黔北地区牛蹄塘组页岩储层裂缝特征

Fracture Characteristics of Shale Reservoirs in Niutitang Formation in Northern Guizhou

以黔北地区下寒武统牛蹄塘组页岩储层为研究对象,通过岩心、薄片、场发射扫描电镜和测试分析等手段,对牛蹄塘组页岩储层裂缝发育特征和主控因素进行了分析。结果表明:研究区牛蹄塘组页岩岩心裂缝以构造裂缝为主,包括高角度剪性裂缝、高角度张性裂缝、低角度滑脱缝,岩心裂缝充填程度较高,以方解石充填为主;镜下微观裂缝以非构造缝为主,包括层间缝、粒内缝和粒间缝。微裂缝的发育对页岩气藏丰度、页岩气井单井产能都有积极贡献。同时,对裂缝发育的主控因素研究发现,构造因素是裂缝发育最主要的控制因素。此外,同一构造背景下受不同岩性、岩石力学性质、矿物组成、有机质含量等因素影响,裂缝发育程度和性质也存在差异。其中总有机碳(total organic carbon, TOC)含量与裂缝发育程度并非单一的线性关系,当TOC过高时(6%~8%),裂缝发育程度明显降低;具有高弹性模量、低抗拉强度的碳质页岩,在区域应力作用下更易形成构造裂缝。

Shale samples were taken from the Niutitang Formations in Northern Guizhou,the fracture development characteristics and main control factors of Niutitang Formation shale reservoir were analyzed by means of core,thin section,field emission scanning electron microscope and test analysis.The results show that the core fractures of Niutitang Formation in the study area are mainly structural fractures,including high-angle shear fractures,high-angle tensile fractures,and low-angle slippage fractures.The core fractures have a high degree of filling,mainly calcite filling.The micro-fractures under the microscope are mainly non-structural fractures,including interlayer fractures,intragranular fractures and intergranular fractures.The development of micro-fractures has a positive contribution to shale gas reservoir abundance and single well productivity.At the same time,the study on the main control factors of fracture development shows that the structural factors are the most important controlling factors for fracture development.In addition,under the same tectonic background,due to the influence of different lithology,rock mechanical properties,mineral composition,organic matter content and other factors,fracture development degree and properties are also different.There is not a single linear relationship between total organic carbon(TOC)and fracture development.When TOC is too high(6%~8%),fracture development decreases significantly.Carbonaceous shale with high Young s modulus and low tensile strength is more likely to form structural cracks under the action of regional stress.