鄂尔多斯盆地三叠系延长组次生孔隙形成机制

Formation Mechanisms of Secondary Porosity in the Triassic Yanchang Formation,Ordos Basin

次生孔隙是鄂尔多斯盆地三叠系延长组砂岩储层的重要储集空间,其中长石溶孔为最主要的次生孔隙类型,其次为岩屑溶孔、晶间孔及沸石溶孔。基于铸体薄片研究,结合扫描电镜分析、阴极发光分析、物性分析以及碳氧同位素分析,自生高岭石和长石含量、长石溶蚀形成的次生孔隙、岩石物性、碳氧同位素组成变化特征研究表明,印支期不整合面之下的延长组上部地层(如长1、长2油层组)次生孔隙的形成机制受控于区域性的大气水溶解作用,而远离不整合面的延长组下部地层(如长8油层组)次生孔隙的形成则更多地受到埋藏成岩过程中与长7烃源岩有关的有机酸的影响,尤其是靠近长7生烃中心区域的长8油层组受到有机酸的影响最为强烈。因此,在进行鄂尔多斯盆地延长组碎屑岩储层质量预测时应针对不同地区不同油层组的次生孔隙形成机制建立相应的预测模式。

The reservoir space of the Triassic Yanchang Formation in Ordos Basin is mainly composed of primary and secondary porosities,wherein the secondary porosity accounts for more than 40% of the total porosity.The secondary porosity is chiefly originated from the dissolution of feldspar,followed by the dissolution of lithic grains and zeolite.Based on the examination of casting thin sections and lots of statistical data,scanning electron microscope,cathodoluminescence（CL）,physical property and carbon and oxygen isotope analysis,the characteristics of the secondary porosity of Yanchang Formation sandstones were studied and the main factors influencing the development of secondary porosity were analyzed.The results show that the secondary porosity in the upper members of Yanchang Formation,such as the Chang-1and Chang-2members,is mainly formed by the leaching of meteoric water related to the unconformity at the base of the Jurassic section;whereas the generation of the secondary porosity in the Lower Yanchang Formation,such as the Chang-8 member,is greatly influenced by the felaspar dissolution as a result of the effects of organic acids during burying diagenesis,especially the Chang-8member in Jiyuan area near the hydrocarbon generation center is mostly influenced by organic acids.Different formation mechanism of secondary porosity for different reservoir in different areas should be considered in the reservoir quality prediction of Yanchang Formation in Ordos Basin.