The carbon and hydrogen isotope composition characteristics of kerogen are one of the important contents of oil and gas geochemistry, especially the study of oil (gas) source rocks. The carbon isotope composition of kerogen not only inherits the carbon isotope composition characteristics of its biological precursors, but is also affected by diagenesis and generation. The carbon isotope composition of kerogen in terrestrial source rocks is similar to the average carbon isotope composition of terrestrial organic matter, ranging from -22‰ to -30‰, while the carbon isotope composition of kerogen in marine source rocks has a wider distribution range, -10 ‰～-50‰, most values ??are -26‰±7‰. Relative to its biological precursor, the carbon isotope composition of kerogen that has undergone diagenesis and thermal maturation will not change significantly. Within the thermal evolution range of petroleum generation, the carbon isotope composition of kerogen becomes slightly heavier (less than 2‰) as the degree of thermal evolution increases. However, after entering the metamorphism stage, as the degree of metamorphism increases, the carbon isotope composition of kerogen may seriously change. Heavy.
Shen Pingping (1991) studied the carbon and hydrogen isotopes of kerogen in several basins in my country and concluded that the carbon isotope composition of type I kerogen ranges from -28.0‰ to -30.2‰, and the δ13C value of type I-II kerogen The δ13C value of type II-III kerogen is -26.7‰~-29.4‰, and the value is -24.0‰~-25.8‰. There are two sedimentary environments for source rocks of kerogen with a carbon isotope composition lighter than -28.0‰. One is freshwater deep lake sediments, such as the third core member of the Paleogene in the Biyang Sag and the Permian on the Yaomo Mountain in the southern margin of the Junggar Basin. ; The other is marine sediments, such as the Sinian System in Shouxian County, Anhui Province. The kerogen in these source rocks is mainly sapropelic kerogen. Kerogen whose δ13C is heavier than -26‰, the sedimentary conditions of the source rock are swamp phase or lacustrine phase and sea-continent transitional phase dominated by terrestrial higher plants, such as the Carboniferous-Permian in the Ordos Basin, the Jurassic in the southern margin of the Junggar Basin, etc. , the kerogen in these source rocks is humic kerogen, δ13C ranges from -26.0‰ to -28.0‰, and the sedimentary phase of the source rock is shallow lake to coastal lake facies or sea-continental transitional phase.
Changes in the H/C atomic ratio of kerogen. As the degree of thermal evolution increases, the kerogen H/C atomic ratio decreases. However, the thermal evolution of source rocks has very little change in the carbon isotope composition of kerogen, within 1. It can be seen that when the degree of thermal evolution is high, the kerogen carbon isotope composition can more effectively distinguish kerogen types than the kerogen H/C atomic ratio.
Schoell (1984) calculated the hydrogen isotope composition of kerogen in the existing literature and found that it ranged from -90‰ to -130‰. Shen Ping (1991‰) based on the hydrogen isotope study of kerogen in Baise Basin, Nanyang Basin, Jianghan Basin and Liaohe Basin in my country, he believed that in source rocks deposited in freshwater-brackish water depositional environment, the kerogen hydrogen isotope composition is -133‰ ~-210‰. In source rocks deposited in salt water environments, the kerogen hydrogen isotope composition is -119‰. The source rock of kerogen with δD lighter than -130‰ is freshwater lake facies or freshwater swamp facies deposits, such as the third member of the Paleogene core in the Biyang Sag, the Permian on the Yaomo Mountain in the southern margin of the Junggar Basin, and the Jura in the southern margin of the Junggar Basin. Tie. The source rocks of kerogen with a δD heavier than -130‰ are brackish to salt water lacustrine facies, sea-terrestrial transitional facies and marine sediments, such as the Paleogene-Neogene in the Huanghua Depression and Jinhu Depression, and the Carboniferous-Neogene in the Ordos Basin. Permian System, Sinian System in Shouxian County, Anhui Province.