Coarse siliciclastic reservoirs with low porosity and low permeability of fan-delta facies are common in the medium-to-deep first and second members of the Eocene Shahejie Formation (Es

1-2

) at the southeastern margin of Qinnan Sag in the Bohai Bay Basin

but there are also some zones with high porosity and high permeability in some intervals.The reservoir spaces are largely dissolution pores and biological coelomopores

and the mechanisms of favorable reservoir development are different from that of the common tight coarse siliciclastic reservoirs.So we integrated data from core samples

casting thin sections

burial history

and fluid inclusion to investigate the characteristics and controlling factors of the high-quality coarse siliciclastic reservoirs in the Es

1-2

.The results indicate that spiral-shell-bearing coarse siliciclastic interbeds are common in the Es

1-2

coarse siliciclastic reservoirs of fan-delta facies in the study area and are interpreted as wave-modified

intermittently abandoned underwater distributary channel deposits.Intensive calcite cementation plays a major role in the formation of tight reservoirs.The spiral-shell-bearing coarse siliciclastic rock is the major rock type for the development of high-quality reservoirs.Both lithological factors and diagenetic factors contributed to the development of high-quality reservoirs.The former include particle supporting

good sorting

coelomopores being rich in spiral shell fossils and early cryptocrystalline dolomite cementation

while the latter are mainly poor compaction and infiltration of diagenetic fluids due to particle supporting

strong dissolution of intermediate-acidic volcanic detritus

dolomite cements and feldspars by acidic pore-space waters as well as protection of pores by early charged hydrocarbons.This mechanism of favorable reservoir formation is of great referential significance for petroleum exploration in tight coarse siliciclastic reservoirs formed in brackish settings on steep-slope zones.