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

500km

2

.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 176km

2

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.