Effect Of The Injection Of CO₂ On Diagenesis And Physical Properties Of Reservoir In Bayantala Tectonic Zone

Bayantala tectonic zone located between Wuerxun sag and Beier sag. There is high positive reserves and huge resouce potence in this district. The main reservoir is Dawsonite-bearing sandstone in Tongbomiao group and Nantun group. This rock type is the most important reservoir of oil and inorganic-origin CO2.In the paper the diagenesis and reservoir character of Dawsonite-bearing sandstone are system studied, then the reservoir physical features are analysed. Therefore not only broaden the research scope in diagenesis study theoretically, moreover provided theory for actual exploration accordingly.Lithological characteristic is the basic of diagenesis study. By appraisive experiment and description of rock sample from Bayantala tectonic zone, the main rock type in the district are lithic feldspathic sandstone and feldspathic lithic sandstone. According to the diagnostic authigenous mineral :Dawsonite,the sandstone is nomenclatured Dawsonite-bearing sandstone. At the meantime, the average content of Dawsonite in Dawsonite-bearing sandstone is less than 5%.By observation through polarizering microscope and scanning electronic microscope,the types of diagenetic of Dawsonite-bearing sandstone in Bayantala tectonic zone consist of mechanical-compaction, agglutination and authigenesis, devitrification and erosion-dissolution. Most of the clastic grain contact with point and line,indicated the level of mechanical-compaction is low and the pores are well conserved.Except Dawsonite the cement and authigenous mineral in the sandstone are quartz overgrowth,authigenic quartz,siderite,other carbonas,authigenic kaolinite and chlorite.Besides,it is usual that Dawsonite replace feldspar and quarz.So is the replacement of carbonate.There is a great deal of illite,kaolinite and a little chlorite and mixed-layer clay mineral also. Erosion-dissolution is the most development diagenesis in the district, especially the erosion of the feldspar.In the paper the foundation used is .In response to the distribution and formation subsequenceof authigenic mineral,the clay mineral composition,the invert between mixed-layer clay mineral,crystallinity of illite,the structure, constitution and the porosity types of rock,organic degree of maturation and so forth.After all the diagenetic stage contains Phase A and Phase B of early diagenetic stage and Phase A in middle diagenetic stage.By distinguishing principle of diagenetic sequence(Petti Jone,1981) and polarizering microscope and scanning electronic microscope,the diagenetic sequence in Bayantala tectonic zone is as follows:the envelope of clay mineral,early siderite,quartz overgrowth,authigenic kaolinite,Dawsonite and carbonate.This can be clasified into 3 diagenetic phase,which are:syndiagenesis-early compaction,agglutination diagenetic phase;dissolution diagenetic phase and late recomposition diagenetic phase.Based on the method of reservoir layer (Dishixiang,1991) and porosity classification(Shamugam,1985),and by the comprehensive study of observation of normal thin section,casting section,scanning electronic microscope and photographic analysis,the porosity of the Dawsonite-bearing sandstones in Bayantala tectonic zone are inducted into the following 4 types:intergranular porosity(protogenesis and remainder),filling porosity,fissured porosity and dissolution porosity.But also the most developed is induced porosity such as feldspar dissolution porosity and fragment dissolution porosity.Through out the study of 22 wells,altogether 1925 porosity statistics in Bayantala tectonic zone,the porosity property is medium-low porosity and low permeability. Integrate the porosity data in casting section,initial porosity resulted from Sneider Plate and the porosity nowadays,the evolution trend of porosity can be recoveried.The trend and the influencing factor represent the control of various diagenesis in different diagenetic phase. Moreover the primary porosity is 32.4% and the induced porosity is 77.6%,so the latter is more developed. At the meantime this phenomenon is congruent with the intensive dissolution diagenesis.Due to the research for the evolution trend of physical features, the diagenesis of the reservoir sandstone in Bayantala tectonic zone control the evolution trend of physical features from beginning to end. Because induced porosity is the main pore in the diatrict and it is formed by erosion-dissolution, erosion-dissolution is the most important influencing factor in the reservoir.It is already known that Dawsonite is an authigenous mineral formed when a mass of CO2 infuse into reservoir. At that time hyperthermia CO2 react with aluminosilicate then catogenic metamorphism occurring product the characteristic cement:Dawsonite. When the content of Dawsonite in the sandstone is less than 5%, the CO2 in the corresponding partial pressure injected form weakly acidic fluid, this fluid chiefly make erosion to the astable clast. Make a comprehensive view :the erosion-dissolution in the district is mainly formed by the acid fluid made form the injected of CO2.So the inject of CO2 is the most ultimate influencing factor. Early burial diagenesis shows the minute effect to the physical features. The sedimentation setting has a little controlling action but inferior to the impression leaved by diagensis. The influence of tectonism behaves supplying a pathway for deep focus CO2.The difference in the porosity caused by hydrocarbon injected is feeble. They can not be the explanation of why the Dawsonite-bearing sandstone has better physical features.