DIAGENESIS AND PHYSICAL PROPERTIES EVOLUTION OF GRAVITY FLOW TIGHT RESERVOIR OF YANCHANG FORMATION IN THE SOUTHERN ORDOS BASIN

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