多级构型界面下致密砂岩储层构造裂缝发育特征——以川西坳陷须家河组二段为例

Development characteristics of structural fractures in tight sandstone reservoirs under multi⁃level configuration interfaces:a case study of second member of Xujiahe Formation in Western Sichuan Depression

裂缝对四川盆地川西坳陷须家河组二段致密砂岩储层天然气的高产稳产至关重要,不同沉积环境下形成的砂体构型界面控制了储层裂缝的发育。通过野外露头勘测、岩心观测、测井识别和岩相精细刻画等方法,提出了川西坳陷须二段砂体构型界面划分方案,识别划分了研究区的2~4级砂体构型界面,厘清了不同级次构型界面约束下裂缝发育的特征及其油气地质意义。不同级次构型界面控制着储层裂缝的产状、开启程度、发育程度和储渗能力;2级、3级界面的有效控制范围分别为0~0.35 m和0~3 m,4级界面控制范围较大(0~11 m)。不同构型界面控制下裂缝发育特征存在差异,其中2级界面下裂缝欠发育,4级界面下裂缝一般发育,3级界面下裂缝最发育,主要为低角度斜缝;裂缝走向主要为NNE—SSW、SEE—NWW和SSE—NNW,开度较大(>0.04 mm)和倾角较大的裂缝较多。多级界面控制下裂缝发育段的岩心孔渗呈明显的正相关,3级界面下裂缝对储层物性的改善作用最强,对油气贡献度最高;4级界面次之,2级界面最弱。明确不同级次构型界面对裂缝发育的控制作用,有利于研究区致密砂岩储层气藏的有效开发。

Fractures are crucial for the high and stable production of natural gas from the tight sandstone reser⁃voirs in the second member of the Xujiahe Formation in the Western Sichuan Depression,Sichuan Basin.The develop⁃ment of these fractures is controlled by the configuration interfaces of sand bodies formed under different sedimentary environments.Through field outcrop surveys,core observations,logging identification,and detailed characterization of lithofacies,a classification scheme for sand body configuration interfaces was proposed for the second member of the Xujiahe Formation in the Western Sichuan Depression.These interfaces were identified and classified into second to fourth levels.The study clarified the characteristics and geological significance of fractures under different levels of configuration interfaces.These interfaces controlled the occurrence,opening degree,development degree,and storage-permeability capacities of reservoir fractures.The effective control ranges of the second and third level interfaces were 0-0.35 m and 0-3 m,respectively,while the fourth level interface had a larger control range(0-11 m).The development characteristics of fractures differed under different configuration interfaces.Specifically,fractures were underdeveloped at the second level interfaces;fairly developed at the fourth level;and most developed at the third level,primarily as low⁃angle dipping fractures.Fracture orientations were predomi⁃nantly NNE-SSW,SEE-NWW,and SSE-NNW,with more fractures having larger openings(>0.04 mm)and larger dip angle.There was a clear positive correlation between core porosity and permeability in the fracture develop⁃ment under the control of the multi⁃level interfaces.Fractures at the third level interface had the strongest improvement effect on reservoir physical properties and made the highest contribution to oil and gas production,followed by the fourth level interface,with the second level interface being the least effective.Clarifying the control of different levels of configuration interfaces on fracture development is conducive to the effective development of tight sandstone gas reservoirs in the study area.