雪峰隆起西缘下寒武统牛蹄塘组页岩裂缝发育特征及主控因素

要:基于雪峰隆起西缘下寒武统牛蹄塘组页岩地表露头、岩心、薄片和扫描电镜下天然裂缝特征参数的观察与统计，并借助相应样品的实验测试数据，对牛蹄塘组页岩裂缝的发育特征与主控因素进行了深入分析，并探讨了裂缝发育与含气性之间的关系。结果表明:研究区牛蹄塘组页岩宏观裂缝以构造缝为主，成岩缝次之，构造缝包括高、中角度剪切缝，低角度滑脱缝及少量张裂缝，主要为6期形成，其中第1期高角度剪切缝最为发育，裂缝优势方位为NWW向、NNW-近SN向与NE向。微裂缝主要为层间缝和顺层缝。岩心宏观裂缝多被充填，后期裂缝的充填程度低于前期裂缝。裂缝的发育受构造作用、岩性和矿物组分、岩层厚度、有机碳含量、成岩作用及异常流体高压等多种因素影响，其中构造应力与构造部位是裂缝发育的主要控制因素，区域构造应力决定了裂缝的性质与整体发育特征，局部构造应力与构造部位使裂缝的分布具有差异性，应力相对集中且变化梯度大、地层变形强烈的构造部位更易形成裂缝。此外，页岩在脆性矿物含量高、单层厚度小、成岩阶段晚、有机碳含量高、存在异常高流体压力时，裂缝更发育。研究区牛蹄塘组天然裂缝的发育及伴生的大量微裂缝有效改善了页岩的储渗性能，对提高含气量起重要作用。 

Development characteristics and major controlling factors of shale fractures in the Lower Cambrian Niutitang

Based on outcrop, coring, thin section and the SEM observation and statistics of shale fracture characteristics in the Lower Cambrian Niutitang Formation, western margin of Xuefeng uplift, as well as the corresponding sample analysis test data, this paper analyzes in detail the development characteristics and major controlling factors of shale fracture in Niutitang Formation and the relationship between fracture development and gas content. The results indicate that macro fractures in Niutitang Formation are dominated by structural fractures, such as high dip-angle shearing fractures, low dip-angle slip fractures and some extensional fractures. Structural fractures are mainly formed in 6 stages. Among them the high-angle shear fractures in first stage are mostly developed, and the predominant orientations of fractures are NWW, NNW-near SN and NE directions. The diagenetic fractures are fewer than structural fractures. The micro-fractures mainly include interlayer and bedding fractures. The macro fractures in the core are mostly filled, and the filling degree of the later fractures is lower than that of the earlier fractures. The fracture development is influenced by many factors, such as tectonism, lithology and mineral composition, thickness, organic carbon content, diagenesis and high fluid pressure, among which structural stress and structural position are the main controlling factors. Regional structural stress determines the nature and overall development characteristics of fractures. Local structural stress and structural position make the distribution of fractures different. Fractures are more developed in the structural position with relatively concentrated stress, large stress gradient and strong deformation. In addition, shale with high content of brittle minerals, small thickness of single layer, late diagenetic stage, high content of organic carbon and high fluid pressure has well fracture development.The development of macro fractures and a large number of associated micro-fractures in Niutitang Formation effectively improve the reservoir permeability of shale and thus play an important role in improving gas content.