Strontium Isotope Composition and Characteristic Analysis of Cambrian-Ordovician Carbonate in the Region of Tazhong, Tarim Basin

The research on the trace elements of Ordovician carbonates plays an important part in the whole work on reservoir in Tazhong (塔中) area. This paper systematically studies the characteristics and sedimentary settings of Ordovician dolomites in Tazhong area, Tarim basin, and debates their enrichment of mechanisms and different element existing patterns. The study makes use of ICP-MS analysis technology to test the strontium and manganese content of 109 samples from four wells in the Tazhong area, Tarim basin and strontium isotope composition tests have also been done on 25 samples from wells Zhong-1 and Zhong-4 on VG354 solid isotope mass spectrograph. By means of analyzing contents of strontium and manganese elements, doing research on the strontium isotope composition characteristics from wells Zhong-1 and Zhong-4 and comparing the results of strontium analysis to the global Ordovician marine carbonate and its evolution trend, in the combination of sedimentary facies characteristics of isolated wells in this area, we can come to the following cognitions: (1) The marine carbonate strontium isotope curve in the Tazhong area of Tarim basin is consistent to the global evolution trend which is overall descending with time, the direct reason of which is the evolution of paleogeographic environment. The Ordovician paleogeographic environment goes through restricted platform to open platform and then shallow marine shelf in Tazhong area, Tarim basin. Dolomitization is another subordinate reason and the inversion of fluid with high manganese can lead to heavy strontium; (2) The fact that the ⁸⁷Sr/⁸⁶Sr ratios of the Upper Ordovician in Tazhong area have an apparent mono-decline trend with the time going by, which is similar to the global strontium isotope ratio, is suggesting that the variation of the Upper Ordovician sea level is starting, the overall trend of which is the rising of the sea level; (3) Compared to the global seawater strontium ratios, the ⁸⁷Sr/⁸⁶Sr ratio of the Lower Ordovician is much high, and ⁸⁷Sr/⁸⁶Sr ratio of the Lower Ordovician is overall increasing with its maximum value reaching 0.709 727, the reasons of which are dolomitization, evaporation of the supratidal and the reflux of high manganese haline, those reasons can lead to the increasing amount of the strontium and the rising of the ⁸⁷Sr/⁸⁶Sr ratio; (4) The manganese element content changes little. Dolomitization has little impact on it; (5) Big variation occurs in the strontium values of Ordovician carbonate in the region of Tazhong. Apart from paleogeographic environment and petrography, the factors which can affect the values of strontium are the content of the terrestrial clastics and the diagenesis modification which may result in the further changing of the strontium values.