陆相致密砂岩自生绿泥石微组构特征及成因

The continental tight sandstone reservoir is one of the important oil and gas reservoirs in China. The reservoir densification and formation of relatively high-quality reservoir are mainly due to the rich content as well as complex structure and composition of authigenic chlorite in the reservoir. The origins of authigenic chlorite in China’s continental tight sandstone can often be found in the results of the researches on chlorites formed in the offshore and sea-land transitional environment, but these origins are lack of researches on the differences of the development characteristics and genesis of authigenic chlorite in continental strata, which limits the deeply understanding of the development mechanism of the ‘sweet spot’ in China’s continental tight sandstone and of the physical and chemical conditions of the key diagenetic events during densification. The reservoir sandstone of Yanchang Formation in Ordos Basin which is the typical continental tight sandstone reservoir in China is studied in this paper. Based on the clear development background of the authigenic chlorite and the researches on the microstructures of it, by analyzing the material composition of different types of microstructures of chlorites, this paper reveals the evolutionary laws of the geochemical characteristics of authigenic chlorite in Yanchang Formation and the key geochemical parameters sensitive to diagenetic responses. The understandings of the formation process and conditions about different microstructures of chlorites are deepened, and the formation model of authigenic chlorite in Yanchang Formation is also established. The results of this paper can enrich the formation mechanism of authigenic chlorite in China’s continental tight sandstone reservoir and they are of theoretical and practical significance to the exploration of favorable reservoirs (sweet spot).

陆相致密砂岩储层是我国重要的油气储集层，储层内部自生绿泥石含量丰富、结构和组分复杂，是导致储层致密化及相对优质储层形成的关键因素。有关我国陆相致密砂岩自生绿泥石的成因解释多借鉴国外海相及海陆过渡环境成因绿泥石的研究成果，缺乏对陆相地层自生绿泥石的发育特征及成因的差异性研究，制约着我国陆相致密砂岩“甜点”发育机理及致密化过程中关键成岩事件发生的物化条件的深入认识。本项目以国内典型的陆相成因致密砂岩储集层——鄂尔多斯盆地延长组储集砂岩为研究对象，在明确自生绿泥石发育背景的前提下，以自生绿泥石的微结构研究为基础，分析不同类型微结构绿泥石的物质组成，揭示延长组自生绿泥石地球化学特征的演化规律与对成岩响应敏感的关键地球化学参数，更深入理解不同微结构绿泥石的形成过程及形成条件，建立延长组自生绿泥石的成因模式。以期丰富我国陆相致密砂岩储层自生绿泥石的形成机理，对有利储层（甜点）勘探具有理论和现实意义。

本项目在前人研究的基础上，利用致密砂岩岩心，采用多种实验分析方法，研究陆相致密砂岩自生绿泥石微结构，分析不同类型微结构绿泥石的物质组成、地球化学特征，并在此基础上讨论绿泥石的成因。. 研究结果标明：以鄂尔多斯盆地晚三叠世延长组为代表的陆相致密砂岩自生绿泥石胶结物主要以孔隙衬里产出，少量以包膜状和玫瑰花瓣状产出。孔隙衬里绿泥石常发育内层和外层的双层结构，内层发育于骨架颗粒表面、厚度约为3μm左右；外层在内层的基础上向孔隙中心呈片状垂直生长，厚度约为8μm左右。外层绿泥石晶片明显大于内层绿泥石晶片，并且晶间孔更为发育。在石英、长石、方解石颗粒表面的绿泥石膜中，外层绿泥石的铁离子和镁离子含量明显高于内层绿泥石的铁离子和镁离子含量。内层绿泥石的生长明显受到其所依附颗粒的影响，颗粒的特征化学成分在内层绿泥石中含量高，且明显高于外层绿泥石。自生绿泥石形成的物质来源主要由黑云母水化和火山岩岩屑溶解提供了铁离子和镁离子。内层绿泥石与骨架颗粒呈锯齿状接触并垂直骨架颗粒生长，内层绿泥石单晶呈片状且为自形或半自型，晶体间见小于0.5μm的晶间孔隙，说明内层绿泥石是在良好空间中生长的，可能是由于成岩物质偏少而导致晶体明显变小。后期由于黑云母和火山岩岩屑的大规模溶蚀，释放了大量铁离子和镁离子而形成外层绿泥石，在有足够胶结空间的条件下，外层绿泥石单井明显变大、晶间孔更为发育。. 相关意义：一是基于不同的方法研究自生绿泥石微组构的化学特征，明确了相关方法针对该研究对象的应用效果；二是精细解剖绿泥石的微组构和化学成分属性，为后期更为深入研究相关矿物提供了借鉴意义。