成岩非均质性对致密砂岩裂缝发育的影响研究—以鄂尔多斯盆地延安地区延长组为例

Natural fractures significantly enhance the reservoir quality of tight sandstones and exert major control on enrichment and high yield of tight oil and gas. The role of diagenesis in fracturing seems to have been largely neglected in previous studies, limiting in-depth understanding of fracture formation and distribution pattern. Diagenetic alterations and evolution proceed along fundamentally different pathways during burial in a reservoir directly related to variations of the original deposit. The contrasting diagenetic evolution causes the inhomogeneity of reservoir microstructures and internal stress state, possibly resulting in complex fracture patterns. The focus of this study is the Yanchang Formation in Yanan Area, Ordos Basin. Based on outcrop analog studies combined with drill-hole data, this study is designed to establish sequence of diagenetic and fracturing events and to examine spatial-temporal relationships between fracture development and diagenetic heterogeneity. The effect of diagenesis on rock mechanical properties and fracture propagation behavior are quantified by carrying out subcritical crack growth experiment and CT imaging. A mechanism explaining fracture development in tight sandstones in tectonically stable region is drawn. This study is of great importance in promoting understanding of fracture development and distribution pattern, and also has important implications for subsurface reservoir modeling and drilling completion and hydraulic fracturing.

天然裂缝发育显著改善了致密砂岩储层的储集和渗流能力，是影响致密砂岩油气富集和高产的关键。前人研究中，对储层内在成岩作用的影响关注不够，制约了对裂缝发育机理与分布规律的深入认识。受控于原始沉积作用的影响，在埋藏过程中储层内砂岩成岩作用与演化表现出强的差异性，导致岩石微观结构和构造、内部应力状态明显不均匀，可能引起裂缝形成及分布型式的复杂性。本项目以鄂尔多斯盆地延安地区延长组为研究对象，将成岩作用研究和裂缝研究相结合，通过典型野外露头和钻井资料的精细解剖，厘定致密砂岩成岩演化与裂缝形成的时间序列，认识裂缝发育与成岩非均质性的时空叠加样式；开展先进的亚临界裂纹扩展试验和CT成像，量化认识差异性成岩作用对岩石力学性质和裂缝扩展行为的影响；提出构造稳定区典型致密砂岩裂缝形成机理及发育模式。本项目研究将提升对裂缝形成与分布规律的认识，对井下精细储层建模、钻完井设计及压裂改造具有重要的应用价值。

天然裂缝发育对致密砂岩油气富集和高产至关重要。前人研究对储层内在成岩非均质性的影响关注不够，制约了对裂缝发育机理与分布规律的深入认识。本项目选择鄂尔多斯盆地延安地区山西组，认识了致密砂岩裂缝发育特征与成岩非均质性的关系，查明了差异化成岩作用对岩石力学性质和裂缝扩展行为的影响，探讨了构造稳定区典型致密砂岩裂缝形成机理。发现延安地区山西组主要发育区域性高角度剪切缝，低角度缝和近水平缝少，整体上裂缝可划分为3期。从碎屑颗粒组成和结构、成岩作用和过程的角度，划分了为五种砂岩成岩相，明确了不同成岩相的成岩演化过程，并认识和表征了其宏观分布的非均质性特征。建立了不同类型砂岩的成岩-裂缝演化的时间序列，明确了裂缝发育与成岩非均质性的时空叠加样式。微裂缝发生具有选择性，主要发生在中-粗粒弱胶结和强溶蚀的石英砂岩（成岩相-A）和中-粗粒贫塑性颗粒的弱胶结和强溶蚀的岩屑石英砂岩（成岩相-C）中。开展三轴压缩试验的进行系统的岩石力学测试，发现不同类型砂岩的岩石力学属性有明显差异。成岩相-A和-C脆性指数普遍大于富塑性颗粒的强压实的砂岩（成岩相-D）和碳酸盐胶结致密的砂岩（成岩相-E），即成岩相-A和-C容易发生破裂形成裂缝。对三轴压缩试验诱导的裂缝扩展路径进行CT扫描成像和表征，发现成岩相-A和-C中破裂面相比于成岩相-D和-E要曲折，且在成岩相-D和-E中，发育了厚度更小的裂缝。微观裂缝扩展路径受岩石骨架组分和成岩胶结物影响。其中在成岩相-A和-C中穿石英颗粒的微裂缝占比最大，穿颗粒（胶结物）边界的微裂缝次之。综合地质解剖和实验研究成果，总结认识了成岩非均质性对致密砂岩裂缝形成的控制作用，探讨了构造相对稳定区裂缝发育机理。项目成果对致密砂岩油气的“甜点”预测和压裂改造具有重要的应用价值。