Abstract: The Baikouquan formation (T1b) of Triassic in Wuerhe oilfield can be divided into one long-stage semi-cycle (LSC), three midstage cycles (MSC) and seven short-stage cycles (SSC) which are in correspondence with the sandbody development period, that is, each period sandbody developed in each SSC correspondingly. The cycle evolvement is characterized by asymmetry-cycle, symmetry-cycle and asymmetry-cycle from bottom to top. The first asymmetry-cycle is resulted from that the sediments of previous stratum descending stage were washed down by post fluvial channels, followed by shaping scour surfaces; the last one is due to the increasing accommodations allowing descending cycles to be under-compensation sediments. The major reservoir units, MSC1and middle-bottom part of MSC2, are located in eary-middle period of LSC and composed of retrograding-upgrading SSC sediments with overlapped sand-gravel lithologies in middle-fan sub-facies of alluvial fan. The quality and productivity of MSC1reservoirs are obviously better than those of MSC2reservoirs.

Key words: Wuerhe oilfield, Baikouquan formation, LSC, MSC, SSC