Yanji Basin is a Mesozoic intermountainous fault-depressed basin situated at the eastern foot of Changbai Mountains in Northeast Jiling Province. During the process of formation and development

the basin underwent three large scales of magmatic activities as early and late Yenshan and Himalayan

and four evolution stuges:fault-subsided

depressed

shrinkage and uplift stages. Its structural framework was controlled by near NS and EW stretching faults

and characterized by zonatesin EW direction and blocks in NS directions. During Variscian stage

the basin received extensive Meso-Cenozoic sediments in which Cretaceous sediments were the main part.The greatest thickness was 3900m. In late Late Cretaceous

the basin was uplifted and denuded resulted by Himalayan movement. The basin has favourable source-reservoir-cap rock assemblage and trap condition.The Dalazhi Formation in Lower Cretaceous has thick sediments (the max. thickness is about 2000m)

and the dark mudstone developed well. Organic abundance and transformation degree of organic matter are high (the average organic carbon is 1.517-2.408%;chloroform extract "A" 0.063-0.072%; the ratio of total hydrocarbon/organic carbon0.232-6.638%; R

o

-value 0.43-1.38% )

and the kerogen is mainly type-Ⅱ

indicating that the mudstone is fine source rock. The formation contains considerable fine to meduim grained sandstone which occupies more than 50% of the total thickness of the formation.The porosity of the Sandstone is about 20%

and the permeability 13×10

-3

μm

2

. This constitutds favourable reservoir rock. The mudstone member of the Dalazhi Formation laterally distributed uniformly; the middle-upper parts of tfie Longjing Formation in Upper Cretaceous contain fine tuff clastic rock and thin gypsum

they both constitute perfect regional caprocks. According to the data of seismic reconmaissance

the trap types of thebasin are mainly lithologic and stratigraphic traps and secondly the drape structure and fault-block traps. The principal exploration turget bed is the Dalazhi Formation; the favourable potential areas are Chaoyangchuan Sag

then the Qingchaguan and Dexin Sags. Fine oil-showings have been found in outcrops of Zhixin Village and from well drilling in Dexin Sag in southeast of the basin.