Development and influencing factors of middle-deep tight sandstone reservoir ‘sweet spots’ in fault basin—Taking the Eocene Shahejie Formation in Suning area of Raoyang Sag in Bohai Bay basin as an example

This study takes the tight sandstone of the lower sub-member of the first member of the Eocene Shahejie Formation (Es₁^x) in the Suning area, Raoyang Sag, Bohai Bay Basin. Furthermore, it determines the rock type of the middle-deep tight sandstone through lithologic profiling, core observation and log data analysis. The genetic control factors of tight sandstone in the study area are analysed using conventional thin section, pore-casted thin section and scanning electron microscope, and the characteristics and genetic control factors of tight sandstone reservoirs are summarized. The research results show that the tight sandstones in this area are primarily feldspathic sandstone and feldspathic lithic sandstone, with high quartz contents, around 61.50%. The average porosity of the reservoir in the study area is 4.69%, and the average permeability is 2.92 mD, which are characteristics of low-porosity and low-permeability reservoirs. Through research, it has been found that the distribution of sedimentary facies and diagenesis in the study area control the development and distribution of ‘sweet spots’. Both compaction and cementation reduce the physical properties of the reservoir, while dissolution can increase porosity, thereby increasing the physical properties of the reservoir; dissolution resulted in an increase of approximately 3.57%–3.69% in the porosity of the tight reservoir in the study area, with an average porosity increase of 3.63%, and is the main controlling factor for the development of ‘sweet spots’ in the reservoir. The results provide an important basis for guiding the exploration and development of tight sandstone oil and gas in the lower sub-member of the first member of the Eocene Shahejie Formation (Es₁^x)in the Suning area.