Characteristics of Slope Break Belt in Large Depression Lacustrine Basin and Its Controlling Effect on Sandbody and Petroleum:Taking the Triassic Yanchang Formation in the Ordos Basin as an example

The viewpoint that slope break belt developed both in the marine basin and the nomarine rift basin played important controlling effect on sequence and sediment frameworks has got extensive concern and been widely accepted in recent years.According to the combination characteristic of seismic profiles and sedimentary microfacies,studies on the Triassic Yanchang Formation in the Ordos basin show that sedimentary slope break belts were also developed in the large depression lacustrine basin,and could be clearly recognized either in the deep water or in the shallow water.Deposits of the deep-water gravity flows such as the sandy debris flows and the turbidity flows were mainly controlled by the slope break belt in deep water,whereas deposits of tractive current like subaqueous distributary channels in the delta-front were controlled by that in the shallow water.Further studies suggest that,except for sandbody origin types,the deep-water and shallow-water sedimentary slope breaks also have obvious controlling effect on thickness,plane distribution,and physical properties of the sandstones.In addition,it is suggested that petroleum distribution was also controlled to a certain extent by the slope break belt.Although oil reservoirs can be found everywhere in the Yanchang Formation,most of the larger lithologic reservoirs are located either below the slope breaks or in the LST(lowstand systems tracts) sandstones neighboring the sequence boundary,or controlled by both of the slop break belt and the sequence boundary.