Development characteristics and major controlling factors of shale fractures in the Lower Cambrian Niutitang
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摘要: 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。Abstract: 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.
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Key words:
- fracture /
- development characteristics /
- major controlling factors /
- Niutitang Formation /
- shale gas /
- Xuefeng uplift
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图 1 研究区构造位置与区内牛蹄塘组裂缝走向分布
Figure 1. Structural location of the study area and outcrop fractures occurrence in Niutitang Formation
图 2 牛蹄塘组野外露头裂缝类型及特征
.a.平面剪切缝，3组共轭，裂缝走向分别为305°、355°、50°；b.平面剪切缝，相互切割成网格状，裂缝走向分别为300°、358°、43°；c.剖面“X”型共轭剪切缝，裂缝走向分别为35°、345°；d.区域性直立剪切缝，裂缝走向为315°；e.褶皱核部张裂缝；f.页岩层理缝(红色箭头指示)，风化作用相关缝(黄色箭头指示)
Figure 2. Types and characteristics of outcrop fractures in Niutitang Formation
图 3 牛蹄塘组野外露头裂缝主方位分布
Figure 3. Main occurrence of the outcrop fractures in Niutitang Formation
图 4 牛蹄塘组岩心裂缝类型及特征
a.高角度剪切缝，两期交切关系，一期无充填，另一期方解石充填，湘吉地1井，1 878.8 m；b.高角度剪切缝与层理缝(蓝色箭头指示)，剪切缝方解石充填，湘吉地1井, 1 860.75 m；c.高角度剪切缝与低角度剪切缝，交切关系，湘吉地1井, 1 838.8 m；d.滑脱缝，镜面现象，湘张地1井, 1 990.0 m；e.滑脱带滑脱缝强烈发育，缝面擦痕明显，岩心沿裂缝破碎，湘吉地1井, 2 009 m；f.张裂缝，方解石充填，见岩石碎屑，湘吉地1井, 2 015.9 m；g.泄水缝，泥质与方解石充填，湘吉地1井, 1 993.2 m；h, i.异常高压相关缝，方解石充填，湘吉地1井, 2 036.8 m、湘张地1井, 1 977.9 m；j~l.多期裂缝相互交切，构成网状裂缝系统，湘吉地1井, 2 040，2 010.7，2 019.1 m(蓝色箭头指示裂缝形成时间早于红色箭头指示裂缝)
Figure 4. Types and characteristics of fractures in Niutitang Formation core
图 5 牛蹄塘组页岩裂缝特征参数统计
Figure 5. Statistical characteristic parameters of fractures in cores of Niutitang Formation shale
图 6 湘张地1井、湘吉地1井牛蹄塘组页岩裂缝纵向分布特征(全岩和黏土矿物X射线衍射分析结果分别为质量分数wB/%和占比/%)
Figure 6. Comprehensive columnar section of the fracture vertical distribution of Niutitang Formation shale in Well XZD-1 and Well XJD-1
图 7 微观薄片观察下牛蹄塘组裂缝特征
a.3期裂缝相互交切，充填方解石、石英及残余泥粒，第3期为“X”型共轭剪切缝，湘张地1井，1 990.3 m；b.2期裂缝相互交切，第4期未充填，湘张地1井，1 990.3 m；c.方解石充填缝内部发育未充填孔隙，湘张地1井，1 950.2 m；d.泄水缝切穿层理，充填泥质，湘张地1井，1 721.0 m
Figure 7. Characteristics of the micro-fractures in casting thin sections of Niutitang Formation
图 8 扫描电镜观察下牛蹄塘组裂缝特征
a.层间缝，湘张地1井, 1 905.5 m；b.矿物定向排列形成顺层缝，湘张地1井, 1 905.5 m；c.糜棱化矿物间裂缝与孔隙，湘张地1井, 1 974.6 m；d.条带状有机质内部裂缝，湘张地1井, 1 895.2 m
Figure 8. Characteristics of the micro-fractures in FE-SEM photographs of Niutitang Formation
图 9 牛蹄塘组构造缝密度与脆性矿物质量分数的关系
Figure 9. Relationships between structural fracture development and brittle mineral content of Niutitang Formation shale
图 10 牛蹄塘组裂缝与单岩层厚度的关系
Figure 10. Relationships between fracture development and the bedding thickness of Niutitang Formation shale
图 11 牛蹄塘组裂缝密度与含气量(a)及孔隙度(b)关系
Figure 11. Relationships between fracture development and gas content (a) or porosity (b) of Niutitang Formation shale
表 1 牛蹄塘组岩心构造缝发育期次与特征
Table 1. Stages and characteristics of structural fracture in Niutitang Formation core
裂缝
期次倾角/
(°)充填程度 充填物 裂缝类型 特征描述 发育程度 1 55~70 全、半充填 以方解石为主，伴随
少量石英、黄铁矿、泥质剪切缝 组内均有分布，延伸较长、开度相对小，缝内
发育未充填次级孔、缝，见共轭缝非常发育 2 5~10 全充填 以方解石为主，伴随少量石英 剪切缝 开度变化大，延伸不长，切割前期裂缝 欠发育 3 70~85 全充填 方解石 剪切缝 一组多条集中发育，开度小，延伸短 发育 4 2~75 全、半充填与未充填 以方解石为主，
伴随少量泥质滑脱缝 滑脱带内密集发育，组成网状系统，以低角度
为主，开度变化大，擦痕明显，岩心沿裂缝开裂局部发育，滑脱带内极为发育 5 60~85 未充填 无 剪切缝 缝面平直光滑，延伸长，岩心沿裂缝开裂 发育 6 5~80 全、半充填 方解石 张裂缝、剪切缝 局部构造缝、延伸短；张裂缝缝面粗糙，开度
较大，分布局限仅局部发育 
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