中下扬子区三叠纪海相碳酸盐岩成岩作用和成岩环境

THE DIAGENESES AND DIAGENETIC ENVIRONMENTS OF MARINE CARBONATE ROCKS OF THE TRIASSIC IN THE LOWER-MIDDLE YANGTZE REGION

中下扬子区广泛分布着三叠纪滩相、潮坪相和岩溶带 3类碳酸盐岩。根据成岩作用类型、成岩组构、孔隙类型和成岩环境等特征对 3类岩石进行比较。滩相碳酸盐岩的主要成岩作用是在海水渗流、潜流和淡水渗流、潜流成岩环境下产生的胶结、云化和溶解作用。潮坪相白云岩和富含石膏的蒸发岩则以高盐度的海水渗流带的准同生毛细管蒸发浓缩云化作用为主 ,在大气淡水渗流或潜流成岩环境下则以去云化、去膏化和溶解作用为主。岩溶带碳酸盐岩—蒸发岩及其岩溶堆积角砾岩主要经历早期海水强烈蒸发作用下准同生云化作用和晚期表生淡水环境下的岩溶作用。上述主要成岩作用使 3类岩石次生孔隙率增加。因此 。

Three types of Triassic carbonate rock, including bank facies, tidal flat facies and karst zone, are widely spread in the Lower Middle Yangtze Region. According to their lithological characteristics, diagenetic types, diagenetic fabrics, porosity properties and diagenetic environments, the diagenetic features of the three types of carbonate rocks are compared. Diageneses of the bank facies carbonate rocks include cementation, dolomitization and solution as a function of vadose and phreatic diagenetic environments of marine water and meteoric fresh water. The carbonate rocks of the tidal flat facies are commonly composed of metasomatic dolomites and evaporites rich in gypsums. The dolomitization is chiefly penecontemporaneous replacement through capillary concentration and vaporation of marine water of high salinity in vadose zone. Dedolomitization, degypsification and solution develop in vadose and phreatic diagenetic environments of meteoric fresh water. The main rock types of the karst zone are carbonate evaporites and karst collapse breccias. The diageneses are characterized by penecontemporaneous dolomitization under the action of strong evaporative marine water during the early stage of diagenesis. The karst dolomitization occurs in the late diagenetic stage under the epidiagenetic freshwater diagenetic environments. Because of secondary porosity development due to main diageneses mentioned above, these diageneses play a dominant role in making the three types of carbonate rocks become prospect reservoir beds.