In this study, 76 samples of fillings (calcareous, argillaceous and calcite) in the Ordovician karst fracture-cave system in Lungu area were collected by layers, and carbon and oxygen isotopes were tested and analyzed (Table 5-3). The analysis results show that the stable carbon and oxygen isotopes of filling minerals obviously reflect the isotopic abundance characteristics of secondary minerals.
Table 5-3 Isotopic Characteristics of Filling Materials in Ordovician Paleokarst Fracture-Cave System in Lungu Area
1. Carbon and oxygen isotopic characteristics of ancient karst fissure fillings in Lunguxi area.
The δ 13C and δ 18O values of the Ordovician carbonate fissure fillings in Lunguxi area vary greatly, with the δ 13C values ranging from-0.70 ‰ to-6.50 ‰, with an average value of-1.76 ‰. The value of δ 1O is -3.75 ‰ ~- 17. 10 ‰, with an average of -9.42%. .
The test results based on PDB show that the δ 133 value of most modern marine inorganic carbonate rocks is close to 0, and the δ 18O value is also close to 0. However, the δ 13C and δ 18O values of carbonate rocks in Lunguxi area are obviously different from this value, and are also obviously different from the average value of original oxygen isotopes of marine carbonate rocks from Devonian to Cambrian (-4 ‰ ~-5 ‰), especially the δ 18O value is obviously negative.
2. Carbon and oxygen isotopic characteristics of the fillings in ancient karst fractures and caves in the east of BG17 well.
It can be seen from the test results (Table 5-3) that the δ 13C and δ 18O values of the fillings in the Ordovician carbonate paleokarst fracture-cave system in the east of BG 17 well vary greatly, with the δ 13C values ranging from 6.03 ‰ to 8.69 ‰; The value of δ 18O is -5.78 ‰ ~- 17.28 ‰. The distribution of δ 13C and δ 18O of the fillings (calcareous argillaceous rocks and calcite) in ancient karst fracture-cave systems (karst caves, dissolved fractures and tectonic fracture) are also different, but the δ 18O value is obviously negative, indicating that the fillings in ancient karst fracture-cave systems are influenced by atmospheric fresh water, reflecting the karst deposition environment of weathered crust at that time. However, the δ 13C value of backfill is relatively negative, and its provenance may come from Carboniferous-Permian.
From the δ 13C-δ 18O diagram (Figure 5-23A), it can be seen that there are at least four different environmental conditions for the formation of carbonate minerals in karst fillings: the first one is formed during the mechanical filling of weathered crust in karst period, and the fillings are argillaceous calcareous sediments, which are carbonate sediments formed by mechanical filling under the action of running water, and are in relatively dry and hot conditions. δ 18O and δ 13C are higher, δ 18O is -9.87 ‰ ~-5.78 ‰, and δ 13C is-4.0/kloc-0 ‰~ 2.1. The second type is chemical deposition filling formed in the later stage, and the δ 18O value of calcite is obviously negative, reflecting that the formation of calcite filling is obviously related to hydrothermal action, while the δ 18O value of calcareous argillaceous material is obviously negative, reflecting that the argillaceous material filled in the early stage was cemented by calcium under the action of heated liquid in the later stage. δ 18O is-17.28 ‰ ~-16 ‰ and δ 13C is -4.78 ‰~- 1.20‰, which is mainly filled in caves. The third type is the karst reconstruction filling in the burial period, and the δ 18O value of the filling is generally in the range of -5.0 ‰ ~- 10.0 ‰, indicating that the formation of the filling in the ancient karst fracture-cave system is influenced by atmospheric fresh water, and the δ 13C value is relatively negative, reflecting that the material source may come from Carboniferous-Permian, δ 655. The fourth type is early filling calcite or calcium, and most of them have no obvious recrystallization, which mainly reflects the isotopic characteristics of early filling environment. δ 18O is-17.74 ‰-12.40 ‰, and δ 13C is 5.82 ‰-6.03 ‰.
Fig. 5-23 Isotopic characteristics of fillings in Ordovician karst fracture-cave system in Lungu area.
