| Dolostones are widely developed in the Middle Permian rocks of East Yunnan and West Guizhou（Upper Yangtze Platform,China）.A series of studies were conducted on the genesis of these dolostones,and various dolostone formation mechanisms have been proposed.However,it remains controversial about the source of magnesium ion,the forming environment characteristics of dolostone,and the main controlling factor of dolomitization.These problems severely hinder the oil-gas extended exploration of East Yunnan and West Guizhou and even the whole Upper Yangtze Platform.To help predict the distribution of dolostone reservoirs and reduce the exploration risk and cost,this study analyses the regularities of distribution of dolostone（longitudinal distribution characteristics and plane distribution characteristics）,the petrographic characteristics of dolostone（macroscopic and microcosmic characteristics）,and the geochemical characteristics of dolostone（cathode luminescence characteristics,order degree characteristics,fluid inclusion characteristics,carbon-oxygen isotope characteristics,strontium isotope characteristics and magnesium isotope characteristics）in detail by the comprehensive use of outcrops,cores and drilling data and proposes a new model of dolomitization:open thermal convection dolomitization.Unlike existing thermal convection models,the dolomitization model proposed here is related to the mantle plume and the scale of dolomitization is very large.In addition,the distribution of dolostones in this thermal convection model is controlled by sedimentary facies and the dolomitization is selective.In this new dolomitization model,dolostones can be divided into two types based on their occurrence in single beds:massive dolostones（complete dolomitization,with dolomite content of 90%～100%）;and patchy dolostones（incomplete dolomitization,with dolomite content of 50%～90%）.The massive dolostones and patchy dolostones are of the same origin,with the only difference being the degree of dolomitization.Mg2+in dolomitizing fluids（high temperature and moderate salinity）originates mostly from seawater,with a minor component coming from deep hydrothermal fluids.The dolomitization should have occurred between the end of the middle Permian period at 259 Ma,and the initial basalt eruption period at 257Ma.The dolomitizing fluid mainly migrates through pores in bioclastic grainstone（shoal facies）and faults and fracture systems.Driven by spatial variations in temperature（due to elevated heat flux in the vicinity of the Emei mantle plume）,the fluid migrates along the circulation flow pathways,resulting in fast and pervasive dolomitization of Middle Permian limestone.The dolostones that have undergone open thermal convection dolomitization are mainly distributed in the Maokou Formation,which primarily contains shoal facies.The proposed model not only explains the characteristics and distribution of dolostones in the study area,but also serves as a reference for predicting the distribution of dolostones in other areas subjected to thermal convection. |
