Eogene Pinghu Formation and Huagang Formation can be divided into four main sedimentary systems of semi-closed bay

lake

delta and river; and six main sedimentary types of shore-tidal flat

lagoon

shore-lake

river-controlling delta

tide-controlling delta and river etc.

and several subfacies and microfacies. The development and evolution of the sedimentary systems in Pinghu Formation and Huagang Formation indicate change characteristics from the sea to the land; namely Pinghu Formation is chiefly composed of semi-closed bay environment with its own feature of development of bay-lagoon facies which is distributed largely in the central belt of working area. Huagang Formation consists mainly sedimentary systems of lake

delta and river which apparently can be divided into two sedimentary systems; the lower section is made up of ingression lake facies

the upper section is composed of fresh water lake sediments and interlayering sediments of meandering river flooding plain. The lake sedimentary system is mainly distributed in the northeast and central part of the working area

the delta system is chiefly developed in the south and southeast part of the working area while the meandering river system developed in the west part. Tidal channel

tidal-flat bar

river mouth bar

front sheet-like sand and microfacies of braiding flow river are main favorable reservoir belts in the area.