Experimental Study on the Characteristics and Diagenetic Processes of Typical Tight Oil Reservoirs

The southern part of the Ordos Basin, in the Fuxian area, where the Yanchang Formation’s Chang 7 Member is a new stratigraphic unit for tight oil development, and there has been little research on its pore structure and diagenetic processes. This paper takes the Chang 7 Member in the Fuxian area as an example and systematically conducted a micro-scale reservoir property study of the Chang 7 tight oil using a large number of thin sections, scanning electron microscopy, physical properties, and CT scanning experiments. The results show that the sandstone types in the Chang 7 Member of the study area are mainly lithic arkose, followed by lithic arkose and sublithic arkose. The quartz content in the Chang 7 Member sandstones is relatively high, ranging from 15% to 47%, with an average of 31%. The types of pores developed include intergranular pores, intercrystalline pores, intragranular dissolution pores, intergranular dissolution pores, organic matter pores, and micro-fractures. Compaction, cementation (silica cementation, carbonate cementation, clay mineral cementation), and metasomatism have a destructive effect on reservoir quality; constructive diagenetic processes mainly include chlorite cementation and dissolution. Diagenetic sequence analysis indicates that the Ro value of the Chang 7 Member in the study area is between 0.79-0.91%, belonging to the late diagenetic stage A period. Early calcite is mostly pore-filling, and late calcite precipitates to fill dissolution pores. Clay minerals are mainly illite-smectite mixed layers, with the content of smectite in the mixed layers being less than 41%, and some less than 15%.