Experimental investigation of pore development of the Chang 7 member shale in the Ordos basin under semi-closed high-pressure pyrolysis

Semi-closed heat simulation experiments were conducted on cylindrical samples, and a series of residue samples of Chang7 shale heated to different temperature were acquired to characterize the evolution and controlling factors of shale reservoir structure. Low pressure adsorption experiments with carbon dioxide and nitrogen as the adsorbents were conducted to characterize the evolution of shale nanopores. Mercury intrusion capillary pressure (MICP) and He-Hg porosities of the raw and residue shale columns were measured to study the evolution of porosities with increasing maturity. Also, the morphology of nanopores and microfractures of Argon ion polished shales was examined using scanning electron microscopy. Decreasing trends of micro- and meso-pores with increasing temperature were mainly caused by masking or occupation of pores by generated oil and bitumens during the oil generation stage. Then the specific surface areas (SSAs) and volumes of micro- and meso-pores increase dramatically with increasing temperature as further nanopores were formed through secondary cracking of the generated oil and bitumens. Destruction of nanopores occurred at the over-maturation stage, and caused decreasing trends of SSAs and volumes of micro-pore, meso-pore and macro-pore from 450 °C, 489 °C and 500 °C, respectively. Shale porosity showed a generally positive correlation with temperature, suggesting that macro-pores and micro-fractures were the predominant reservoirs. That was also observed with focused ion beam scanning electron microscopy. The insights into the evolution of shale nanopores presented in this study are generally in accordance with previous research in North America and China on the properties of these structures in shales at different maturation level.