Study On The Relationship Between Reservoir Evolution And Oil Accumulation Process Of Chang 8 Tight Sandtone Oil Reservoir In Jiyuan Area, Ordos Basin

The Chang 8 reservoir of Yanchang Formation in the Jiyuan area Ordos basin is typically tight sandstone oil reservoir. It has a certain theoretical and practical significance to clarify the relationship between reservoir evolution and oil accumulation process for understanding the oil enrichment regularity and guiding the exploration and development in this area.Based on core observation, combined with rock thin section, casting thin sections, scanning electron microscopic, grain size analysis, X-ray diffraction, constant-pressure mercury intrusion, constant-speed mercury intrusion, electron microprobe, carbon and oxygen isotopes and oil inclusions, the paper made thorough research on reservoir characteristics, diagenesis characteristics, the relationship between reservoir evolution and oil accumulation process. The paper also revealed the main control factors of relatively favorable reservoirs, set up a comprehensive evaluation standard of tight sandstone reservoir in the research area, and made a comprehensive evaluation and prediction on reservoir.The research showed that the main type of the sandstone is debris-feldspar and feldspar-debris sandstone, the amount of interstitial material is high, compositional maturity of the reservoir is low while textural maturity of the reservoir is middle. The physical property of the reservoir is totally low porosity and ultra-low permeability, the average porosity is 8.67% and average permeability is 0.46×10-3μm2. The reservoir pore structure can be divided into four categories, and the dominant pore structure is low displacement pressure and small pore-throat type. The distribution of the pore radius mainly ranges from 80μm to 250μm, and it belongs to large scale pore. With the increase of permeability, the distribution of throat radius ranges become wider and the high value of pore throat radius ratio reduces. The sorting coefficient, maximum mercury saturation, pore and throat mercury saturation have a positive correlation with reservoir porosity and permeability, while structure coefficient and physical property have a negative relationship.The types of the diagenesis of Chang 8 reservoir in Jiyuan area is rich and complex, it mainly includes compaction, carbonate cementation and dissolution. Diagenetic evolution is generally up to stage A of the middle diagenetic stage. Quantitative calculation of visual compaction ratio is 0.42 on average, while visual cementation ratio is 0.58, the intensity of compaction and carbonate cementation is medium to strong. Average visual dissolution ratio is 0.34, the intensity of dissolution is medium. Without considering the abnormal pressure, the upper limit of the throat critical radius is 0.592μm when buoyancy and capillary pressure is equal, and it determines the upper porosity of tight sandstone reservoir was 12%. The quantitative calculation of pore evolution shows that the distribution of residual porosity rang from 19.7% to 25.56% after compaction and porosity is generally less than 12% after the cementation. Compaction causes a substantial reduction in primary pore. However, the late cementation is the primary cause of Chang 8 reservoir to become tight sandstone reservoir in the research area.The research on Chang 8 reservoir tightening evolution and oil accumulation process shows that after middle-Jurassic low-mature oil began to form gradually. In the early period of Early Cretaceous, hydrocarbon accumulation was small, while in the middle stage of Early Cretaceous, hydrocarbon filling and accumulation became larger. Reservoir tightening and large-scale hydrocarbon accumulation are synchronous.On the whole background of low permeability and low porosity tight sandstone reservoirs, favorable reservoirs still develop relatively, and their dominated factors are deposition, diagentic reservoir facies and tectonic fractures. On the basis of traditional reservoir evaluation, the paper seclected new parameters which reflected the pore structure features of tight sandstone reservoirs, such as displacement pressure pore texture types and throat radius, and also reveald the distribution of the relatively favorable reservoirs in the research area. The result of reservoir evaluation can guide the following exploration and development.