Tectonic Transformation of Changling Fault Depression in the Southern Songliao Basin and Its Significance of Hydrocarbon Accumulation

Three typical near EW-trending cross-sections of Changling fault depression in the Songliao basin were studied using the theory of balanced cross-sections. The analysis indicates that rifting of the basin started in Huoshiling period and subsided intensively in Shahezi and Yincheng periods,with extension rates of 907.50 m/Ma,1373.90 m/Ma,and 1100.00 m/Ma respectively,peaking at the Yincheng period. Structural shortening also took place at the end of the two extension periods,with a maximum and ratio of shortening 2.11 km and 2.89% in the former and 2.26 km and 3.21% in the latter. The basin started to transform from faulting depression to subsiding depression in Denglouku period,to post-rifting cooling subsidence in ing the Quantou-Nenjiang period,characterized by weak extension,and to a stage of inverse structure development,characterized by compression at the end of the Nenjiang period. The Shahezi and Yingcheng Groups,as both major rock,and cover/reservoir rocks,formed a unique assemblage for authigenic and self-storing hydrocarbon accumulation. Therefore,large-scale low-magnitude folding structure formed by tectonic activities at the end of Shahezi and Yincheng periods is a favorable zone for accumulation of oil and natural gas.