Diagenesis and Physical Property Evolution of Gravity Flow Tight Reservoir of Yanchang Formation in Southern Ordos Basin

The research of diagenesis and physical property evolution is important to tight sandstone reservoir and tight oil exploitation. The tight reservoir was detailedly studied in the study area by the analysis of cast thin section, scanning electron microscopy (SEM), X-ray diffraction and homogenization temperature analysis of fluid inclusion in this study. The results show that deep-water sand body is a typical ultra-low porosity and permeability reservoir, which has fine granularity, poor reservoir physical properties, with thin pore-micro throat dominated pore-throat combination. The reservoir in the B sub-stage of mesogenetic diagenesis has experienced the following complex diagenesis evolution sequence: compaction/calcite cementation→tuffaceous hydrolyze alteration/mica hydration/illite cementation/chlorite cementation/quartz dissolution→early feldspar dissolution/quartz overgrowth/carbonate plaster recrystallization/early carbonate-cement dissolution→late feldspar dissolution→ferroan calcite cementation. The diagenetic environment underwent an evolution of alkalescence→acid→alkalescence. The test data of fluid inclusions show that Early Cretaceous (99-118 Ma) is the main hydrocarbon accumulation period with two hydrocarbon injections. The reservoir physical properties evolution show that early diagenesis had great influence on the reservoir, and it is inferred that the sandstone had already been compacted very tight before the hydrocarbon accumulation period.