鄂西渝东区构造裂缝发育特征及力学机制

针对鄂西渝东区隔档式褶皱发育的构造特点,结合区域构造演化史,分别以隔档式褶皱和正弦曲线几何形态的褶皱建立模型,用有限元法模拟了褶皱不同构造部位的应力场分布特征。褶皱外侧为张应力集中区,内侧为压应力集中区,且应力矢量与地层产状平行,二者以中部既无明显挤压、亦无明显拉张的中和面为分界。通过野外露头和岩心观察,总结了鄂西渝东区构造裂缝发育的三种形式:具多层介质的非能干层中发育顺层裂缝;向斜内侧、背斜外侧的能干层中发育高角度剪切缝;背斜外侧、内侧具多层介质的非能干层中发育顺层裂缝与高角度缝共生的网状缝。这种特征与模拟结果呈现较高程度的一致性,可根据应力场模拟成果和裂缝发育地层的能干性进行合理解释。复向斜中的隆起区可作为页岩气勘探的重要目标。

Characteristics and Mechanism of Structural Fractures in Western Hubei-Eastern Chongqing Area

According to the ejective folds developed in the study area, two geometry models including ejective fold and sine-curve shaped fold are suggested, and the finite element method is used to simulate the distribution of stress fields in different structural parts of folds. It is indicated that the direction of stress vectors are parallel to the formation occurrence. The outer sides of folds are tensile stress concentration and the inner sides of folds are compressive stress concentration, which are distinguished from a middle compromise boundary where neither compression nor tension is obvious. Based on the observation of field outcrops and drilling cores, three types of structural fractures exist in Western Hubei-Eastern Chongqing area. They are the bedding fissures developed in ductile rocks with multiple layers, the high-angle shearing fissures developed in brittle rocks in the inner sides of synclines and outer sides of anticlines, and the mesh cracks associated with bedding fissures and high-angle fissures in multiple-layered ductile rocks in the inner and outer sides of anticlines. This feature is well coincident with simulation results, which can be reasonably interpreted by the results of stress fields and brittle-ductile property in sedimentary formations in mechanism of structural fractures. It is suggested that the uplifts in synclinoria are the prospective target of shale gas exploration.