Study On The Geoligical Structure And Evolution Of The Fault-depressions In The Deep In North Songliao Basin

From the Late Jurassic to the Early Cretaceous, the deep fault-depressions in North Songliao Basin had gone through three developing stages, the initial faulting stage during Huoshiling Period, the intense fault-subsiding stage during Shahezi Period, and the fault-depression depauperation stage during Yingcheng Period.All the depression-controlling faults in the deep in North Songliao Basin had generally gone through the dynamic processes from the independent development, interactional development, to the united development. The faults during the independent developing stage appeared as the en-echelon arrangements along NNW and NNE strike under the shear-extending. And finally, under the controlling of the depression-controlling faults in the deep in North Songliao Basin, there developed the great NNW strike fault-depression zone, An'da-Xujiaweizi fault-depression zone.The present characteristics show that the developing degrees of the depression-controlling faults are very different. The faults with the high developing degree are long, great in the vertical displacements, with the concordant characteristics along the strikes. And the fault-depressions controlled by the high-degree faults are generally very big. But the faults with the low developing degree always have the completely contrary characteristics from the high-degree faults.The dynamic developing faults controlled the dynamic developments of the depressions during Shahezi Stage during the Early Cretaceous. The present depression structures in North Songliao Basin show that the developing process of the entire depression generally goes through five phases: the embryo phase (such as Gulong-Changjiaweizi Fault-depression), the infancy phase (such as Shuangcheng Fault-depression), the youth phase (such as Yingshan Fault-depression), the adult phase (such as Xujiaweizi Fault-depression), and the united phase (such as Xujiaweizi Fault-depression and Anda-Changwu Fault-depression having been united together). Any fault-depression perhaps terminates at any developing phase. The high-phasefault-depressions always went through the low developing phase, and were always left with some imprints of the low-phase fault-depression. Although the original characteristics of the low-phase fault-depressions in the high-phase fault-depressions are difficult to be distinguished, the present low-phase fault-depression can show their original characteristics.In Songliao region, there developed three basins with the different formation mechanisms and different boundary conditions during the Mesozoic-Cenozoic, which are the fault-depression basin during Huoshiling-Yingcheng Period, the great depression basin during Denglouku-Yi' an Period, and the present geographic basin under the basin-range extending since Neogene Period. Between the development period of the fault-depression basin and the great depression basin, the regional geologic setting changed revolutionarily, but not with the inherited development. There happened the major structural reformations on the deep formations during the gestation period, growth period, and the reversion-depauperization period of the great depression basin. The regional compressive folding and denudations happened during the gestation period. The subsiding during the growth period resulted in the burial patterns of the deep formations. During the reversion-depauperization period, the deep formations in the east were embroiled in the regional thrust-folding system.The fault-depression prototypes during Shahezi Stage controlled the distribution of the source rocks. According to the 'Source-controlling Theory', the burying-denudating histories and present distributions of Shahezi source rocks controlled the distributions of the natural gas reservoirs. Xuxi Extending Fault Belt, Fengle Flexure Belt, Xunan Depression, Shengping-Xingcheng Transforming Slope, and Maoxing Fault-depression are the better exploration zones for the natural gas in the deep.