Reservoir porosity characteristics and controls of the Shanxi Formation shale reservoir, Yanchang area, Ordos Basin

The micro-scale shale reservoir porosity type and the corresponding pore size distribution of the Shanxi Formation in the Yanchang exploration area of the Ordos Basin were analyzed using argon ion polished sections for SEM observation. Helium gas expansion, mercury injection, low pressure nitrogen adsorption, carbon dioxide adsorption, nuclear magnetic resonance (NMR) tests and full pore-size quantitative characterization were applied to study the reservoir physical properties and the pore structure of various rock types. Various geological factors influencing the reservoir pore development and preservation of the Shanxi Formation were discussed from different aspects combined with the mineralogical and geochemical characteristics of reservoir rocks. Results showed that there were differences in pore types, pore sizes and porosity of different rock types of the Shanxi mud shale formation, and there were differences in pore size and development characteristics of different pore types of the same rock type. The organic matter content (asphalt content) of silty shale was decisive for porosity and middle-large pore development. In low TOC shale, quartz feldspar content is a main factor that affects porosity, showing a positive correlation. Organic matter content has a positive influence on the porosity of the high TOC argillaceous shale, while rigid particle size and argillaceous mineral content show a clear negative relationship.