The sandbodies of offshore turbidite fan in the Zhanhua Depression with a buried depth of 3200-3640m are located in a sealed environment around by argillic source rocks. The reservoir spaces are of secondary origin

and porosity is relatively high and stable within the extent of studied depth. The reasons that porosity does not vary with the buried depth are that:1. The existance of carbonate cements and the replacing minerals in the early and middle stages of diagenesis provided large amounts of spaces for the formation of the secondarypores;2. A nearly closed sedimentary system prepared abundant acidic water solution;3. The minerals themselves produced the condition of physical-chemical solution. At about the buried depth of 3211m

plagioclase dissolution reduced due to albitization while the dissolving pores of potash feldspar increased remarkably. This relationship between growth and extinction constituted the main cause of the relative stability of the porosity. The microfacies of braided channels in the offshore turbidite fans are the major targets for hydrocarbon exploration owing to sandstones with great thickness

lower content of mud and higher isotropy.