decomposition temperature of different carbonates is different. The decomposition temperature of siderite (FeCO3) is much lower than that of calcite. In fact, when the pressure is equal to 0. 1MPa, the decomposition temperature of FeCO3 is only 280℃. According to the indoor experiment, the required temperature of calcite (CaCO3) is 773℃. Carbon dioxide can be formed with siderite and water at about 200℃, and the carbon dioxide gas reservoir in Shatouxu structural area of Sanshui basin in China is this kind of carbon dioxide gas reservoir.

Only in the metamorphic zone where deep high-temperature magma is in direct contact with carbonate rocks can a large amount of inorganic carbon dioxide be produced. The δ 13CO2 of such gas fields is between-3.7 ‰ and+3.7 ‰; In addition, because the parent carbonate of carbon dioxide is formed in the sedimentary layer, and the process of carbon dioxide generation generally occurs in the shallow crust, the 3He/4He ratio of helium shows the characteristics of typical sedimentary crust source, that is, n× 10-8.

At present, the discovered gas reservoirs with high carbon dioxide content are mainly caused by mantle. However, not all volcanism and magmatism can form carbon dioxide gas reservoirs, and only non-degassed mantle magma can become an effective gas source of carbon dioxide gas reservoirs. The carbon dioxide in the rift basin in the east of China belongs to the carbon dioxide contributed by the degassing of non-degassed mantle magma, and its content is more than 60%.

To sum up, carbon dioxide has important industrial value and theoretical research significance. The content of organic carbon dioxide is very low (generally less than 1% ~ 2%), so it has no practical use value; The content of inorganic carbon dioxide is high (up to 99%), which has industrial utilization value.