Pore characteristics and their evolution in Paleogene mud shales, Dongying Sag, Bohai Bay Basin

The pore characteristics of mud shales from the lower part of the third member and the upper part of the fourth member of the Paleogene Shahejie Formation in the Dongying Sag were studied through modern experimental methods.The relationships between pore size distribution, porosity and burial depth were analyzed.Five major pore types of the Paleogene mud shales in the study area were identified, including intercrystalline pores in clay minerals, intercrystalline pores in calcites, intergranular pores in sands, intercrystalline pores in pyrites, and organic pores.Nano pores were well developed in the mud shales, and the pores smaller than 25 nm accounted for 49.7% to 86.3% of total pores.The changing modes of porosity with depth are discussed.An model of how the pores in the mud shales evolved in the Paleogene in the Donging Sag was established from their characteristics and knowledge of their controlling factors.There were three evolution stages for pores in the Paleogene mud shales, including the original formation stage, the transformation stage and the hysteresis stage, among which the second and the third contributed more to the hydrocarbon storage capacity of the shales.Greater thickness, high organic abundance, rich pyrite and active thermal sulfate reduction effect were favorable conditions for reservoir development in mud shales.