The formation environment of carbonate minerals in type ⅰ and type ⅱ karst fillings is relatively common in Lungu area, which belong to the main period of filling and reconstruction of ancient karst fracture-cave system respectively. The third type is mainly distributed in higher areas, such as BN/KOOC-0//KOOC-0/2, BN/KOOC-0/4/KOOC-0/4, BN/KOOC-0/8, etc. Class Ⅳ is mainly distributed in buried karst areas.
There are three kinds of environmental conditions for the formation of karst caves and crevice fillings, mainly Class I and Class II (Figure 5-23B, c). However, calcite filled by structural fractures is obviously related to hydrothermal process (Figure 5-23D).
3. Environmental significance of carbon and oxygen isotope indication in Lungu area.
Environmental significance of (1) oxygen isotope
The δ 18O values of calcite and calcareous argillaceous fillings in ancient karst fracture-cave system in Lungu area are obviously negative, indicating that the fillings in ancient karst fracture-cave system in carbonate rocks are obviously influenced by atmospheric fresh water, reflecting the karst diagenetic environment of weathering crust at that time. On the one hand, carbonate rocks are rich in 18O, while atmospheric water is seriously poor in18o; On the other hand, the oxygen isotope exchange between meteoric water and carbonate rocks obviously depends on the change of temperature, that is, with the increase of temperature, the exchange effect is not good for filling the 18O ancient karst fracture-cave system, while meteoric water is enriched to 18O, and the increase of overburden thickness and temperature in the later period is also beneficial to this effect. Therefore, the δ 18O value has obviously decreased during the transformation of atmospheric water (weathering crust karst) and deep-buried hot water for hundreds of millions of years, and δ 18O has the law of simultaneous migration to low value and negative value with the increase of dissolution intensity, indicating that there is isotope fractionation effect from weak to strong in the dissolution process.
(2) Environmental significance of carbon isotope
Due to the low carbon content of atmospheric water, the volume of carbon in carbonate rocks is much larger than that of pore water that reacts with it during diagenesis, and the carbon isotope exchange between pore water and carbonate rocks is not affected by temperature change, so the composition of δ 13C in carbonate matrix and calcite cement has not changed significantly since its formation, although it is negative overall, it is still close to its original value. However, the δ 13C value of the fillings in the ancient karst fracture-cave system is relatively negative, which is because the fillings mainly come from the Carboniferous-Permian denuded strata in the karst period of weathering crust (early Caledonian-Hercynian and late Hercynian). Compared with the environment when the Ordovician carbonate matrix was formed, the relative proportion between the two foreign carbon reservoirs, namely the oxidized carbon reservoir and the reduced carbon reservoir, has changed. Carbon oxides are mainly stored in carbonate sediments, which is characterized by being rich in 13C. The reduced carbon pool is organic carbon, which is characterized by being rich in 12C. The average value of Carboniferous δ 13C in Lunnan-Donghetang area is -3. 17 ‰, which is very close to the δ 13C value of Ordovician paleokarst fillings in the east of Lungu 7 well area. Therefore, the Ordovician paleokarst filling in the east of Lungu 7 well area comes from Carboniferous-Permian. Obviously, the organic carbon content of Carboniferous-Permian is higher than that of Ordovician. Therefore, the δ 13C value of cave sediments in the east of Lungu 7 well area is lower than that of carbonate matrix in the west of Lungu. On the other hand, it also proves the material source of fillings in ancient karst period and ancient karst fracture-cave system.
(2) Carbon and oxygen isotopic characteristics of Ordovician paleokarst fillings in the outcrop area of northern Tarim.
The stable carbon and oxygen isotopes of Ordovician ancient karst filling minerals in the outcrop area of northern Tarim clearly reflect the isotopic abundance characteristics of secondary minerals (Table 5-4). Compared with the isotopic abundance of marine carbonate rocks [δ 13C(PDB) is generally-1 ‰ ~ 2 ‰, with a range of 3 ‰ ~ 5 ‰], the δ 13C in the Ordovician karst fracture-cave fillings in the outcrop area of northern Tarim is mostly less than-1.
Generally, the δ 18O value of marine carbonate is-1.5 ‰-10 ‰, and that of freshwater carbonate is δ 10‰. The δ 18O value of carbonate fillings in the outcrop area of northern Tarim is -6.46 ‰ ~- 14.54 ‰, which is obviously lower than that of bedrock, indicating that the formation environment of karst fillings is more complicated, and the content of 18O is reduced due to partial heating fluid or isotope replacement.
