An oblique progradational sequence occurs in the 1

st

Member of Liushagang Formation in Weixi'nan Sag

Beibuwan Basin.A comprehensive analysis of single well facies

multi-well correlation

and seismic facies based on geological

logging and seismic data reveals that it belongs to lacustrine forced regressive deposits.It is characterized by obvious upward shallowing in respect of singe well facies

vertical overlapping and lateral sequential migration of multi-phase deltas in multi-well correlation

and significant oblique progradation reflection configuration on seismic profiles.Local uplifting of the basement is the main controlling factor for the development of lacustrine forced regression.The identification of the lacustrine forced regressive deposits helped to eliminate the disputes concerning stratigraphic division and correlation of the 3

rd

Member of Weizhou Formation and the 1

st

Member of Liushagang Formation in Weixi'nan sag.Meanwhile

it also led to the recognition of lithologic traps such as turbidite

sublacustrine fan

and progradational delta and the discovery of several reservoirs within the lacustrine forced regression system tracts

thus expanding new domains for oil and gas exploration in this sag.