Abstract: The fracture-cave reservoirs of weathering crust on the top of Middle-Lower Ordovician in the Tahe Oil Field of Tarim Basin are the major production layer, and large scale reservoirs are also developed underneath the reservoirs. In order to discuss the genetic mechanism of formation of these reservoirs, carbon and oxygen isotope, strontium isotope, rare earth elements and cathodoluminescence were tested in the well A area. The δ¹⁸O and δ¹³C values of calcite in caves and structural fractures are obviously negative than those in limestone background. The mean value of δ¹⁸O_PDB is -14.74‰, showing a trend of constant δ¹⁸O and variable δ¹³C, which are the calcite lines of atmospheric water formed by degassing. The ⁸⁷Sr/⁸⁶Sr ratios of calcites from caves and structures range from 0.709 622 to 0.709 968, which are obviously higher than the background values, and are mainly affected by crustal strontium. There are no positive anomalies in Ce and Eu elements, indicating that there is no hydrothermal fluid involved. The cathodoluminescence of calcites are mainly orange and dark brown, which represents weak oxidation-reduction environment. The results showed that the fracture-cave reservoirs of the Midde-Lower Ordovician Yingshan Formation in the well A area were formed in the deep slow flow zone of atmospheric water during the Early Hercynian, showing the characteristics of pressure bearing, slow flow and not controlled by the karst drainage base level.