长宁地区海相页岩天然裂缝发育特征及其对含气性的影响

Development characteristics of natural fractures in marine shale in Changning area and their influence on gas-bearing properties

天然裂缝是页岩储层中重要的储集空间和渗流通道,控制页岩气的富集和保存条件。利用地表露头、岩心、EMI成像测井、薄片和扫描电镜资料,在研究四川盆地长宁地区五峰组—龙马溪组页岩储层天然裂缝发育特征和主控因素的基础上,分析了不同成因类型天然裂缝对页岩含气性的影响。研究区宏观裂缝以构造裂缝和成岩裂缝为主,微裂缝主要为构造微裂缝和粒间缝。构造裂缝的优势方位为北西—南东向、北北东—南南西向、北东东—南西西向和东西向,以中高角度裂缝为主。构造裂缝的发育程度受构造、脆性和岩石力学层厚度3个主要因素控制。与断层距离越近,页岩构造裂缝越发育;褶皱核部的构造裂缝比翼部更发育;页岩脆性矿物质量分数越高,构造裂缝越发育;岩石力学层厚度越小,层内张裂缝的发育程度越高。不同成因类型的天然裂缝对页岩气含气性影响不同,穿层剪切裂缝影响页岩气的保存条件,层内张裂缝和页理缝有利于页岩气的富集,因而需要分类型进行页岩储层天然裂缝的评价和预测。

Natural fractures are important storage spaces and seepage channels for shale reservoirs,which affect the enrichment and preservation of shale gas.Using outcrops,cores,EMI imaging logging data and thin slices and scanning electron microscopy data,by studying the characteristics and main controlling factors of natural fractures in shale reservoirs from Wufeng Formation to Longmaxi Formation in Changning area of Sichuan Basin,the effects of natural fractures of different genesis on shale gas-bearing properties are analyzed.In the study area,macrofractures are mainly structural and diagenetic fractures,and microfractures are mainly structural microfractures and intergranular fractures.The dominant orientations of structural fractures are NW-SE,NNE-SSW,NEE-SWW and EW directions,mainly with middle-angle and high-angle fractures.The development of structural fractures is controlled by three main factors:structures,brittleness and rock mechanics layers.The closer to the fault,the more developed the structural fractures in the shale.The fractures in the core of the fold are more developed than in the wings.The higher the mass friction of brittle minerals in the shale,the more developed the structural fractures.The smaller the thickness of the brittle shale layer,the more developed the intraformational tensile fractures.The different types of natural fractures have different effects on the gas-bearing properties of shale gas.Transformational shear fractures have a negative effect on shale gas preservation conditions,while intraformational tensile fractures and lamellation fractures are conductive to shale gas enrichment.Therefore,it is necessary to evaluate and predict natural fractures of shale according to different types.