Diagenesis and evolution of deep tight reservoirs: A case study of the fourth member of Shahejie Formation (cg: 50.4-42 Ma) in Bozhong Sag

This study focused on the deep tight sandstone reservoir (DTSR) of the fourth member of the Shahejie Formation in the Bozhong Sag, Bohai Bay Basin, a special type of reservoir. To reveal the diagenesis and evolution of the reservoir in the study area, cores observation, thin section identification, scanning electron microscopy, grain size analysis, and petrophysical properties measurements are available to analyze the mechanics of diagenesis and densification processes. The recognition is agreed on that (1) the lithology of the fourth member is mainly composed of lithic arkose and feldspathic lithic sandstone with low compositional maturity, and grain sizes vary from middle to coarse; (2) the porosity of reservoir ranges from 4 to 11.5% (av. 6.8%), which belongs to the medium low porosity sandstone reservoir; the pore structure is complex and the type of pore spaces is mainly secondary pore, while original pores are less developed due to the deep distribution of strata; (3) based on the quantitative calculation of porosity of the DTSR, it was identified that compaction is the main reason for sandstone reservoir densification, with an average porosity reduction of 62%, followed by cementation filling intergranular pores with an average pore reduction rate of 25.1%. Dissolution plays a constructive role in improving porosity, with an average increase rate of 18.5%.