Pore Structure Characteristics and Main Control Factors of Sandstone in the Jurassic Zhiluo Formation in the Northern Ordos Basin

The Jurassic Yan’an Formation in the Ordos Basin is one of the main coal seams mined in the basin, and the enrichment of water bodies in the upper part of this coal seam is closely related to the sand bodies in the Zhiluo Formation. This study is based on the use of core observations in the northern part of the Ordos Basin for studying the pore characteristics of the permeable sand layer of the Zhiluo Formation in the study area through testing methods such as ordinary thin sections, cast thin sections, scanning electron microscopy (SEM), mercury intrusions, and physical property analysis. The results indicate that the primary pores of the Zhiluo Formation sandstone in the study area include primary intergranular pores, residual intergranular pores, and interstitial micropores. The secondary pores are mainly intergranular pores, feldspar dissolution pores, and rock debris dissolution pores. The throat-type pores are mainly variable fault contractions, sheets, curved sheets, and bundle-shaped throats. The pore structures of the Zhiluo Formation sandstone in the research area are complex, and the permeability is influenced by the throat characteristics. The main controlling factors of the pore structure characteristics of the Zhiluo Formation sandstone in the study area are sedimentation and diagenesis. Compaction and cementation are the main factors that destroy the sandstone pore structure, while later dissolution plays a certain role in the improvement of the pores. Section 1 of the Zhiluo Formation is greatly affected by diagenesis, and section 2 is greatly affected by sedimentation.