Early Cretaceous Sedimentary Systems and Depositional Process during the Period of Transition from Faulted Depression to Sag,Changling Faulted Depression,Songliao Basin

Recently,tight sand gas plays have been discovered in the Lower Cretaceous of Changling fault depression,southern Songliao Basin.By analyzing abundant materials,it is concluded that the dominant provenance is in the south and southwest of the depression.During the transition from a fault depression to a sag,the dominant sedimentary facies are braided river and braided river delta,with other facies such as alluvial fan,fan delta and lacustrine.In the early stage of the deposition progress,several separated depressions form.Faulted steep slope controls the distribution of fan deltas and also the distribution of depocenters.Gentle slope controls the distribution of braided rivers and braided river deltas.During the late stage of depositional progress,only one depocenter remains in the north.The abundant sediments from southern provenance prograde to the north.During the deposition of Quan I member,the only depocenter immigrates to the north.Braided rivers fill the main area of Changling Sag,and the river channels stacked in a large area.Based on the analyses of sedimentary systems,we predicted two potential reservoir belts:one is the braided river sand belts of Denglouku Formation on the south low uplift,the other one is braided river sand belts of lower Quan I member in the southern Changling sag.