砂岩中的凝灰质填隙物分异特征及其对油气储集空间影响——以鄂尔多斯盆地西北部二叠系为例

DIFFERENTIATION OF SANDSTONES' TUFF FILLINGS AND ITS EFFECT ON POROSITY—An Example from the Paleozoic Sandstones in Northwestern Ordos

运用铸体薄片、扫描电镜、能谱分析及溶蚀实验等多种测试手段对鄂尔多斯盆地西北部二叠系砂岩填隙物进行了成份和成岩演化分析,认为砂岩中的填隙物主要为凝灰质填隙物,而不是普通意义上的粘土矿物。在有残余粒间孔隙或裂缝沟通的较开放性水环境里,凝灰质填隙物蚀变程度强,Si、Al、K、Na及Ca等离子因活性不同而发生分异,蚀变为“脏”高岭石及硅质团块等产物;封闭环境下,凝灰质填隙物保存完好,偏光显微镜下显示出似非晶态波状消光,活性强的K、Na元素含量仍然很高。凝灰质填隙物分异过程中,可以产生一定数量的溶蚀孔隙及晶间孔隙,砂岩储集性能得到了明显改善。对声波时差异常带砂岩进行大量的铸体薄片观察,证实了本区砂岩粒间溶蚀孔隙发育,凝灰质填隙物发生了强烈溶蚀。综合分析26口井的资料,发现垂向上凝灰质填隙物溶蚀强度呈箱型展布,箱内“脏”高岭石及溶蚀孔隙发育,箱顶部凝灰质填隙物含量高,保存好,并缺少“脏”高岭石团块出现。

The Paleozoic sandstones in northwestern part of the Ordos Basin were characterized by the abundant tuff fillings. Based on the analyses of cast slice, SEM and EDX, the components and diagenetic evolution of the sandstones＇ tuff fillings under the burial environment were analyzed. The acid bed water induced the split-up of tuff fillings, which usually evolved into impure kaolinite or/and autogenetic quartz shoot. The intense dissolution usually took place in the tuff fillings, which easily communicated with the acid water in virtue of remnant pores or microcracks, and differentiation diversity of various elements, such as silicon, aluminium, potassium, sodium and calcium, depended on their activities. By contrast, the tuff fillings in the occluded burial environment could be saved to some extent, which was not qualified with crystal features in microscope, and had much contents of some elements with high activities. The dissolution of tuff fillings could produce much intergranular secondary pores, and the gas reservoir was enhanced intensively. Cast slice of the sandstones with higher interval transit time verified that the dissolution intensity of tuff fillings took on trunk. The tuff fillings of sandstones were dissolved strong, and much secondary pore and impure kaolinite presented in the trunk. By contrast, the tuff fillings of sandstones could be saved well, and impure kaolinite rarely presented on the top of the trunk.