四川盆地九龙山构造致密砾岩储层裂缝特征及其贡献

Characteristics of fractures and their contribution to the deliverability of tight conglomerate reservoirs in the Jiulongshan Structure,Sichuan Basin

致密块状砾岩作为油气储层比较少见,其裂缝的成因类型、分布特征及控制因素尚不清楚。为此,以四川盆地九龙山构造下侏罗统珍珠冲组致密砾岩储层为例,根据野外露头、岩心、成像测井和岩心测试分析资料,在研究砾岩储层裂缝成因类型和分布特征分析的基础上,就裂缝对该类储层及产能的贡献大小进行了评价。珍珠冲组致密砾岩储层主要有构造裂缝、成岩裂缝和原岩裂缝3种成因类型,并以构造剪切裂缝为主,可分为东西向、近南北向、北西西向和北东东向4组裂缝;裂缝非常发育,岩心裂缝面密度平均为31.4m/m2,溶蚀以后的裂缝在地层围压条件下的平均开度为221.28μm;裂缝及其溶蚀孔洞的平均总孔隙度为2.26%,占该区岩心总孔隙度的67.4%,裂缝的渗透率主要分布在20.0～160.0mD,比储层孔隙的渗透率高2～5个数量级,说明裂缝及其溶蚀孔洞是珍珠冲组致密砾岩储层的重要储集空间和主要渗流通道,控制了该区天然气藏的分布和单井产能。

Tight clumpy conglomerate reservoirs are so rare in China that the genetic types,distribution features and controlling factors of their fractures are still unknown.In view of this,such reservoirs of the Lower Jurassic Zhenzhuchong Formation in the Jiulongshan Structure of the northwestern Sichuan Basin are studied as an example in this paper.Various data including outcrops and core observation,image logs and core test results are integrated to evaluate the contribution of fractures to the deliverability of these tight conglomerate reservoirs based on the analysis of genetic types and distribution characteristics of fractures.There are structural fractures,diagenetic fractures,and original fractures in the tight conglomerate reservoirs of the Zhenzhuchong Formation.Among these three types,the structural shear fractures are dominant and can be grouped into four sets,i.e.EW-,approximately NS-,NWW-,NEE-trending fracture sets.Fractures are well developed in the study area.The average areal density of fractures in cores is 31.4 m/m2 and the average aperture of dissolved fractures under hydrostatic conditions is 221.28 μm.The average porosity of fractures and their dissolved pores and vugs is 2.26%,which accounts for 67.4% of the total core porosity.The permeability of fractures is in the range of 20.0-160.0 mD,which is 2-5 orders of magnitude higher than the matrix permeability.This indicates that fractures and their dissolved pores and vugs are not only important storage spaces and flow pathways in the tight conglomerate reservoirs of the Lower Jurassic Zhenzhuchong Formation,but one of the most important factors of controlling the distribution of gas reservoirs and the deliverability of a single well in the study area.