Sequence Stratigraphy and Depositional Characteristics of the 4th Member of Yingcheng Formation of Lower Cretaceous, Rifted Depression of Xujiaweizi, Songliao Basin 

Key words: 
• sequence stratigraphy

Abstract: The 4th member of Yingcheng Formation of lower Cretaceous is a series of stratigraphy of coarse clastic rock deposited in a background of volcano erupting of rift basin of Xujiaweizi faulted depression. It is the reservoir for natural gas pool. Using 3D seismic data of high resolution, cores, drilling logs and log curves, the framework of sequence stratigraphy and depositional systems of the 4th member were established by this paper based on sequence principle and the way of lithofacies analysis, respectively. Two thirdorder sequences were identified in the fourth member of Yingcheng Formation of lower Cretaceous in Xujiaweizi rifted depression. The sequence 1of lower fourth member (Sq1) of Yingcheng Formation was deposited in three geographically isolated volcano lakes formed by volcano erupting; the sequence 2 of the upper fourth member (Sq2) of Yingcheng Formation was deposited in a relative whole lake, because of the depression subsidence. Lakes during Sq1 period were well divided, in great disparity of depth, and mostly developed sedimentary systems of alluvial fan and fan delta being adjacent to the uplifts and characterized by lithofacies associations of cobble conglomerate and pebble conglomerate. Sq2 was formed in a relative whole lake, and developed a large scale sedimentary system of braided delta in the northern part of the depression and a sedimentary system of fluvial delta in the southern part of the depression. The two kinds of sedimentary system are characterized by lithofacies assemblages of pebble conglomerate, granular conglomerate, and coarse sandstone. The depositional system of braided delta is characterized by braided delta plain, composed of conglomerates, the thickness of which is great and the area of which is large. The depositional system of fluvial delta is characterized by proximal meandering river of conglomerate. Because of evolution of paleogeomorphology background from three small isolated lakes during Sq1 period to a relative whole lake in Sq2, depositional systems of alluvial fan and fan delta during Sq1 period changed to depositional system of braided river delta and depositional system of fluvial delta during Sq2 period. The front of delta is good reservoirs favorable for nature gas. 