Characteristics of oxygen and carbon isotopes of karst fissure-cave fillings in the Yingshan formation, TazhongⅡarea，Tarim basin and their implications for environment

This work is based on analysis of logging curves, core observations and thin section identification to fissure-cavity filings from Yingshan formation of Ordovician in the Tazhong II area, Tarim basin. The results show that the fillings are of high GR values in logging curves and their lithology is dominated by calcareous-argillaceous material and breccias. Comparison of oxygen and carbon isotopes with bedrock indicates that the calcareous-argillaceous filling contains 0.6‰ to 1.04‰ ofδ13C and -6.8‰ to -5.5‰ of δ18O, of which theδ13C value is obviously more positive. However，theδ13C andδ18O values of small breccia fillings are -0.13‰ and -6.29‰, respectively, consistent with that of bedrock. Both the fluid nature and ancient hydrological setting were variable when the fillings formed, resulting in different oxygen and carbon isotope values. Thus such values can be used to estimate fluid properties and ancient hydrological environment. Based on previous studies and the results of this test, we consider that the calcareous mudstone in caves was filled in a marine environment during the Lianglitage period and breccia fillings might be formed by collapsed or broken bedrock around caves. This recognition provides evidence for the study of filling mechanism in ancient karst fissure-cave reservoirs and establishment of paleokarst models.