Through fine core observation and description

eleven different lithofacies were recognized in the 1

st

member of Dainan Formation and the 1

st

member of Sanduo Formation.According to the characteristics of logging data and lithofacies associations

and in combination with the regional depositional environment

this paper documents that the 1

st

member of Dainan Formation is of braided delta sedimentary system and the 1

st

member of Sanduo Formation is of fluvial sedimentary system with braided-river in the lower part and meandering-river in the upper part.During the deposition of the 1

st

member of Dainan Formation

braided-river delta front were well developed in the northern fault terrace zone and the south-eastern slope zone of Qutang Sub-sag and the south-western slope zone of Haibei Sub-sag

and sandbodies of underwater distributary channels constitute the framework of the braided-river delta fronts at this time.During the deposition of the lower part of the 1

st

member of Sanduo Formation

the study area was dominated by the braided-river channel and flood plain deposits

and channel bars were well developed.During the deposition of the upper part of the 1

st

member of Sanduo Formation

the sedimentary subfacies changed to meandering river channel

bank and floodplain

with point bar deposits developed in the convex bank of channel.The sandbodies of underwater distributary channels

channel bar in braided river and point bar in meadering river have relatively good poroperm characteristics

and thus are favorable facies for hydrocarbon accumulation.