During sedimentation of the Jurrasic Sanjianfang Formation in Taibei sag of Tuha basin

northern part of the sag developed into a fan-delta due to its steeper topography whereas southern part of the sag evolved into a braided channel delta owing to its gentle topography. Clastic rock compounds as a main sedimentation body deposited by means of distributary channel sediments are well developed in the sedimentation of the fan delta and braided channel delta. Mechanical compaction

cementation and solution effects are chief diagenesis of Sanjianfang Formation. Clastic rock body in front of the fan delta in northern part of the sag is manly characterized by mechanical compaction effort and weaker solution effect. Most of primary interparticle pores would disappear in the reformation of intense mechanical compaction of clastic rock body and become a reservoir body of low porosity and permeability with the main feature of cementation of autogenic minerals and solution effect of unstable compositions. Whereas the braided channel delta in southern part of the sag has not experienced mechanical compaction because of its higher content of rigid material

preserved with a large amount of remaining interparticle pores which become a reservoir of good physical property with the reformation of solution effect.