Physical properties and influencing factors of the reservoir rocks in the lower part of the third member of the Palaeogene Shahejie Formation in the Shulu depression

The reservoir rocks in the lower part of the third member of the Palaeogene Shahejie Formation in the Shulu depression are characterized by less-developed original pores,dominant secondary solution openings and fissures with extraordinary low porosity and permeability.These reservoir rocks interpreted as pore-fissure reservoir rocks consist of sandstones and conglomerates of gravity flow origin and marls of mixed origins.The factors influencing the reservoir quality include lithology,thickness,sedimentary facies,diagenesis and tectonic stress.The fissures generally occur in the brittle medium to thin-bedded sandstones and conglomerates as the matrix-poor debris flow deposits with lower contents of mud and under the tectonic stress rather than in the plastic thick-bedded gravity flow deposits and marls with high contents of mud.The compaction and cementation during the diagenesis are responsible for the decrease of porosity,and the dissolution leads to the formation of secondary porosity in the reservoir rocks.