Characteristics Of Diagenesis And Diagenesis-gas Accumulation For Upper Paleozoic Tight Reservoir In Eastern Ordos Basin

According to core observation, analysis of casting thin sections and conventional thin sections, scanning electron microscope, conventional mercury injection testing, rate-controlled mercury injection testing, core NMR testing, illite K-Ar aging, temperature and component detection of fluid inclusion and other modern testing method, combined burial and thermal evolution data of Ordos Basin, this paper made a further research on the diagenesis and diagenetic facies of Upper Paleozoic tight sand reservoirs of Eastern Ordos Basin, and predicated the physical upper and lower limit of tight reservoirs on the basis of geological characteristics of gas accumulation. Besides, based on analysis of gas injection periods and rebuilding of pore tighten process, it also can be inferred the correlation between diagenesis and accumulation of Upper Paleozoic tight sand reservoirs with different lithology.The results indicate that multi-diagenesis develop in tight reservoirs, which experienced long and complex diagenesis evolution, and up to stage B of middle diagenesis. Furthermore, diagenesis of tight sand reservoirs is uniquely characterized by grain size, sorting degree, mineral component and pore-throat size, etc. According to diagenesis and pore combination, diagenetic facies can be divided into three types, i.e. facie of siliceous cement-dissolved pores, facie of mixed cement-dissolved pores and tight mixed facie, besides, gas testing of different diagenetic facies were still analyzed.With empirical approach, found general acceptance, this paper made a research on the reservoir limit of tight gas. Based force state analyzing of gas in micro-pore-throat, a linear regressive correlation between pore-throat radius and physical attributes can be built, and then physical limit also can be calculated by corresponding pore-throat radius in different forceOn the basis of primary porosity restoration, porosity losing, caused by compaction, can be rebuilt, and then porosity losing, caused by different cementation, can also be calculated interms of cement formation sequence and content. Similarly, porosity augment, caused by dissolution, can be calculated so. Coupling analysis was accomplished to the effect between gas injection period and tighten period, defined by calculated limit of tight reservoi, and shown that reservoir tighten period of lithic-quartz sand reservoir, developed in He8Section and Taiyuan Formation, is prior to gas accumulation. However, for quartz sand reservoir of Shan2Section, reservoir tightening is almost simultaneous with gas accumulation.Detailed study on accumulation conditions of Upper Paleozoic gas, Eastern Ordos Basin, was carried out, which focuses on the gas-generation intensity, seal closing state and reservoir tight difference. The results show that, with favorable gas generation condition, three-dimensional arrangement, composed by reserve with different diagenesis evolution and mud stone, controls gas accumulation, which lead to "quasi-continuous" reservoir, characterized by bordering or overlaying distribution.