鄂尔多斯盆地中部奥陶系马家沟组五段白云岩储层成岩作用及孔隙演化

Diagenesis and pore evolution of dolomite reservoir in the 5th member of the Ordovician Majiagou Formation, central Ordos Basin

通过岩心、薄片、阴极发光、扫描电镜、能谱和地球化学等分析测试工作,对鄂尔多斯盆地中部奥陶系马家沟组五段(马五段)白云岩的白云岩化作用、胶结作用、溶蚀作用和压实、压溶作用等开展研究,将研究区马五段白云岩储层成岩作用分为近地表-浅埋藏和中-深埋藏两个阶段。研究表明,马五段白云岩储层可分为泥晶-微晶白云岩型、晶粒白云岩型和颗粒白云岩型3大类;准同生期近地表环境中的溶蚀作用是储层孔隙形成的关键因素,白云岩化作用是储层孔隙保存的重要有利条件,胶结作用是孔隙破坏的主要成岩作用;含石膏结核和含石膏团块的泥晶灰岩在海平面短暂下降期间,受大气淡水影响,石膏溶解,形成膏模孔和不规则膏溶孔,后经浅埋藏期白云岩化作用,岩石抗压性得到增强,利于孔隙保存,中-深埋藏期经历胶结作用,部分孔隙空间被充填;颗粒灰岩在沉积期形成粒间孔,在准同生期经胶结作用,在浅埋藏环境下经历白云岩化作用,残余粒间孔得到保存,形成良好的储层。

The study focuses on the dolomitization,cementation,dissolution,compaction and pressure-solution of the 5th member of the Ordovician Majiagou Formation in the central Ordos Basin,by means of core and thin section observation,as well as analyses of catholuminescence,scanning electron microscope(SEM),energy spectrum and geochemistry.The diagenetic process of the dolomite reservoir in the study area thus can be divided into two stages:a near surface-shallow burial stage and a medium-deep burial stage.The dolomite reservoir in the study area can be categorized into three types:micritic-microcrystalline dolomite,crystalline-grained dolomite and dolomite grainstone.The dissolution in the near surface environment during the penecontemporaneous period serves as the key for pores’development.Dolomitization is a significant factor conducive to porosity preservation,while cementation is a major diagenesis detrimental to early-developed pores.The micritic with gypsum nodules and gypsum clumps have developed gypsum-moldic and irregular dissolved pores under the impact of meteoric water during a short period of exposure caused by temporary sea level fall;then dolomitization during the shallow-burial period enhanced the compaction-resistance of the limestones,being favorable for pore preservation;but the cementation during the medium-deep burial period occluded part of the pore spaces.The grainstone developed intergranular pores during deposition,and their residual inrergranular pores were preserved after cementation during the penecontemporaneous period and dolomitization during shallow-burial period,resulting in high-quality reservoirs.