Turbidite fans in the 3

rd

member of Shahejie Formation in central Dongying depression

mainly distributed in the deepest sag of the depression

can be divided genetically into three types:slump turbidite fan

slope-sliding turbidite fan and sublacustrine fan.Their formation was closely related to the fluctuation of base level.Slump turbidite fans were formed during the short-term falling of base level

as a result of redeposition of the slumped frontal sediments of the prograding delta.Slope-sliding turbidite fans were formed during the short-term rising of base level

as a result of redeposition of sediments sliding from marginal slope of the lake basin to deep lake area

due to erosion and washing of delta front and plain by dynamic force of lake-and flood-water.Sublacustrine fans were formed during the medium-term rising of base level and located in the sequence stratigraphy corresponding to the lowstand fans in the lowstand system tract in classical sequence stratigraphy.Of these three types of turbidite fans

the sublacustrine fans and slope-sliding fans

with large thickness

good poroperm characteristics and petroliferous properties

are the main reservoirs in this area

while slump turbidite fans are characterized by smaller sizes and relatively poor poroperm characteristics.