鄂尔多斯盆地中西部下白垩统露头区裂缝研究

通过对研究区野外露头宏观裂缝的详细观测与统计,研究了鄂尔多斯盆地中西部下白垩统裂缝系统的发育特征与成因类型,同时依据构造解析（Tectonic Analysis）的相关理论与方法,分析了裂缝形成的构造应力场。其结果表明：研究区下白垩统主要发育了4组高角度或近直立裂缝,其走向分别为北东东向、北东向、北北西向及北西向;裂缝的横向延伸长度比纵向切深大得多,两者的主体布范围（或峰值）分别介于为3～20m与0．1～0．6m;裂缝间距总体呈对数正态分布,大小主要集中在0～0．8m,约占85％;从裂缝地质成因来看,上述4组裂缝均为直接受控于区域构造应力场的区域构造裂缝,而从裂缝力学性质角度看,则又普遍表现出剪切裂缝的典型特征,都归属于剪裂缝范畴,并且在区域上构成早晚两期平面X型共轭剪切裂缝体系,其中北东东-北西向共轭剪切裂缝主要形成于盆地燕山期构造应力场,北北西-北东向共轭剪切裂缝则主要在盆地喜山期构造应力场的控制下形成．

Study on fractures of outcrop area in Lower Cretaceous in central and western Ordos Basin

Through the detailed observation and statistics of the outcropping fractures in the region of interest, the development characteristics and genetic type of fracture system are studied in Lower Cretaceous in central-west Ordos Basin. Meanwhile, the tectonic stress field of fracture generation is analyzed according to the theories and methods of tectonic analysis. Study results show that tbur groups of high-angle or near-vertical fractures were developed in Lower Crataceous of the study area. The fracture strikes are NEE, NE, NNW and NW respectively. Fracture lateral length is much longer than the vertical extent. Both the peaks are separately between 3-20m and between 0.1-0.6m. Fracture interval shows a logarithmic normal distribution in general. Its size is mainly 0-0.8m, accounting for about 85%. From the geologic origin of fracture, above-mentioned fuur groups of fracture are all the regional structure fractures which are directly controlled by the regional tectonic stress field. However, from the mechanical properties of fracture （mechanical origin）, they also generally show the typical features of shear fracture, belonging to the category of shear fracture. Moreovex, they constitute the two-step （early and late stage） plane-X conjugate shear fracture system, in which NEENW conjugate shear fractures are mainly formed in Yanshanian tectonic stress field in the basin, while NNW-NE conjugate shear fractures are mainly created under the controlling of Himalayan tectonic stress field in the basin.