In order to understand reservoir characteristics and controlling factors of the lower 3

rd

Member of Shahejie Formation(E

2

s

3L

) in Bohai A oilfield

we studied the lithological composition

pore types and structure

porosity and permeability distribution and discussed the controlling factors of petrophysical properties of the reservoirs from sedimentary

diagenetic and tectonic perspectives based on thin section

scanning electron microscopy (SEM)

mercury injection and other conventional lab test data.The results show that the reservoirs of fan delta facies in E

2

s

3L

have low to ultra-low poro-sity and ultra-low permeability.They are composed of lithic arkosic sandstone with low compositional maturity

their main pore throat types are primary intergranular pore

dissolution pore and residual pore

and the pore throat structure is fine to micro.The reservoir properties were controlled by various factors

including sedimentation

dissolution and tectonic movements etc.Sedimentary environment of distributary channel and mouth bar at fan delta front controlled the grain size and sorting of the reservoir

resulting in relatively good reservoir properties.Compaction and cementation were the main factors causing the low permeability

however

dissolution of feldspar and detritus components by organic acids formed secondary pores

and the abnormal high pressure in the reservoir weakened compaction and helped the preservation of primary intergranular pores.Fractures are well developed near the faults formed by various tectonic movements

which also helped to improve the physical properity of the reservoirs.Through comprehensive analysis of all available data

we can conclude that strong hydrodynamic depositional environment

later dissolution and well-developed fracture network are the main factors that control the formation of the reservoirs with relatively high porosity and permeability.