川西拗陷须家河组第四段致密砂岩高岭石及其对储层物性的影响

Characteristics of kaolinite in tight sand reservoirs of Member 4 of Upper Triassic Xujiahe Formation in west Sichuan depression and its influence on physical properties of reservoirs

研究川西拗陷上三叠统须家河组第四段致密砂岩中高岭石的发育特征与形成机理,探讨其对储层物性的影响。采用扫描电镜、铸体薄片、黏土矿物X射线衍射测试以及物性资料等,结果表明须四段自生高岭石呈书页状、蠕虫状,主要分布在深度小于3.8 km的须四段中上部;高岭石的形成与长石的溶蚀有关:(1)大气淡水和煤系地层酸性水使得长石在早成岩阶段发生了第一期溶蚀,形成了自生高岭石;(2)随着埋藏深度的增加和地温的升高,部分高岭石与临近的残余钾长石发生反应形成了伊利石。自生高岭石及其伴生矿物伊利石有利于须四段储层的孔隙发育;但它们阻塞了孔喉、降低了储层渗透率,不利于油气的运移。

The development characteristics and formation mechanism of the tight sandstone of the Member 4 of Upper Triassic Xujiahe Formation(T 3 x 4)in the western Sichuan Depression are studied and its influence on the physical properties of the reservoir is discussed so as to provide a theoretical basis for oil and gas exploration in this area.Comprehensive analyses of scanning electron microscope(SEM),rock thin section observation,clay mineral X-ray diffraction and physical property data,etc.,show that the authigenic kaolinites are lamellar shaped and vermicular shaped,mainly distributed in the upper part of T 3 x 4 with a depth less than 3.8 km.It is showed that the formation of kaolinite is related to the dissolution of feldspar.In fact,atmospheric fresh water and acidic water in coal measure strata bring about the first stage dissolution of feldspar in the early diagenesis stage and eventually lead to the formation of authigenic kaolinite.With the increase of burial depth and ground temperature,some kaolinites react with adjacent residual potassium feldspar and lead to the formation of illite.Based on physical properties and mercury intrusion data,it is considered that although the development of kaolinite in the rocks can enhance the reservoir porosity,kaolinite and its associated illite also can block the pore throat and reduce the reservoir permeability,creating an unfavorable condition for the migration of oil and gas.