川东北地区茅二段顶部玄武岩淋滤型白云岩形成机理

Discussion on the Formation Mechanism of Dolomite in the Basalts Leaching at the Top of Maokou Formation of Lower Permian in Northeast Sichuan

根据野外剖面观测、井下岩屑、测井等资料,分析了川东北地区下二叠统白云岩形成与岩溶、玄武岩淋滤之间的关系。川东北地区下二叠统茅口组二段顶部白云岩具有“整体含量低、局部发育好”的特点,多由粉-细晶白云石晶体组成,含有钠长石等玄武质残余成分,白云岩的形成与岩溶、玄武岩息息相关。茅口组沉积后,地层边抬升边淋滤,建立渗流通道;华蓥山和梁平一带在东吴期喷发溢流的玄武岩,水解淋滤后形成硅酸真溶液以及富含Mg 2+的流体沿岩溶层的各种裂缝和节理向下渗流,使茅口组顶部的岩溶层发生硅质交代或充填以及白云石化现象;玄武岩淋滤白云石化的发育程度主要取决于龙潭组玄武岩和茅口组顶岩溶面的接触关系,直接接触则白云岩较为发育,有泥岩隔挡则白云岩发育较差,如有泥灰岩夹层但有裂缝沟通则形成少量白云岩;对于川东北地区茅口组风化壳而言,只有经过玄武岩淋滤白云石化改造后才能形成良好的储层。

According to the field profile observation,analysis of downhole debris and logging data,the dolomite at the top of Maokou Formation of Lower Permian in Northeast Sichuan Basin has the characteristics of low overall content and good local development,it is composed of powder-fine crystal dolomite,containing basaltic residual composition,such as sodium feldspar,etc.The formation of dolomite is closely related to karst and basalt.After the sedimentation of the Maokou Formation,the strata are leached to form a seepage channel.Eruption of basalt overflows from Huayingshan and Liangping Area at Dongwu Stage,forms a silicate after hydrolysis leaching solution and fluid which is rich in Mg 2+with various cracks and joints to the lower percolation along the karst layers,induces the siliceous metasomatism or filling and dolomitization of karst at the top of Maokou Formation.The development degree of basaltic leaching dolomitization mainly depends on the contact relation between the basalt of Longtan Formation and karst at the top of Maokou Formation:for the direct contact,dolomite is relatively developed,the development of dolomite is poor in mudstone,a small amount of dolomite is formed if there are cracks in the marl.For the weathering crust of the Maokou Formation in northeast Sichuan,only after the transformation of the basaltic leaching dolomitization,it can form good reservoirs.