STUDY OF SEISMIC AND SEDIMENTARY FACIES OF THE 2ND AND 4TH MEMBERS OF THE XUJIAHE FORMATION IN THE GREAT CENTRAL SICHUAN

Oil and gas exploration started in the 1940s in the Xujiahe Formation,Upper Triassic,Sichuan Basin,of which the 2nd,4th and 6th members are not only the major targets,but also the most favorable horizons in search for new gas sources.According to the seismic and sedimentological studies,in combination with drilling data,there are seven seismic sequence boundaries in the Xujiahe Formation in the Great Central Sichuan area,namely SB1-SB7,corresponding to six large lake flooding surfaces,MFS1-MFS6 respectively.In this paper,the seismic reflection features of the sequence boundaries are summarized.Taking the advantage of seismic reflection internal structure,external form and reflection termination type,five kinds of seismic facies were identified and described,including the parallel,sub-parallel,foreset,hummocky,lenticular and divergent shaped seismic facies.Interpretation of seismic facies and sedimentary facies for 18 2D seismic sections was made and the distribution map of seismic facies for the 2nd and 4th members of the Xujiahe Formation compiled.Through the conversion of seismic facies to sedimentary facies,the Xujiahe Formation could be divided into several sedimentary systems,such as alluvial fan,fan delta,braided river delta and lakes.Based on the data,sedimentary facies maps of the 2nd and 4th members were compiled,and the history of tectonic evolution of the great central Sichuan area discussed.