| The Tamtsag basin, located in eastern Mongolia, is one of a widespread basin system which formed during Late Jurassic and Cretaceous period. The basement of the Tamtsag basin is composed of metamorphic rocks and sedimentary rocks of pre-Paleozoic, Paleozoic, and Triassic, as well as intrusive rocks of Jurassic system. The basin fillings consist of mainly terrigenous detrital rocks of Jurassic, Cretaceous, Tertiary and Quaternary system. The dawsonite-bearing pyroclastic rock occurred in Tongbomiao formation of the Early Cretaceous of Tanan depression in Tamtsag basin, which has been considered as a natural analogue of the CO2-pyroclastic rock interaction.By integrating the multi-discipline methods of petrology, stable isotopic geochemistry, CO2 fluid-pyroclastic rock interaction experiments and geochemical simulation, on the basis of research on CO2 fluid-rock interaction in geological record, the dissolution of pyroclastic rock and the CO2 mineral trapping of it. I have clarified the characteristics of the CO2-pyroclastic rock interaction and revealed the mechanism of CO2 mineral trapping, and evaluated the capability of CO2 mineral trapping in pyroclastic rock, which can provide basic geological information for CO2 geological storage and research of pyroclastic rock oil and gas reservoirs. Dawsonite has been recognized in tuff, sedimentary tuff and tuffaceous sandstone, especially in tuff as a cement and replacement mineral. Besides dawsonite, the cements in dawsonite-bearing pyroclastic rocks include calcite, microcrystalline quartz, kaolinite, ankerite and clay minerals. Their diagenetic paragenesis includes 7 ordinal diagenetic processes. From earliest to latest, the diagenetic processes are:(a) clay coating formation, (b) calcite formation (c) early generation microcrystalline quartz formation, (d) early generation kaolinite formation, (e) dawsonite formation, late generation microcrystalline quartz formation, late generation kaolinite formation and (f) ankerite formation.The stable isotope composition of dawsonite and ankerite are investigated by MAT251, MAT252 and MAT253 stable isotope mass spectrometer, which have been used to investigate the origin of cleat dawsonite and ankerite in the Lower Cretaceous Tongbomiao formation pyroclastic rocks in the Tamtsag Basin, northeast Mongolia. Dawsonite 813C values (-8.15‰～1.3‰) and the calculated values of CO2 gas in isotopic equilibrium with dawsonite (-13.68%o～-3.83%o) are similar to the values in Hailaer basin and BGS basins suggests that CO2 for forming dawsonite almost belong to mantle origin-magma. Ankeriteδ13C values range form-7.29%o to-0.99‰, which are similar to dawsonite, combined with related geological and geochemical simulation examples, suggests that CO2 for forming ankerite also belong to mantle origin-magma.Hydro-thermal experiments on CO2-H2O-pyroclastic rock and CO2-H2O-basalt have been carried out to be as supplementary study for petrological observations. CO2-H2O-pyroclastic rock interaction experiments show that feldspar, pyroclastic materials and chlorite are dissolved in different degrees,and microcrystalline quartz, zeolite and carbonate precipitated. CO2-H2O-basalt interaction experiments show that feldspar and pyroxene are dissolved in different degrees, and contemporarily boehmite,kaolinite and some carbonate minerals containing siderite, calcite and dolomite formed. The two experiments verified silicate minerals and feldspar are easily dissolved in CO2 fluid which can supply ion for the formation of carbonate.The difference between petrology observation and experiment on the formation of carbonate is having no dawsonite precipitation in experiment, the reason may be the formation of dawsonite require a longer reaction time which is difficult to measure in experiment.Long-term CO2-pyroclastic rock process of interaction has been simulated.Numerical simulation of CO2-pyroclastic rock interaction reveals that after 3200 years, under CO2 injection conditions, K-feldspar, plagioclase and chlorite have been hardly dissolved, and some ion released. After CO2 injection, carbonate minerals include dawsonite, calcite and siderite have formed after 10000 years, the volume fraction of which are3.3%,2.03%and 0.28%respectively, and ankerite is rare. And non-carbonate minerals include albite, Na-montmorillonite, Ca-montmorillonite, illite, microcrystalline quartz and kaolinite formed. Albite volume is up to 15.62%,volume of newly formed illite, kaolinite and quartz isl.227%,3.066%and 2.85%respectively, sodium montmorillonite is reach 4.091%and calcium montmorillonite is rare.Through comparing petrology study, I have the identified authigenic minerals formed after mantle-magmatic CO2 flooding contain authigenic quartz, kaolinite, dawsonite and ankerite.CO2-pyroclastic rock interaction experiment have shown that injection of CO2 leading to the precipitition of microcrystalline quartz, zeolite, calcite and dolomite. Numerical simulation of CO2-pyroclastic rock interaction have confirmed that authigenic minerals formed after CO2 injection includes kaolinite, quartz, siderite, calcite, dawsonite, ankerite, illite and montmorillonite. The conclusions derived from these three research approaches are broadly consistent.The CO2 storage capacity of pyroclastic rocks has been calculated through natural analogs and simulation. CO2 mass captured by natural analogs in pyroclastic rocks can reach 75.8 kg per stere pyroclastic rock. Numerical simulation showed that CO2 that may be sequestered by precipitation of dawsonite, calcite, siderite, and ankerite. After 10 years CO2 mass captured islkg per stere,100 years later is about 3.56kg per stere, after 1000 years is 30 kg per stere, after 10000 years CO2 mass captured is up to 63kg per stere pyroclastic rock, which is coincident with CO2 captured by natural analogs.Although CO2 charge leads to porosity and permeability increasing, porosity and permeability in vicinity of CO2 injection are relatively high. Numerical simulation of CO2-pyroclastic rock interaction shows after 10,000 years, the porosity down from 10%to 6.957%and permeability from the 4E-15 m2 reduced to 1.347E-15m2. Porosity and permeability is proportional with the radial distance, away from the CO2 injection point, porosity and permeability decreases in large scale. This means porosity and permeability of the reservoir near the CO2 injection are relatively high which is favorable areas for finding high quality reservoirs.
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