The Coupling Relationship of Reservoir Densification History and Hydrocarbon Emplacement in Tight Sandstone Reservoir: A case study of the Chang 8 Oil Member, Yanchang Formation, southern Ordos Basin

Key words: 
• Ordos Basin /
• oil member /
• diagenesis /
• quantitative /
• porosity evolution

Abstract: The Chang 8 oil member of the Upper Triassic Yanchang Formation in Zhenjing area in the southern Ordos basin is typically a tight sandstone reservoir. Based on the analysis of cast thin sections, cathode-luminescence thin sections, fluorescence thin sections, SEM, XRD and fluid inclusions test etc., the reservoir characters, diagenesis and hydrocarbon emplacement and its phases are studied, the quantitative study on porosity evolution is conducted as well as porosity evolution is recovered with the coupling of diagenesis and hydrocarbon accumulation. These analyses indicate the sandstones are mostly feldspathic litharenites and lithic arkoses with low composition and textural maturity. The reservoir mainly develops carbonate cement, silica cement and authigenic clay minerals. It mainly develops intergranular dissolution pores and residual original intergranular pores. The reservoir is in the middle diagenetic stage B and develops intensive mechanical compaction and cementation but weak dissolution. Mechanical compaction and cementation play an important role in reducing reservoir porosity, resulting in decreases of porosity by 22.2% and 14.6%, respectively. Dissolution and replacement add 22.2% and 14.6% in porosity, respectively. During the early and middle oil emplacement phases, the reservoir is in good properties, while in late oil emplacement phase, the reservoir is in the densification stage. The coupling relationship between reservoir densification history and hydrocarbon emplacement is "densification after hydrocarbon accumulation".