Distribution characteristics of micro pore throat size of turbidite sandstone reservoir in middle sub-member of 3rd member, Shahejie Formation in Dongying Depression

Key words:  Dongying Depression  Shahejie Formation  turbidite sandstone reservoir  microscopic pore structure  full pore size distribution

Abstract:

Taking the turbidite sandstone reservoirs in the middle sub-member of 3rd member of Shahejie Formation in Shishen 100 area of Dongying Depression as an example, the microscopic pore structures of the reservoirs were studied by casting thin section, scanning electron microscope, and mercury injection experiment. Specifically the full pore throat size distribution characteristics of turbidite sandstone reservoirs were characterized by the combination of high-pressure mercury injection and constant pressure mercury injection. The results show that there are a variety of pores and throat types in the low-permeability turbidite sandstone reservoir in the study area. The main pore types are residual intergranular pores, feldspar dissolved pores and intercrystalline micropores. The throats are mainly sheet-like throat and tube-like throat, and the throats are relatively fine. There are many unconnected pores surrounded by narrow throats, and the pore-throat configuration is complex. The turbidite sandstone reservoirs have strong reservoir capacity, but weak seepage capacity. Compared with the conventional sandstone reservoir, the pore throat size is quite different. The full pore size distribution is between several nanometers and several hundred microns, and the curve is in multimodal shape. Large pores whose pore throat size is greater than 10 μm, and nanopores whose pore throat size is less than 0.1 μm are mainly developed. The key factors that determine the physical properties of low-permeability turbidite sandstone reservoirs are the size, quantity of effective throat and the configuration between pores and effective throats. Effective throat directly controls the reservoirs permeability by affecting the seepage capacity of reservoir.