Geochemical Research on Straticulate Dolostone and Spatulate Dolostone in Lower Ordovician Strata of Well Tazhong-1,Tarim Basin

A large amount of dolostone occurs in Cambrian-Ordovician strata of the Tarim basin,in which abundant of oil and gas were reserved.Because of the weak previous research on the dolostones,the explorations and developments were strongly restricted here.In this paper,the straticulate dolostone and spatulate dolostone in Lower Ordovician strata of Well Tazhong 1 were studied using petrologic and geochemical methods.The macroscopic conformation and microscopic structure of the straticulate dolostone and spatulate dolostone suggest that the dolostones formed in a high-energy environment;the low degree of order with average value of 0.6,consistent with previous values of dolostones in tidal platform,indicates that the studied dolostones might also be formed in tidal platform.Meanwhile,the contents of high Fe,Mn and low U,Mo support that the dolostones formed in an oxidized environment,consistent with the above results.The values of oxygen isotope compositions are concentrated with δ18O values range in-7.4‰～-5.8‰,and 87Sr/86Sr values were also in a narrow range of 0.709015～0.709276.Both δ18O values and 87Sr/86Sr values in this research are consistent with those of the Late Cambrian-Early Ordovician seawater published previously.Furthermore,the PAAS-normalized REE patterns of the straticulate dolostone and spatulate dolostone are similar with those of Ordovician limestone.Therefore,the necessary magnesium for dolostone might have come from the vaporized seawater.Moreover,the positive correlation between CaO and MgO contents indicates that the straticulate dolostone and spatulate dolostones were not formed by replacement of limestone but by precipitation from vaporized seawater.Therefore,the straticulate dolostone and spatulate dolostones were rapidly formed by direct precipitation from vaporized seawater in tidal platform.