鄂尔多斯盆地中生代地层天然裂缝发育特征与构造应力场演化

The natural fracture characteristics of Mesozoic strata in Ordos Basin and evolution of the tectonic stress field

鄂尔多斯是我国重要的油气盆地，而绝大多数油气储层都会受到天然裂缝的影响。文中通过野外系统观测和室内的统计分析，详细描述了盆地中生代地层中的天然裂缝发育特征和主要控制因素，为裂缝性油气藏勘探开发提供基础理论支持。通过研究得知，盆内中生代地层中主要发育有6组裂缝：E—W向、ENE-WSW向、NE—SW向、N—S向、NNW—SSE向、NNE-SSW向。其中E-W向、ENE—WSW向和NE-SW向为系统裂缝；N—S向、NNW—SSE向和NNE—SSW向为非系统裂缝。在裂缝组合中，有两组正交裂缝系统（E—W向和N—S向，ENE-WSW向和NNW—SSE向），其中E—W向和N—S向裂缝构成的正交裂缝系统只出露于三叠和侏罗纪地层。同时，还有两组共轭裂缝（ENE—WSW向和NNE—SSW向，ENE-WSW向和ESE-WNW向），其中ENE—WSW向和NNE-SSW向裂缝构成的共轭裂缝出露于整个中生代地层，而另外一组只出露于上三叠统延长组中。此外，裂缝间距分析表明：（1）裂缝间距与力学层厚之间的关联性相对较低；（2）E—W向和ENE—WSW向两组系统裂缝的发育强度好于N—S向非系统裂缝组；（3）岩层厚度越小，对应裂缝密度则越大。（4）除了岩层厚度，区域应力场对裂缝的发育具有很大的影响。研究区内裂缝主要形成于两期区域应力场：第一期是晚侏罗世近E—W向的挤压应力场，由古太平洋板块向欧亚板块的俯冲和碰撞作用所致；第二期是新生代NE—SW向的挤压应力场，由印度板块向欧亚板块的俯冲和碰撞作用所致。

Ordos Basin is an important hydrocarbon basin in China, yet the majority of hydrocarbon reservoirs were affected by the natural fracture. Based on the systematic field observation and indoor statistical analysis, we descript the natural fracture characteristics and major controlling factors in Mesozoic strata in basin. Six trends of fracture sets （E-W-trending, ENE-WSW- trending, NE-SW-trending, N-S-trending, NNW-SSE-trending and NNE-SSW-trending） are found in Mesozoic strata within Ordos Basin. Among them, E-W-trending, ENE-WSW-trending and NE-SW-trending sets are systematic fractures, other three sets are nonsystematic fractures. As for the fracture assemblage, there are two main groups of orthogonal fracture system（ E-W- trending and N-S-trending, ENE-WSW-trending and NNW-SSE-trending）, the orthogonal fracture system consisted of E-W-trending and N-S-trending is only outcropped in the Triassic and Jurassic strata. Meanwhile, there are two main groups of conjugate fractures（ ENE-WSW- trending and NNE-SSW-trending, ENE-WSW-trending and ESE-WNW-trending ）, the conjugate fracture consisted of ENE-WSW-trending and NNE-SSW-trending is outcropped in entire Mesozoic strata, yet the other set is only outcropped in the Yanchang Formation. In addition, fracture spacing analysis indicates that： 1 ） Layer thickness has a low effect on the fracture spacing. 2） Fracture intensity of systematic fracture （ E-W-trending and ENE-WSW- trending） is more than nonsystematic fracture （N-S-trending）. 3 ）The thinner the layer thickness, the greater the fracture intensity. 4）In addition to the layer thickness, regional stress field has important effects on the fracture formation. The fractures in the study area are mainly formed in the two phases regional stress fields. The first is Late Jurassic E-W-trending compressional stress field caused by the subduction and collision of paleo-Pacific plate and Eurasia plate. The second is Cenozoic NE-SW-trending compressional stress field caused by the subduction and collision of India plate and Eurasia plate.