This block is located on the eastern side of the Daxing’anling Range and covers an area of approximately 260,000 km2, which is roughly equivalent of the Songliao Plain. In general, its shape resembles an irregular rectangle with a NNE trend. There is currently no agreement about the basement of this block. Some believe that this block has Precambrian crystalline basement, but others believe that the base is a massif that formed due to Hercynian folding. During the Cenozoic, this area was a large basin with thick continental sediments; hence, it has become an important oil-producing area in China.