The Study Of Diagenetic Characteristics Of Upper Paleozoic Tight Sandstone Reservoirs In The Central Part Of Tianhuan Depression Of Ordos Basin

The central part of Tianhuan Depression in Ordos basin is a hot area of tight sandstone gas exploration.Upper Paleozoic Xiashihezi Formation and Shanxi Formation are the main target zones of tight sandstone gas exploration.The obejective of the thesis is to explore diagenetic features of the Upper Paleozic in the central part of Tianhuan Depression.Through research methods such as field survey,core observation,conventional thin sections and casting thin sections observation,cathodoluminescence analysis of thin section,scanning electron microscope,mercury injection experiment and clay minerals X-ray diffraction test,petrologic characteristics,pore characteristics and reserving property of tight sandstone reservoir of He 8 member and Shan 1 member in study area are researched deeply,as well as diagenetic characteristics and the influence of diagenesis on reservoir performance.This paper also analyzes the spatial distribution of the favorable diagenetic facies and predicts the favorable zone of gas reservoir.The main results and understanding achieved are as follows:1.The tight sandstone reservoirs of He 8 member and Shan 1 member in study area are mainly lithic quartz sandstone,lithic sandstone and quartz sandstone,with clastic particle composition mainly quartz and rock debris,featuring high content of quartz,relatively high content of rock debris and very low content of feldspar on the whole.The content of the interstitial fillings including matrix and cements varies in a wide range.2.The pores of the tight sandstone reservoirs of He 8 member and Shan 1 member in study area are mainly secondary pores such as dissolution pores and intercrystalline pores,with four categories of pore structures.Lower He 8 member and Upper He 8 member are both low-porosity and low-permeability reservoirs,when Shan 1 member is very-low-porosity and dense reservoir.The petrophysical property of Lower He 8 member is better than Upper He 8 member's while Upper He 8 member's petrophysical property is better than Shan 1 member's.3.The types of destructive diagenesis of the tight sandstone reservoirs of He 8 member and Shan 1 member in study area are mainly compaction,pressure solution,and cemention,while the types of constructive diagenesis are mainly dissolution and fracturing.Carbonate cements and clay mineral cements reduce the reserving ability observably.With the relative percentile of illite and mixed-layer minerals of illite and smectite increasing,the reserving ability of reservoir becomes worse,while with the relative percentile of kaolinite increasing,the reserving ability becomes better.High level of relative percentile of kaolinite in tight sandstone is a potential marker for relatively high-porosity and high-permeability reservoirs.The diagenetic evolution stage is in middle diagenesis stage B.The diagenetic sequence is significantly featured with two phases of dissolution.The secondary pores are formed by dissolution of feldspar,tuffaceous material,intermediate-basic rock debris and carbonate minerals under the corrosion of acid.4.The Tight sandstone reservoirs are categorized into four types of diagenetic facies:High porosity and permeability,weak cemention-feldspar dissolution facies;Relatively high porosity and permeability,kaolinite cementation-feldspar dissolution facies;Low porosity and permeability,strong cemention-weak dissolution facies;Very low porosity and permeability,strong cemention and compaction,dense facies.The distribution area of favorable diagenetic facies including Type ? and Type ? is relatively widest in Lower He 8 member while it is wider in Upper He 8 member than in Shan 1 member.