Evolution of pore fluid of low porosity and permeability reservoir in Qinnan Sag:a case study of 29-2 structure in Qinhuangdao area

The southeast depression of Qinnan Sag is a potential oil-gas exploration region in the Bohai Sea area.With the analysis of large quantity of rock thin sections, scanning electron microscope and the physical property data of reservoir, the authors studied the petrological characteristics and the evolution of pore and fluid of sandstone in the deeper strata in 29-2 structure in Qinhuangdao area.The results show that the evolutionary tendency of Paleogene sandstone reservoir porous fluid in research area is changed from alkaline porous fluid to acidic porous fluid, and back to alkaline porous fluid.There are three stages of reservoir porous evolution in Qinhuangdao area, namely sharp decrease in porosity due to mechanical compaction, increase in porosity because of corrosion and dissolution, and remarkable reduction owing to carbonate cementation.