渝东南—黔北地区下寒武统牛蹄塘组页岩裂缝发育特征与主控因素

依据渝东南—黔北地区下寒武统牛蹄塘组黑色页岩露头、岩心、薄片和扫描电镜下裂缝系统的观察描述、特征参数和对应样品分析测试数据,深入分析并探讨了牛蹄塘组页岩裂缝的发育特征与主控因素及其对页岩含气性的影响。结果表明:牛蹄塘组页岩中发育不同类型的宏观裂缝和微裂缝,宏观裂缝以构造缝为主,主要为高角度的剪切缝、张剪性裂缝、压扭性裂缝及低角度滑脱缝;微裂缝主要为非构造成因的层间缝、粒间缝和粒内缝。岩心中宏观裂缝大部分被充填,充填物主要为方解石、泥质及黄铁矿。裂缝的发育主要受构造应力及构造部位、有机质含量、矿物成分及含量、岩性与岩石力学性质等多种因素的影响,其中构造因素是裂缝发育最主要的控制因素,脆性矿物与有机质含量对页岩岩石力学性质与裂缝的发育具有重要影响。在良好保存条件下,裂缝的发育对牛蹄塘组页岩含气性与渗透性均具有贡献,而对渗透性的改善是其最主要的贡献。压扭与滑脱性裂缝缝面糜棱状矿物内部孔隙、裂缝发育,有效改善了页岩的孔渗能力。裂缝中方解石、石英等充填物内部发育开启的次级裂缝和溶蚀孔缝亦增加了充填裂缝的有效性。

Development characteristics and major controlling factors of shale fractures in the Lower Cambrian Niutitang Formation,southeastern Chongqing-northern Guizhou area

Based on outcrop, coring, thin section and the SEM observation and characteristic parameters of fracture system of black shale in the Lower Cambrian Niutitang Formation, the southeastern Chongqing-northern Guizhou area, as well as the corresponding sample analysis test data, the development characteristics, major controlling factors and influences on the shale gas-bearing property of shale fracture in Niutitang Formation were deeply analyzed and discussed. The results indicate that multiple types of macro and micro fractures are developed in the shale of Niutitang Formation, of which the macro fractures are dominated by structural fractures, such as high dip-angle shearing fracture, tension-shearing fracture, compresso-shear fracture and low dip-angle slip fracture. The micro-fractures mainly include interlayer, inter-particle and intra-particle fractures with the non-structural genesis. Most of macro core fractures are filled by calcite, pelite and pyrite. The fracture development is mainly influenced by structural stress, structural position, TOC content, mineral component and content, lithology, rock mechanical properties and other factors, of which structural factor is the most controlling factor of fracture development. Besides, the contents of brittle minerals and organic matters have strong influences on the rock mechanical properties and fracture development of shale. Under the favorable preservation conditions, the development of fracture can make contributions to the gas-bearing property and permeability of shale in Niutitang Formation, mainly in terms of permeability improvement. The development of the pores and fractures in mylonitic minerals on the interface of compresso-shear and slip fracture can improve the porosity and permeability of shale effectively. Secondary fractures and dissolution pore-fractures developed in the calcite and quartz fillings of fractures can also increase the effectiveness of fracture filling.