The Formation Mechanism Of Dawsonite Veins In Sandstone Of Southern Songliao Basin

CO₂ is greenhouse gas,it can lead to global warming.CO₂ capture and storage technology（Carbon Capture and Storage: CCS）can effectively solve this problem.The geological storage of CO₂ is the most important technical means of this method.The cracks caused by geological tectonic movement cannot be ignored for the safety and stability of CO₂ geological storage.In the natural geological environment,CO₂ gas reservoirs or traps CO₂ had existed can be called "natural analogues" or natural analogy.It has has important implications for the safety of storage of geological CO₂（CGS）.Dawsonite is the main carbon mineral for CO₂ geological storage.Sandstone with dawsonite bearing veins were found in Quantou Formation forth of Xin 351 well in Fuxin uplift,southern Songliao Basin of.This is of great significance for supercritical CO₂ self-sealing and effect of CO₂ geological storage.The rock type of the Xin 351 well is fine-grained feldspar lithic sandstone.The depth of veins is 1295-1300 m.Take the core of 1295.9m as the object of study.Hand specimens were observed,dawsonite aggregates were white,radial,hemispherical,half-filling or complete-filling.It attached to the wall of the crack and grew inwards,to the wide part of the crack dawsonite is half-filling,to the narrow part for completely-filling.X-ray diffraction analysis shows that the authigenic minerals of surrounding rock are clay minerals（mainly illite smectite）,authigenic quartz（including quartz overgrowth and microcrystalline quartz）,dawsonite and ankerite.From early to late,the sequence of diagenetic symbiosis in the wall rock is: percolating clay,authigenic quartz,dawsonite,and ankerite.The carbon and oxygen isotopic analyses show that the carbon isotope in the fractures is between 0.3‰ and-0.2 ‰（PDB）.The d13C values of dawsonite in the surrounding rock is between-3.41‰ and-2.16 ‰（PDB）.The d13C values of ankerite is between-4.62‰ and-4.18 ‰（PDB）.By calculation,the CO₂ carbon isotope ranges between-7.50‰ and-8.74‰（PDB）.CO₂ derived from an inorganic mantle source.The numerical simulation results show that CO₂ would preferentially seize the fracture,change the fluid environment,and more severe water rock reaction would occur.Specific performance: during injection time,the concentration of supercritical CO₂ in crack was the highest,p H decreased rapidly.Mineral mainly dissolved are albite and K-feldspar,dissolved quantity of cracks was significantly higher than that of surrounding rock.Dawsonite is the main mineral as deposit,precipitation of cracks was significantly higher than that of surrounding rock.Secondly,because of the obstruction of mudstone,the boundary of sand and mudstone is also the high value area of feldspar dissolution and dawsonite precipitation.There are three kinds of dawsonite in the Xin 351 well: 1,fill the fractures;2,fill the pore of wall rock;3.metasomatic feldspar and debris.The forming process of the crack filling dawsonite are: crack formed earlier than the injection of CO₂;crack is water-saturated;wet supercritical CO₂ fill the fractured,dissolve and enlarge the fissure;due to dissolution,ions near the fractured wall concentrate,and form the crystal nucleus of dawsonite,reaction water in crack is sufficient,continuous injection of supercritical CO₂ in open environment ensure the dissolution and ions supply.Basised on the calculation of d18O and the formation temperature of microcrystalline quartz,it is found that the formation temperature of dawsonite is 73-85℃.In the wall rock,dawsonite is either filling pores or replacing feldspar.From the distribution of the content of dawsonite,the influence of CO₂ injection is even larger.Dawsonite is a hydrothermal crystalline mineral.it also shows that under the condition of sufficient formation water,supercritical CO₂ can be isolated as mineral trapping and achieve self-sealing.The surrounding rock contains a large number of mafic debris,and its dissolution brings ultramafic ions,which can be the source for the formation of ankerite.