Fluid origin of cave-filling giant crystalline calcites in the Early Paleozoic in Tahe Oilfield, Tarim Basin and the model of the fluid action

Trace elements, δ¹³C, δ^l8O and ⁸⁷Sr/⁸⁶Sr of cave-filling giant crystalline calcites and fluid inclusion homogenization temperatures (HTs) were analyzed from each formation in the Eopalaeozoic carbonate reservoirs. Result shows that the giant crystalline calcite samples in different wells may have various fluid origins. The shared characters of giant crystalline calcites in well T417 (Yingshan Formation) , S76 (Yingshan Formation), T904 (Yijianfang Formation) and TS1 (Qiulitage Formation) are: U-rich, high V/Cr ratios and at least 20 °C higher than the ambient rocks. It reflects that there is a hot fluid in these wells and these fluids show high ⁸⁷Sr/⁸⁶Sr ratios and low δ¹⁸O values. The giant crystalline calcite samples in well T904 (Lianglitage Formation) are charactized by U-deplete, low V/Cr ratios and temperature(75 81 °C), which indicates a low temperature fluid. The characters of ⁸⁷Sr/⁸⁶Sr-rich and low δ¹⁸O values suggest that this low temperature fluid should be derived from meteoric water. The migration of the hot fluid and meteoric water is likely controlled by various elements : faults, cracks, and unconformity ,etc. So the giant crystalline calcites in different formations and wells may origin from various fluids.