The isotopes of carbon

oxygen and strontium

REE

and the geochemical features of the fluid inclusions in the Middle-Upper Cambrian and Middle-Lower Ordovician side-beds and fracture fillings of the well Yingdong-2 were systematically analyzed.Based on the results

we traced phases and sources of the reservoir fluids

determined chronogenesis of fluid charging

explored the impact of the fluids upon reservoir modification and discussed the dynamic preserving conditions for oil and gas in such settings.The geochemical study of the fluids suggests that there are three charging phases of allogenic fluids with abundant Sr from the crust in the Middle-Upper Cambrian and Middle-Lower Ordovician .The fluids charged in the first phase have a

87

Sr/

86

Sr ratio of 0.710 8

the fluids charged in the second phase have a

87

Sr/

86

Sr ratio in the range of 0.709 8-0.710 0

and the fluids charged in the last phase are hydrothermal fluid from the deep crust and have a

87

Sr/

86

Sr ratio of 0.713 8.The fluids charged in these three phases all played a destructive role in the forming of reservoirs

but the hydrothermal fluids charged in the last phase were constructive to the reservoir formation.The Middle-Upper Cambrian reservoirs had relatively better hydrocarbon preserving conditions during the first phase of fluid charging

but lost them when the second and the third phase occurred.Geochemical tracing of fluids provides a new method for dynamic evaluation of deep fluid activities in the Yingdong tectonic belt as well as the whole eastern Tarim Basin.