白云岩储层中蒸发矿物的赋存形式与成因演化——以鄂尔多斯盆地中部气田区马家沟组为例

鄂尔多斯盆地中部气田区马家沟组马5¹—马5⁴亚段粉晶白云岩中广泛发育毫米级、厘米级的硬石膏柱状晶和小结核等蒸发矿物,是储层发育的关键。通过详细的井下地质调查、典型岩心剖面写实、系统取样配套测试和室内比对分析等方法,首次针对硬石膏矿物的赋存形式与成因演化进行精细解剖。研究发现:硬石膏柱状晶和小结核赋存于粉晶白云岩中,其中柱状晶多呈星散状分布,而硬石膏小结核发育有中-高成核数量中核径、中-高成核数量小核径及低成核数量大核径3种典型结构。上述硬石膏矿物均为局限(内)陆棚环境准同生期或早成岩浅埋藏期交代先期白云岩的产物,其数量多少取决于硬石膏开始析出时海水含盐度的高低,个体大小取决于海水中适合于硬石膏生长的含盐度持续时间的长短。不同蒸发矿物组合在成岩阶段将经历不同的改造过程。其中,含中-高成核数中核径硬石膏结核的粗粉晶白云岩中,残余溶模孔及伴生的裂碎缝极其发育,最终演化为区内最为优质的裂缝-溶孔(洞)型储层。

Occurrence,genesis and evolution of evaporite minerals in dolomite reservoir: a case study of the Majiagou Formation in gas fields of the central Ordos Basin

Anhydrite columnar crystal and nodules of millimeter to centimeter sizes are widely developed in the crystal powder dolomite of Ma 5¹-5⁴ (the 1^st to 4^th submembers of the 5^th member of Majiagou Formation) in the gas fields of the central Ordos Basin,and are critical to reservoir development.The occurrence,genesis and evolution of anhydrite were finely analyzed for the first time through detailed subsurface geological survey,observation of typical core section,system sampling and testing,indoor comparative analysis,and so on.The anhydrite columnar crystals and small nodules occur in crystal powder dolomite.The columnar crystals are scattered in the dolomite,while the small nodules have three typical distribution forms,namely medium nuclear diameter in middle-high nucleation number,small nuclear diameter in middle-high nucleation number and large nuclear diameter in low nucleation number.All of the anhydrite minerals mentioned above are the products of metasomasis of early dolomite during penecontemporaneous or shallow burial periods in an restricted (inside) continental shelf environment.of the amount of anhydrite minerals depends on the seawater salinity at the beginning of anhydrite precipitation,while their sizes are controlled by the duration of seawater salinity that is suitable for the growth of anhydrite minerals.Moreover,different evaporite mineral associations may experience different process of modification.Residual moldic pores and associated cracks are highly developed in the coarse crystal powder dolomite of medium nuclear diameter in middle-high nucleation number,which finally evolved into the high quality fractured-vuggy reservoir in the area.