准南逆冲褶皱带超压与逆冲断层持续活动

Activity thrust faults and overpressure in the thrust and fold belt of southern Junggar Basin

天山北缘准南地区的褶皱带为自新生代以来一直持续活动的逆冲构造带,由于逆冲断层的持续活动,形成了现今断层和相关褶皱。钻井资料显示,准南逆冲褶皱带内的超压层主要发育在古近纪安集海河组泥岩和紫泥泉子组泥岩之中,而该泥岩同时又成为逆冲断层发育的主滑脱面。通过多年来对准南地区地面地质调查、二维地震和三维地震资料的解释以及钻井证实,我们统计出准南逆冲褶皱带现存的逆冲断层倾角分别集中在两个区间:30±5°和50±5°区间。应力分析表明,在持续挤压应力作用下,超压层(泥岩、页岩和煤系地层)中和超压层之下地层中发育的早期逆冲断层与晚期最大主压应力之间的夹角处在30±5°之间时,作用在断层面上的最大主应力与最小主应力比达到最小值,因此该断层最容易再次活动,形成最大的流体压力,因而断层周围的流体就会沿着最大主应力方向发生流动,断层本身就会成为流体运移的主要通道;而早期逆冲断层与晚期最大主压应力之间的夹角处在50±5°之间时,作用在断层面上的最大主应力与最小主应力比较大,断层重新活动所需要的流体压力较高,导致断层作为流体运移的通道因被挤压而闭合。应力分析和钻井实测应力均指出,准南逆冲褶皱带发育的超压为挤压构造应力形成的超压。这些研究表明,准南逆冲褶皱带的逆冲断层持续活动,导致早期发育的断层在晚期应力作用下,断层倾角聚集在两个优势区间,油气沿最大主压应力方向运移,聚集油气则沿断层滑动面发育形成构造超压,导致该区域油气长期处于运移与聚集的动平衡状态。

The thrust and fold belt of south Junggar Basin, north Tianshan, is still activity since Cenozoic time, and the development of an activity fault and fault-related folds. Well data show that overpressures are developed in the Paleocene Anjihai Formation mudstones and Ziniquanzi Formation mudstones, which are the main detachment surfaces of thrust faults. Based on the surface survey and interpretations of 2D/3D seismic data as well as well data for many years, dip distribution of thrust fault in the thrust and fold belt of south Junggar Basin has a selection of statistically preference, with mainly concentrated peaks at 30 ± 5° and 50 ±5°. Under continuing compressional stress,analysis of the stress show that the optimal orientation for fault reactivation,at which the ratio of effective principal stresses attains a positive minimum （i. e. maximum fluid pressure） , when thrusts dipping at 30 ±5° to maximum principal compressive horizontal stress,σ1. Frictional lockup is expected when a reactivation angle of thrusts to σ1 equals 50±5°, steep reverse faults require more supra-lithostatic fluid pressures for continued reactivation. The stress analysis and actual measurement of strata pressure data by wells show that overpressures mechanism is resulted from tectonic stress, i.e. tectonic overpressures. These researches indicate that a continuously activity faults in the thrust and fold belt of south Junggar basin has resulted in the reactivation of the earlier formed faults by the later compressive forces, and tectonic overprcssure along the surface of the faults of accumulating petroleum, in which hydrocarbon migration has a dynamic balance with hydrocarbon accumulation over a long period of time.