Oil & Gas Geology ›› 2018, Vol. 39 ›› Issue (2): 318-329.doi: 10.11743/ogg20180211

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Typological analysis and genetic mechanism of dolomite in the lower Cambrian Longwangmiao Formation,eastern Sichuan Basin

Sun Haitao1, Zhang Yuyin2, Liu Huilin3, Xie Rui1, Yang Xueqi1, Ren Ying1   

  1. 1. School of Geosciences, China University of Petroleum(Beijing), Beijing 102249, China;
    2. Petroleum Exploration & Production Research Institute, SINOPEC, Beijing 100083, China;
    3. Geophysical Exploration Academy of Hebei Province, Langfang, Hebei 065000, China
  • Received:2018-01-05 Revised:2018-02-08 Online:2018-04-28 Published:2018-05-16

Abstract: The dense sampling of the dolomite from the Bandenggou outcrop of Longwangmiao Formation in the eastern Sichuan Basin was carried out with the samples selected through X-ray diffraction.The classification and genesis of the dolomite were demonstrated based on test results such as major and trace elements,rare earth elements (REEs) and carbon and oxygen isotopes.The results show that the dolomitization of the study area could be divided into two types in terms of origins.One is the micritic dolomite with low order degree,which developed in evaporating environment and was influenced by para-syngenetic Sabkha dolomitization,seepage-reflux dolomitization,and terrigenous clastic minerals.For this kind of dolomite,MgO and CaO contents are positively correlated,and Mg/Ca ratio is high in general,with low content of Sr,high content of Fe,Mn and REE+Y,and significant δEu & δCe negative anomalies.Additionally,the δ13C & δ18O values are consistent with those of the limestone deposited in coeval sea water.The other is the powder fine crystal dolomite,formed by shallow burial dolomitization,with negatively correlated MgO and CaO content,low Mg/Ca value,high content of Sr,Fe and Mn,low content of REE+Y,δEu & δCe negative anomalies,and the same δ13C value of the limestone deposited in coeval sea water.However,the value of δ18O is a little bit lower than that of the limestone in coeval sea water.According to the petrographic characteristics of the dolomite,we believe that the rock has undergone the syndepositional dolomitization (including Sabkha effect and reflux infiltration)superimposed by shallow burial dolomitization at later time.As a result,the values of carbon and oxygen isotopes became different through the processes.

Key words: REE, carbon and oxygen isotopes, dolomite, Longwangmiao Formation, Sichuan Basin

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