Diagenesis and Diagenetic Lithofacies of Tight Reservoir of Chang4+5 Member of Yanchang Formation in Zhenbei, Ordos Basin

Key words: 
• relatively high porosity and permeability reservoir /
• diagenesis /
• diagenetic lithofacies /
• Ordos Basin

Abstract: Though the prospective of oil reserves of tight reservoir of Chang 4+5 reservoir of Yanchang Formation, which contains tuffaceous component, is great in Zhenbei area, Ordos Basin, but the mechanism and distribution of relatively high porosity and permeability is not clear causing the speed of oil exploration and development slowly. Diagenesis and favorable diagenetic facies of Chang 4+5 Member of Yanchang Formation are researched by casting slice, scanning electron microscope, electron probe microanalysis, cathodeluminescence, micro fluorescence, X-ray diffraction and fluorescence, fluid inclusion, high pressure mercury, and physical properties technique. The results show that the quartz overgrowths of siliceous cementation and iron dolomite of carbonate cementation caused the background of low porosity and ultra low permeability reservoir of Chang 4+5 reservoir in the study area, and the diagenetic stage has reached the "A" middle now. Chlorite film cementation and early oil accumulation effectively protected pores, and limited the mechanical compaction and cementation. The intense dissolution of feldspar components could greatly improved the porosity and caused the relatively high porosity and permeability reservoir. There are mainly divided into four kinds of diagenetic facies, including mechanical compaction facies, carbonate cementation facies, chlorite cementation facies and feldspathic solution facies. The most favorable diagenetic facies is feldspathic solution facies, which mainly distributes in the middle sandbody of the underwater distributary channel, and it is favorable for the formation of relatively high porosity and permeability reservoir, which surface porosity is more than 6.0%, porosity is more than 14.0%, permeability is more than 1.30×10^-3 μm². Comprehensive studies have shown that the combination of sedimentation and diagenesis control the distribution of relatively high porosity and permeability reservoir.