Abstract: Utilizing the practical study on the continental basins,the authors divide the whole third-order sequence into four systems tracts, which are lowstand systems tract (LST), transgressive systems tract (TST), highstand systems tract (HST) and regressive systems tract (RST),named sequence type I. If LST is undeveloped in a thirdorder sequence, it forms the assemblages of TST, HST and RST，named sequence type II. LST occurred in the rapid declining period of lake level (base level). TST appeared with first lake flooding surface and maximum lake flooding surface. HST formed in the relative static high lakelevel period. RST formed in the gradual lake-level fall stage, and the supplying rate of sediments was more than the increasing rate of accommodation space at that time. Generally, LST consists of small scale progradational parasequence sets, vertically, the sedimentary environment is shallower, and river downcutting forms in the basin margin. TST consists of retrogradational parasequence sets, and the sedimentary environment becomes deeper from bottom to up. HST consists of aggradational parasequence sets, and the sedimentary environment changes little or is in standing water. RST consists of large scale progradational parasequence sets, and the sedimentary environment becomes shallower from bottom to up. Finally, the practical study on Songliao basin has been introduced to respectively expound the distribution regularities of oil and gas reservoirs in different system tracts in depression basin and fault basin. LST develops mainly lens-shaped lithologic reservoirs in the steep slope side of fault basin and deep-water area of depression basin. TST forms mainly updip thin away type lithologic oil & gas reservoirs in steep slope belt of fault basin, and forms stratigraphic overlap lithologic oil & gas reservoirs in the gentle slope belt of fault basin and slope zone of depression basin. HST mainly consists of lens-shaped lithologic reservoirs in deep-water area. RST develops mainly stratigraphic unconformity screened oil and gas reservoirs in fault basin, and developes fault block reservoirs and fault screened oil & gas reservoirs in depression basin.Thus, in the combination of theory and the practice, the practicality of the four division about continental sequence stratigraphy has been illustrated.

Key words: four division of system tracts, genetic mechanism, identification marks, distribution regularity of oil and gas reservoirs, petroleum, Songliao basin