The Sequence Characteristics and Sedimentary Facies of the Second Member of Nantun Formation in Urxun Sag

Based on the results of previous studies(nantun formation is divided into the first member and the second member of nantun formation),combined core logging and logging data and seismic data,the second member of nantun formation can be divided into three 4th-order sequences(SQ1、SQ2、SQ3).The sedimentary characterristics in different periods are mainly controlled by tectonic evolution and sediment supply.In SQ1 period,the whole southern Urxun sag began to sink,and the area of the lake rapidely expanded,sedimentary facies mainly develop fan delta,braided river delta systems,while developing shore,coastal-shallow lacustrine,semi-deep lake-deep lake and the sublacustine.In SQ2 period the lake base level raise continuously,and sedimentary characteristics like the SQ1.In SQ3 period,controlling fault weakened,the lake decreased,reducing the scale of the fan delta devolopment,but the braided-river delta system still developed at a large scale.