GEOCHEMICAL CHARACTERISTICS OF A ORDOVICIAN CARBONATE CAVE IN KEPING AREA, TARIM BASIN

Affected by multi-stage tectonic movement and fluid activity events, the Ordovician carbonate strata develop complex karst in Tarim Basin. The geochemical characteristics of carbon oxygen isotope and strontium isotope of typical outcrops in Yijianfang in Keping area are analyzed. The results show that the range of filling δ¹⁸O value is -10 ‰ ~ -6 ‰, and the range of δ¹³C value is -0.3 ‰ ~ 0.5 ‰, which is slightly different from the surrounding rock.The paleosalinity value is more than 120, indicating that the sediments are marine origin and formed in the middle and late Caledonian. The δ¹³C value of the filling in the cave is less than -0.5 ‰, and the paleosalinity value is less than 120, indicating that the sediments are formed by atmospheric freshwater. The obvious negative δ¹⁸O may be due to the influence of late Hercynian hydrothermal activity or the rise of temperature under burial conditions. According to the strontium isotope test, the ⁸⁷Sr/⁸⁶Sr value of the filling in the cave is between 0.70955 and 0.7106, which is larger than the ⁸⁷Sr/⁸⁶Sr value of the O₁ formation of the Tarim Ordovician. The ⁸⁷Sr/⁸⁶Sr value of surrounding rock ranges from 0.7089 to 0.70922, which is close to the ⁸⁷Sr/⁸⁶Sr value of Ordovician O₁ stratum, and the formation environment of fillings in this area can be judged accordingly. When the ⁸⁷Sr/⁸⁶Sr value is significantly higher than 0.7091, it indicates that the filling material and siliceous layer are formed under the action of late Hercynian hydrothermal activity. When the value is slightly higher, the filling is formed in the shallow burial stage due to the effect of formation water or the dissolution fluid carrying strontium. To sum up, the formation of cave is affected by Caledonian movement and Hercynian movement. The filling of surrounding rock and cave is the product of late Hercynian heating fluid, which can provide important research ideas and basis for the prediction of carbonate cave reservoir and reservoir development.