Experimental means,including ordinary slices,casting sections and electron microscope,were used to study samples from reservoirs in the Shahejie Formation of Qibei slope in Bohai Bay Basin.The results were then combined with logging data to analyze the features(lithological composition,porosity and permeability distribution,pore texture and re-servoir quality index(RQI))and forming mechanism of the reservoirs.The analyses show that the sandstone reservoirs in the formation are typical low-porosity and low-permeability reservoirs featuring in low maturity,various types of pores,Ⅳ type of pore structure,small-to micro-sized pore throat and secondary intergrannual pores.The forming mechanisms of the reservoirs include sedimentation,secondary dissolution and deep overpressure.It also indicates that sedimentary setting of the channels controlled the particle size,cement properties and content,pore throat size and sorting coefficient of the re-servoirs.Sandstone in the main channels offered better conditions for the forming of reservoirs as it is thicker and has a porosity of 3%-5% higher than that at the side edges.Primary pores in the reservoirs were almost killed during compaction and cementation,while dissolution of feldspar and other minerals created secondary pores and provided reservoir space.Overpressure in the reservoirs restrained compaction but boosted dissolution and enlarged reservoiring space.A comprehensive analysis of all the information suggests that secondary pore zones in gravity flow channels and fan delta fronts be the most favorable locations for reservoirs to develop.