According to the relationship diagram of δ 13C-δ 18O (Figure 5-24), the ancient karst filling minerals in this area were formed under five different environmental conditions: the first type is argillaceous calcium mud deposition and filling, which is formed under the condition of relatively dry heat with carbonate deposition accompanied by mechanical filling under the action of running water, and δ 18O is-8. The second type is early calcite, and the baked edge invaded by magma vein is observed in the field, with δ 18O being-13.32 ‰ ~-1.46 ‰ and δ 13C being-1.24 ‰. The third type is chemical deposition filling formed in the later stage, which is mainly filled in karst cracks or dissolved structural cracks. The filling process is slow and lasts for a long time, showing multilayer shape; The fourth type is also the chemical deposition filling formed in the later stage, which is mainly found in the cave wall and coexists with gypsum; The fifth category is the early paleokarst fillings, which experienced recrystallization. δ 18O is-15.86 ‰-15.05 ‰, and δ 13C is-4.33 ‰-3.23 ‰, which is obviously lower than other types.
Table 5-4 Test Results of Carbon and Oxygen Stable Isotopes of Ancient Karst Filling Minerals in the Northern Margin of Tarim Basin
Figure 5-24 Relationship Diagram of Paleokarst Filling Minerals δ 13C-δ 18O in the Northern Margin of Tarim Basin
(3) Carbon and oxygen isotopic characteristics of Ordovician ancient karst fillings in Tahe area.
From the test results (Table 5-5), it can be seen that the δ 13C and δ 18O values of the fillings in Ordovician carbonate paleokarst fracture-cave system in Tahe area change greatly, and the δ 13C values are 1.70 ‰ ~-6.67 ‰, δ/kloc. The δ 18O value of the fillings in the ancient karst fracture-cave system is obviously negative, indicating that the fillings in the ancient karst fracture-cave system are influenced by atmospheric fresh water, reflecting the karst sedimentary environment of the weathering crust at that time.
From the δ 13C-δ 18O diagram (Figure 5-25), it can be seen that there are two different environmental conditions for the formation of carbonate minerals in karst fillings: the first one is formed during the mechanical filling of the weathered crust in karst period, and the fillings are argillaceous calcium argillaceous deposits, which are carbonate deposits formed by mechanical filling under the action of running water, and are in relatively dry and hot conditions. δ 18O and δ 13C are higher, δ 18O is -9.88 ‰ ~-6.60 ‰, and δ 13C is -0.74 ‰ ~-2.05 ‰. The second type is chemical sedimentary filling formed in the later stage, and the δ 18O value of calcite is obviously negative, which reflects that the formation of calcite filling is obviously related to hydrothermal process. The value of δ 18O of calcareous argillaceous materials is also negative, reflecting that the argillaceous materials filled in the early stage are cemented by calcium under the action of heated liquid in the later stage, and δ 18O is-12.27 ‰ ~-10.17 ‰, which is mainly filled in caves, karst cracks or.
The formation environment of carbonate minerals in Class I and Class II karst fillings in Tahe area is universal, which belong to the main period of filling and reconstruction of ancient karst fracture-cave system respectively.
Generally speaking, the carbon and oxygen isotope values of sedimentary fillings produced by fresh water leaching, leaching and organic acid intervention in karst process are obviously negative. Among them, the oxygen isotopes in the karst fillings produced in the early supergene and bare environment are rich, and the δ 18O value is obviously negative, while the δ 13C value has little change. The content of CO2 in the sediments produced by organic acids in the released water in buried environment is high, and the δ 13C is obviously negative, and the δ 18O value is also low. Under the action of hot water, organic matter decomposes and methanates, and heavy isotopes in water are enriched, resulting in higher δ 13C in sediments, some of which have higher positive values.
Figure 5-25 Carbon-oxygen Isotopic Relationship Diagram of Filling Materials in Paleokarst Fracture-Cave System in Tahe Area.
Table 5-5 Carbon and oxygen isotope characteristics of fillings in ancient karst fracture-cave system in Tahe area