Study on the relationship between tight sandstone reservoirdiagenetic evolution and hydrocarbon reservoirs filling:A case from the Yanchang Formation,Ordos Basin

Abstract

Diagenetic densification process of dense reservoirs in Yanchang Formation of southwest Ordos Basin since burial has been deeply studied based on a large number of core observation,slice identification,scanning electron microscopy and core physical analysis.The compaction was the main effect of the densification,and the cementation caused the reservoir porosity and permeability to deteriorate further on the basis of compaction.Calcium,siliceous cementation has a significant effect on the porosity reduction,and cementation of illite in clay minerals blocked the throats has a great effect on permeability.According to the changes of detrital minerals and interstitial fillings during diagenesis,the changes of reservoir physical properties were discussed from the microcosmos view.The porosity of each diagenetic period was calculated and the permeability was estimated by curve fitting between porosity and permeability.The reservoir permeability decreased to 0.27×10-3 μm2 at the end of the acidic dissolution stage(mid-diagenetic stage A1),which was lower than the critical dense reservoir permeability of 0.3×10-3 μm2,and at which time the tight reservoir had been formed.Combined with the inclusions test,coupled dense history and buried history of reservoir,it was presumed that a large-scale ripening oil filling had begun before the reservoir reached permeability threshold.After the formation of dense reservoirs,oil and gas filling did not stop immediately.Densification and reservoir-forming should be almost at the same time,and oil accumulation continues after densification.

Keywords

• ordos basin
• tight sandstone
• diagenesis and pore evolution
• dense accumulation sequence