Research On Characteristics Of Chang 9 Oil Reservoir Of The Triassic Yanchang Formation In Zhidan Area, Ordos Basin

Chang 9 oil reservoir is in the situation of low exploration degree and great prospecting potential. Under this situation, we do this study aiming at Chang9 oil reservoir of Yanchang formation in Zhidan area, Ordos basin. Guided by reservoir sedimentology and reservoir geology, applying outcrops observation, core observation, logging data, and multiple advanced testing mesures, we study the reservoir characteristics comprehensively. The reservoir characteristics cover petrology, physical property and heterogeneity, pore type and microscopic pore structure, diagenesis and micro-seepage characteristics. Then we analyze main controlling factors on the reservoir, and summarize the development mechanism of relatively high permeability zone. Based on these study, we evaluate the chang 9 oil reservoir. In the end of the study, we establish oil accumulation mode and predict favorable area for further exploration.Zhidan area developed delta front and shallow lake subfacies in Chang 9 period of Yanchang stage. Sandstones of underwater distributary channel in delta front are distributed along northeast to southwest. Sand bodies are superimposed. The main rock type is lithic feldspar sandstone. Interstitial materials include mainly chlorite, illite, laumonite, carbonate minerals and siliceous minerals. The reservoir properties of Chang 9 are poor with low porosity and permeability. By contrast, reservoir properties of Chang 91 are better than Chang 92. The reservoir has strong heterogeneity.Residual intergranular pore and feldspar dissolution pore are the main pore types in Chang 9 reservoir. High pressure mercury injection experiment shows that the structure of pore and throat has strong heterogeneity. The results of constant-speed mercury penetration experiment show that pore throat structure has the characteristics of large pore, thin throat and inhomogenous distribution. Throat characteristics are the main control factor of the reservoir permeability. The heterogeneity of pore and throat structure is the key factor of reservoir heterogeneity.After experienced four diagenesis which are compaction, cementation, metasomatism and dissolution, Chang 9 reservoir is now in the phase A of middle diagenetic stage. The constructive diagenesis for the reservoir property are mainly chlorite cementation and dissolution. The diagenetic facies of Chang91 and chang92 reservoir in study area are divided into four types respectively.NMR experiment shows that movable fluid saturation of Chang 9 reservoir is low as a whole. According to the movable fluid saturation, the corresponding reservoir category is Ⅱ-Ⅳ. The influence factors of movable fluid saturation are diagenesis and microscopic pore structure characteristics. The oil displacement efficiency can be obtained from the microscopic water flooding experiment. The oil displacement efficiency shows that water flooding is in favour of Chang 9 reservoir. The key factors of oil displacement efficiency is the heterogeneity of the pore structure.Sedimentary microfacies, diagenesis and pore structure characteristics collectively control the characteristics of Chang 9 reservoir in the study area. Factors that controlling relatively high permeability reservoir are sandbody in the underwater distributary channel, relative coarse deposition of local region, chlorite cementation and dissolution. We set up the standard of comprehensive evaluation on Chang 9 reservoir. According the standard, Chang 9 reservoir is divided into four categories. By analyzing the character of source rock and comparing different source rocks, we conclude the oil sources of Chang 9 reservoir. In the southeast of the study area the oil sources are from Chang 9 source rocks. In the other area the oil sources are from Chang 7 source rocks mainly, and from both Chang 7 and Chang 9 source rocks partly. Finally, we select Ⅰ,Ⅱ type and risk favorable area of Chang 91 and Chang 92 reservoir in study area which provide theoretical guidance for further oil exploration.