Characteristics and tectonic evolution of the fault system in the north sag of Northern Depression of South Yellow Sea basin

The north sag of the Northern Depression of the South Yellow Sea basin is a Meso- Cenozoic sag since Late Cretaceous. It is a composite dustpan-like sag with fault and deep basement in the north, and overlapping on shallow basement in the south. There are three secondary tectonic units within the sag, i.e. the northern step-fault zone in the north, the central deep depression zone in the center and the southern slope belt in the south. Normal faults dominate the region with a small amount of overthrust faults locally. The faults could be classified into three groups in EW, and NE, and NW directions according to fault strike. Besides, the faults could be also grouped into three stages, i.e. the early stage fault, mid-term stage fault and late stage fault. Many types of faults have been discovered in this sag, including flat plate fault, shovel-like fault, slope-like fault, and so on. On the basis of fault scale and its control over strata, the faults could be sorted into three levels. The sag is controlled by the first level fault, the zones within the sag are controlled by the second level fault, and the traps are controlled by the third level fault. The activity rate of the Qianliyan Fault passing through the center of the north sag is the highest where rapid depositional rate and thicker deposits occur, corresponding to the core region of the northern step-fault zone and the central deep depression zone. However, the activity rate is smaller relatively on the margins of the sag and slope zones. The tectonic evolution of the northern sag is controlled by four tectonic movements, i.e. the Yizheng movement, Wubu movement, Zhenwu movement and Sanduo movement. The tectonic evolution of the sag may be divided to two stages, i.e. the rifting stage and depression stage. The sag is filled by the Taizhou Formation, Funing Formation, Dainan Formation, Sanduo Formation and Neogene deposits in a time span from Late Cretaceous to Quaternary. On account of above research achievements, the hydrocarbon accumulation model of north sag is described in this paper. Oil and gas are accumulated in complicated fault block traps close to source rock and fault. Deep faults are the key element to drive hydrocarbon moving into traps. The middle step-fault zone and the fault nose region of the deep depression zone are recommended as favorable exploration targets where a batch of favorable traps occur and close to the center of source rock.