A group of huge faults overthrusting from north to south along the southern sides of Mts.Saishiteng,Luliang,Xitie and Aimnik cn the northern margin of Qaidam Basin are found in outcrops with dip angles of seventy degree or more,but seismic data available show that the dip angles of these fault planes in the lower part of Tertiary are gradually reduced and finally turned to twen-ty or lesser within Prepaleozoic sediments.Another huge fault,Maxian fault,overthrusting from south to north is located at the north side of Mahai and Nanbaxian anticlines on the southern margin of eastern Saishiteng depression with a dip angle of thirty degree.Between these opposite dipping faults,there lays Mahai-Pingtan depression with sediments of Meso-Cenozoic system which measures to 10,000m or more in thickness,among them,more than 1,200 mthick sediments belong to Mesozoic,in which the 5th and 7th members of the Dameigou Formation of Middle Jurassic,the source rock in this area,might be existed and covering a area of about 1,500km2.Within this depression,some petroleum fields have been found out,e.g.western Yuka Oilfield with Upper Jurassic pay zone is in the north,and Mahai Gasfield with Oligocene reservoir in the south.It is proved by seismic prospecting that the concealed Mahai-Pingtan anticli-nal nose has a trap area of 176km2 in upper Mesozoic,and supposedly,other traps might exist somewhere eastward.These subtle traps,together with Mahai and Nanbaxian anticlines,might respectively belong to the footwall and hanging wall of the Maxian fault.This area,being close to the petroleum generating depression,having good traps,is therefore favorable for oil prospecting.In western Saishiteng depression,there are a series of concealed anticlines along the footwalls of Tuonan and Quenan faults.The 5th member of the Dameigou Formation,the source rock of Lenghu Oilfield,may distribute in this region,which could be 1,000-1,300m thick,and is potential for oil gene-ration,and as it is near the Lenghu 4th and 5th Oilfields with Tertiary reservoir and Lenghu 3rd Oilfield with Jurassic pay bed,this area is also very hopeful in petroleum exploration.
