四川盆地东部寒武系洗象池组白云岩成因研究

Research on the Genesis of Dolomites in the Cambrian Xixiangchi Formation in the Eastern Sichuan Basin

白云岩地层对成岩环境具有较高的敏感性,使得其在长期沉积和埋藏的地质历史时期极易受到各种成岩作用的改造,进而能形成良好的储层。但是,白云岩问题至今都是困扰国内外学者的重要科学问题,其成因尚未有明确定论。以四川盆地东部寒武系洗象池组为研究对象,通过钻井、野外露头、岩心、薄片以及地球化学分析等资料和手段,探讨白云岩的成因。通过研究区白云岩的稀土元素可以看出,其成岩流体来源主要为海水。研究区洗象池沉积期,受蒸发作用以及海平面频繁变化的影响,孔隙水不断向空中散发,同时海水又通过毛细管作用不断补充到沉积物的颗粒之间,导致孔隙水盐度增高,沉淀出石膏。石膏的沉淀使孔隙水或者表层积水Mg/Ca比率大大增加,形成高镁粒间水,进而交代其他碳酸盐岩矿物,形成白云岩。

The dolomite strata is quite sensitive to the diagenetic environment.Based on such a feature,during the long-term sedimentation and burial geological history,the dolomite strata was extremely susceptible to various diagenesis transformations,which was conducive to the formation of good reservoirs.However,the dolomite problem has been an important scientific topic that challenges scholars at home and abroad,and there is still no clear conclusion on the cause of dolomite formation.Focusing on the Cambrian Xixiangchi Formation in the eastern Sichuan Basin as the research object,based on data and method such as drilling,field outcrops,cores,slices,and geochemical analysis,this study explored the genesis of the dolomites in that area.The results of a comprehensive study showed that the main source of diagenetic fluids was seawater.During the sedimentation period of the Xixiangchi in the study area,affected by evaporation and frequent changes in sea level,pore water continued to evaporate into the air,and at the same time,seawater was continuously filled between the particles of the sediment through capillary action.That caused the salinity of the pore water to increase,and gypsum was deposited.The precipitation of gypsum greatly increased the Mg/Ca ratio in pore water or surface water,forming high-magnesium intergranular water,which in turn replaced other carbonate minerals and formed dolomite.