鄂尔多斯盆地华庆地区延长组长6砂岩绿泥石膜的形成机理及其环境指示意义

Forming Mechanism and Their Environmental Implications of Chlorite-coatings in Chang 6 Sandstone(Upper Triassic) of Hua-Qing Area,Ordos Basin

通过薄片鉴定和扫描电镜等技术手段,对鄂尔多斯盆地延长组砂岩的成岩现象进行了分析,发现绿泥石黏土膜为该区重要的成岩现象。在对该区砂岩绿泥石黏土膜的产状、结构特征及与其它成岩矿物之间的关系等特征分析的基础上,对其形成机理进行了详细讨论,认为绿泥石黏土膜与储层原生孔隙具有一定的关联作用,其形成与同沉积时期水动力条件关系更为密切,属于强水动力沉积条件的一种标志,主要发育于三角洲前缘水下分流河道和河口坝等微相中,具有较强的环境专属性;绿泥石黏土膜具有双层结构,里层的等厚环边胶结物相对致密,是早期成岩阶段形成的。外层的自生绿泥石垂直于环边表层生长,晶形较好,是晚期成岩阶段形成的;绿泥石黏土膜保护砂岩孔隙能力有限。绿泥石黏土膜发育的砂体物性较好,主要是由于强水动力条件下沉积的砂体本身岩石学特性所决定,与绿泥石黏土膜关系不大。由此提出绿泥石黏土膜形成模式,认为其形成经历了5个阶段:原始颗粒沉积阶段、原始颗粒表面黏土吸附成膜阶段、颗粒初期压实阶段、环边绿泥石化阶段和自生绿泥石形成阶段。鉴于此,在华庆地区延长组石油勘探过程中,应寻找绿泥石黏土膜相对发育的强水动力条件下的沉积砂岩储集体。

With thin section analysis and scanning electron microscopic analysis,the diagenetic phenomenon of Yanchan Formation sandstone in the Ordos basin were systematically studied,and it shows that chlorite film was the important diagenetic phenomenon in this area.Depending on the studies relevant to the features including occurrence,structural characteristics,relations between other diagenetic minerals of sandstones in this area,the formation mechanism was discussed in detail.Chlorite rim had some relevance with the original porosity of reservoirs,its formation had more closely relationship with hydrodynamic conditions during the contemporaneous deposition.They belonged to a mark of water power conditions for they occur mainly in the subaqueous distributary channel and mouth bar in the deltaic front,and had strong environmental specificity;chlorite rim had double layer structure in which isopachous rim cement of internal layer was relative dense,and formed during the early diagenetic stage.Outerlayer authigenic chlorite grown on the rim surface vertically with better crystal form was formed during the late diagenetic stage;chlorite film was of a limited ability to protect the sandstone porosity.The reason for this is that the porosity of sandstones with chlorite clay rim was better mainly depended on the petrological characteristics of sedimentary sandstones under strong hydrodynamic condition.It had little relation with the chlorite clay rim.So the formation model of chlorite clay rim was put forward.It was considered that the formation of the chlorite film experienced five steps： sedimentary stage of original particles,stage of clay adsorption and film formation on the surface of original particles,early compaction stage of particles,stage of rim chloritization,authigenic chlorite forming stage;In view of this,the sedimentary sandstone reservoirs that had chlorite clay rims under strong hydrodynamic conditions should be the target in the process of oil exploration.