Abstract：According to seismic, well-logging and core data, the micro-facies and type of the third Member of Paleogene Hetaoyuan Formation in Bidian area of Biyang sag were determined on the basis of sedimentary structures, lithological textures of profiles and thicknesses of sandstone bodies. Using the new methods for seismic inversion, the authors obtained the thickness of the iso-chronological single sandstone body and drew the distribution map of sedimentary micro-facies by tracing thin sandstone reservoirs from three-dimensional data. It is shown that there exist widely-distributed deposits of turbidite fans and sliding turbidite fans characterized by the turbidity-flows sediments, and sedimentary frameworks of north and south turbidite fans interbedded with the deep-water lacustrine sediments at the center. Sedimentary evolution, rich early turbidite sandstones and late deep-water mudstones suggest relative rise of the lacustrine level. In the light of favorable facies zones, the authors predicted and evaluated subtle traps of the sag, thus providing a basis for petroleum exploration.