Nanometer Pore Structure Characterization of Chang 7 Member Continental Shale in Northern Shaanxi Area, Ordos Basin Based on the Nitrogen Adsorption Experiment

摘要

The Northern Shaanxi Area, Ordos Basin is an important continental shale gas exploration area in China. The Chang 7 Member of Upper Triassic Yanchang Formation is a main exploration interval for this area. In order to strengthen research on nanometer pore structures of Chang 7 Member shale reservoirs in the Northern Shaanxi Area, the microscopic pore structure characteristics are studied through the nitrogen adsorption experiment. The main controlling factors of pore development are discussed and the significance of pore structure for shale gas storage is analyzed. Results show that the pores of shale reservoirs of Chang 7 Member are dominated by nanoscale ones and the average pore size of shale samples is between 5.7 and 7.9 nm. According to IUPAC classification, shale pores of Chang 7 Member are dominated by mesopores, accounting for 90.8%-95.5% of the total pore volume. The nanometer pore structure is relatively uniform and the adsorption/desorption curves are dominated by Type B in the De Boer classification. The micropore volume and the macropore volume have similar proportions, and they account for 4.5%-9.2% of the total volume, averaging at 7.2%. Characteristics of Type E indicate that most pores are open type ones, and slit-like pores and 'ink bottle' shaped pores are irregularly configured. Total organic carbon (TOC) has a weak negative correlation with specific surface area of the shale, and has a weak positive correlation with total pore volume. Maturity (Ro) has a weak negative correlation with both specific surface area and total pore volume. The content of brittle minerals and content of clay minerals have no clear correlation with the specific surface area and the pore volume.