Oil and gas resources are abundant in the southern margin of Junggar Basin with multi-layered geological structures and complicated and variable structural styles

which features the most complex petroleum geological conditions and the most tortuous exploration history. In recent years

with the improvement of the geological theories

the progress of seismic and drilling technologies

and the shift of exploration strategies for hydrocarbon exploration of ultra-deep clastic rocks in the foreland at the southern margin of Junggar Basin

a large gas area of ultra-deep clastic rocks has been discovered in the middle section of the southern margin of Junggar Basin. Exploration practices and researches show as follows. (1)Five sets of source rocks

i.e.

the Permian

Triassic

Jurassic

Cretaceous

and Paleogene

are developed in the southern margin of Junggar Basin. Among them

the Jurassic and Permian as the main source rocks have entered the stage of large-scale gas generation and possess the material basis for the formation of large and medium-sized gas fields. (2)The lower assemblage has developed multiple sets of large-scale reservoirs such as the Cretaceous Qingshuihe Formation

and the Jurassic Kalazha Formation

Toutunhe Formation

Sangonghe Formation

and Badaowan Formation. The superimposition of pore-type and multi-scale fracture-type reservoirs forms a large-span fracture body

providing reservoir conditions for high natural gas production. (3)The regional thick quilt-like ultra-high pressure mudstone caprock of the Cretaceous Tugulu Group has good preservation conditions

which is the key to natural gas accumulation. (4)The large-scale anticlinal structural trap group distributed in rows has developed in a belt-like pattern. According to the superposition pattern of structures

the southern margin of Junggar Basin can be divided into three rows of tectonic belts (thrust belt

detachment fold belt and frontal slope belt)

including seven structural belts (buried structural belt

Dongwan structural belt

Wuchang transfer belt

Huomatu structural belt

Du’an structural belt

Hutubi structural belt and Fangcaohu structural belt). It is clear that Dongwan structural belt

Hutubi structural belt

Wuchang transfer belt and buried structural belts are most favorable anticlines for natural gas preservation. During exploration of the southern margin of Junggar Basin

a sequence of techniques such as high-precision seismic acquisition

full-depth modeling and migration imaging

and fine description of traps for the "double complex" structures (complex surface and subsurface structures)in the ultra-deep piedmont has been established

as well as the key supporting technologies involving fine evaluation of ultra-deep

ultra-high pressure and ultra-high temperature clastic rock reservoirs

optimal fast drilling and completion

and safety testing. The breakthrough of natural gas exploration in the middle section of the southern margin of Junggar Basin has effectively promoted the exploration and development of natural gas in ultra-deep clastic rocks

thus enriching the petroleum geological theories of continental ultra-deep clastic rocks.