Abstract:

Cambrian-Ordovician dolostones in the Tarim Basin are hydrocarbon reservoir rocks of vital importance. Based on detailed petrographic investigations, four types of dolostones have been recognized, which are, respectively, mud-silt-sized dolostone, algal laminated dolostone （ALD）, prophyritic dolostone, and recrystalized dolostone. Determination of rare earth element （REE） composition, combined with petrographic observation, has been employed aiming to gain insights into the property of diagenetic fluids and fluid-rock ratios. The ΣREE of the four types of dolostones distinctly differentiates from each other. However, their REE patterns are all rich in LREE, depleted in HREE and have Eu negative anomalies. These characteristics are comparable to those of finely crystalline limestone, and are mainly influenced by the sea water. These four types of dolostones show similar REE mobility behaviour and no significant fractionation, although they have been subjected to evidently different diageneses. Mud-silt-sized dolostone has an average ΣREE of 51.55×10^-6, close to that of the finely crystalline limestone, suggesting the REE primarily inherited from its precursor limestone, and the dolostone probably experienced relatively low fluidrock ratio during diagenesis. The ΣREE of ALD is 21.37×10^-6 which less than 1/2 of that of the mud-silt-sized dolostones. The petrographic and geochemical characteristics reveal that algae, which has very low ΣREE, possibly played an important role during its diagenesis. So the low ΣREE of ALD was likely resulted from the involvement of algae during the dolomitization, further more, it was not modified significantly during epigenetic diagenesis. The ΣREE of prophyritic dolostone is 28.89×10^-6, much less than that of the finely crystalline limestone and mud-silt-sized dolostone, indicating that it experienced diagenesis of higher fluid-rock ratio than the mud-silt-sized dolostone. The ΣREE of recrystalized dolostone is 13.64×10^-6, less than 1/4 of that of finely crystalline limestone, and ranks the lowest in the 4 types. Petrographic and geochemical characteristics show that this kind of dolostone formed as results of dolomitization and recrystallization of pre-existing dolomite during deep burial diagenesis, both of which require high fluidrock ratios. The REE composition characteristics were probably caused by dolomitization and superimposed by recrystallization of pre-existing dolomite. 

Key words: rare earth element （REE）, dolostone, diagenetic fluid, Tarim Basin, fluid-rock ratio