Chang-7 Member in the Triassic Yanchang Formation served as the most important source rock of oil in the Mesozoic Ordos Basin

and it is also the realistic interval for the exploration and development of shale oil. The Chang-7 Member represents the climax depositional period of the Triassic lake basin. It is composed of a set of fine-grained clastic rocks

including mudstone and oil shale etc. The thickness of the Chang-7 Member is 100~120 m

which can be divided into three sub-members. Among them

the Chang-7

1

and Chang-7

2

sub-members h

ave been proved as the key intervals for exploration and development of shale oil. To deepen the understanding of the sedimentary environment and oil and gas geological characteristics of the Chang-7 Member

measurement of the field sections are combined with the previous work. The well-exposed sections of the Chang-7 Member in Yishicun

Beiyincun

Yaoqu

Bawangzhuang and Tangnihe in Tongchuan area are described in detail. Furthermore

the geological background of the formation of hydrocarbon-rich depression in the middle-late Triassic stages is discussed. Our results show

the lake basin went through gradual shrinkage and declining lake level from Chang-7

3

sub-member to Chang-7

1

sub-member

and the sandstone content gradually increase to the upper part. The study area is located in the deep lake area of the Yanchang Formation deposition. The sedimentary types in the Chang-7 Member include not only normal deep-water fine-grained sedimentary rocks

but also diverse deep-water gravity flow deposits

volcanic tuff

and carbonate rocks which might source from hydrothermal fluid in the deep part. These important phenomena record the special process of the formation of the hydrocarbon-rich depression

and have important implications for understanding the hydrocarbon formation and enrichment mechanism

as well as the exploration and development of shale oil.