Abstract:

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^-3μm². 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.