层厚与岩性控制裂缝发育的力学机理研究——以鄂尔多斯盆地延长组为例
Study on the Development Law of Structural Fractures of Yanchang Formation in Longdong Area, Ordos Basin

摘要 以鄂尔多斯盆地南部的延长组长6~7 段为例, 重点研究层厚和岩性控制构造裂缝发育的规律, 并对其裂缝面密度进行定量化研究。结果表明, 层厚影响构造裂缝发育, 薄层比厚层更易发育构造裂缝, 这与不同层厚中裂缝尖端应力集中情况不同有关。在一定厚度范围内, 裂缝面密度与层厚之间有幂指数关系; 当厚度超过临界值(约250 cm)时, 裂缝面密度基本上不变。在相同层厚和不同岩性条件下, 裂缝面密度由小到大依次为中砂岩、细砂岩、粉砂岩和页岩, 即在相同构造环境和等厚地层情况下, 碎屑岩粒级越小, 面密度越大, 构造裂缝面密度与岩石粒径之间呈负幂指数关系, 这与不同粒级的颗粒间应力不同有关。通过多元分析和力学机理探讨, 认为本区控制裂缝发育的两个因素中, 层厚是主控因素。
Abstract In order to study the distribution of fracture controlled by layer thickness and lithology, clastic rock of 6-7th Member, Yanchang Formation in Longdong area is selected as investigated subject, and the areal density of their structural fractures is meassured. Measured fracture densities show that the layer thickness of clastic sequence has an effect on fracture density. Compared with thicker layer, it is easier for thinner layer to develop structural fracture, which is related to the different stress concentration near fracture tip in different layer thickness. Within a certain thickness range, fracture density has an exponential relationship with layer thickness, while the fracture density remains mostly unchanged when thickness exceeds 250 cm. Under the condition of same layer thickness and different lithology, the value of fracture densities from small to large follow the order of medium sandstone, fine sandstone, siltstone and mudstone, which means that in the same tectonic setting and layer thickness, the smaller the size range is, the larger the fracture density will be. Besides, there is an exponential relationship between fracture density and grain size, which may be caused by the different stress between grains of different size. By multivariate statistics and mechanism analysis, layer thickness is the key factor in controlling fractures’ development compared with lithology.