Abstract: In order to clarify the formation mechanism of effective reservoir in the Xihu sag and implement the strategy for the development of marine economy effective exploration, we used thin section identification, scanning electron microscopy, constant rate mercury injection, X-ray diffraction and DST tests to analyze the genesis and development of effective reservoirs in the Xihu sag from three aspects including source, sink, and lithology, and discusses the lower limit of their development depth. The result shows that the formation of effective reservoir is controlled by provenance, sedimentation, and diagenesis. The igneous (especially granite) and metamorphic rocks occur around the Xihu Sag. The strong hydrodynamics of large braided river-delta- tidal flat sedimentary environment provides conditions for the development of effective reservoirs under deep low permeability in the Xihu Sag, while transformation of pore throat structures by diagenesis determines vertical distribution of effective reservoirs. Favorable sedimentary microfacies, strong dissolution and early chlorite development are the main factors controlling effective reservoir development in the Xihu Sag. Clean medium-coarse and gravel-bearing medium-coarse sandstone reservoirs under low geothermal and overpressured development determine the lower limit of effective reservoir development.

Key words: Xihu Sag, Pinghu Formation, effective reservoir, main controlling factors