In the Yuejin一2 structure of the western Qaidam basin，Palaeogene organic reefs are developed in the lowermember of the lower Ganchaigou Formation(E；)，and two types of reefs are recognized：matrix—supportedreefs(stromatolites)and skeleton—supported reefs (frame reefs)． The reefs have complex pores，such asinterparticle pores，skeletal pores，coelomic pores，intraparticle pores and corroded cavities，which are excellentreservoirs in the study area． Reservoir characteristics of the reefs in the Yuexi-'4 well are thoroughly tested。whose average porosity is 26．1％ and average permeability is 97．2×10一。 m。．The organic reefs show distinctcharacteristics of seismic response and well logs，SO the reefs may be identified and predicted according to theirgeophysical characteristics． Besides biotic factors， their development is affected by palaeoclimate，palaeolatitude， palaeo—hydropower， palaeo—aqueous m edia and terrigenous clasts． On the other hand， theirdevelopment is controlled by tectonic structure and landform of the lakebed． The discovery of Palaeogeneorganic reefs in the Qaidam basin is of important practical significance for the exploration and development ofthe Qinghai Oilfield．