Abstract:Immature continental shale samples collected from the outcrops of the Chang 7 member in Ordos Basin were analyzed by hydrous pyrolysis experiment. Seven samples with different simulated buried depth were obtained in the laboratory. The characteristics and pore size distribution were observed using field emission scanning electron microscope and low pressure nitrogen adsorption experiments. The results indicated there were mainly five types of pores in primary oil shale, including: primary pores, secondary dissolved pores, intergranular pores between clay minerals, intracrystal pores and shrinkage pores. Some of them were filled with residual hydrocarbon. Organic pores began to form with increasing temperature and pressure, they contributed to the whole porosity. The porosity began to decrease after drastically increasing- from original 3.8% to 17.53%, and then reduce to 8.15%. Peak pore sizes appeared between 50~200 nm. It had not yet reached gas window as a low thermal evolution for the primary sample, immature source rocks with high organic matter abundance had not begun to generate a great deal of hydrocarbons. The linear, slit pores developed a lot at the contact edge of organic material and skeletal grains. When the temperature reached 350℃and above with a depth deeper than 2500 m, pores began to form in the organic matter, and especially, interlayer pores generated with the decomposition of organic material within the clay. With the increasing simulated depth, compaction, phase transformation of minerals and less organic pores made a remarkable porosity reduction by 5.68%, compaction is the key issue of the organic-rich shale reservoir.