The Evolution Of Diagenetic Fluid In Dawsonite-bearing Sandstone, Changling Depression-Huazijing Step, Southern Part Of Songliao Basin

Abundant of CO2 occurred in southern part of Songliao Basin, and as reported, most of the CO2 gas reservoirs distributed in Qian'an, Gudian and so on. As an active gas, CO2 can dissolve into water and form the acid fluid. Theoretically, ankerite, siderite and dawsonite can all become the effective products with the function of CO2-H2O-sandstone interaction, but recently, geological observation, synthetic experiment and numerical simulation are inclined to that dawsonite would make more sense. Numerous evidences show that, in the background with CO2 gas reservoir, as the trace mineral, dawsonite can make record on the migration, accumulation and seepage of CO2.Nowadays, abundant of dawsonite was found in Changling Depression- Huazijing Step, southern part of Songliao Basin, which producing an ideal natural laboratory for doing research on fluid evolution in daswonite-bearing sandstone. Combining with petrology, stable isotopic analysis and fluid inclusion methods, the petrologic characteristic and genesis of CO2 for forming the dawsonite are studied; then we focus on the coupling relationship among volcanic activity, CO2 fluid and diagenesis of sandstone in the material contents and time scales; at last the evolution of diagenetic fluid in dawsonite-bearing sandstone is investigated.The diagenesis sequence for dawsonite-bearing sandstone in research area are as follows: illite and smectite mixed layer, micrite calcite, authigenic kaolinite, quartz overgrowth, late calcite, illite, dawsonite and ankerite. Among them, dawsonite and ankerite are the authigenic mineral combination which formed after CO2 injection. In the late CO2 injection period, the weak acid fluid turns to be alkaline, and if the pressure of CO2 in porosity is higher, dawsonite formed, otherwise, the authigenic mineral is ankerite. These specific mineral combinations can keep track for transformation, aggression and seepage of CO2, so they can make contribute to the research on CO2 gas reservoir.Dawsoniteδ13C values are -5.3‰～1.13‰, and the calculated values of CO2 gas in isotopic equilibrium with dawsonite are -4.55‰～-11.72‰, meanwhile,δ13C values of the accompanying CO2 in Qian'an and Honggang are -5.32‰～-6.76‰, so these calculated data is coincided with the values of free gas. Moreover, these data appears as an inorganic CO2 source, especially in the range for mantle- magmatic CO2 (-8‰～-2‰). The ratios of 3He/4He(R) of natural gas samples range from (3.16±0.09)×10-6 to (6.94±0.20)×10-6,with the R/Ra of 2.26～4.96, showing a mantle source characteristics. Combining with the truth that there is a good relationship between dawsonite-bearing sandstone and CO2, and both of them are controlled by basement faults, a same carbon source for forming dawsonite and free gas phase CO2 in middle-shallow layers can be suggested, and this kind of carbon source seems to belong to mantle-magmatic origin mixing with some organic CO2.Sequencing and timing are two new methods can be used to estimate the infilling time of CO2 which forming the dawsonite in Changling depression- Huazijing step.Through fluid inclusions and diagenesis sequence studies, two terms of oil&gas infilling were determined, and dawsonite was found developing between these two infilling stages. The analysis of isotopic dating of autogenetic illite shows that the time for hydrocarbon injection is 92.5～58.8Ma, which is coincide with the first phase inclusions (developing in quartz overgrowth). The other stage of inclusions mainly developed in calcite cements, microfissure which cutting quartz debris and overthgrowths. Through homogenization temperature and buried&thermal history studies, the second phase of hydrocarbon injection was calculated to be between 37 and 20Ma. Then the filling time for CO2 which inducing the dawsonite genesis can be fixed to be 58.8～37Ma. After a series of research on the volcanic activities after late cretaceous, it is found that there is a great time coincidence among Shuangliao volcanic activity, inorganic CO2 infilling and genesis of dawsonite. The fluid inclusions data showed that abundant of CO2-melt inclusions were caught in alkaline olivine basalt by both phenocryst and xenoliths, Shuangliao volcanic cluster. Therefore, through the analysis on activities time and materials of the volcano, the volcanic groups in Shuangliao and the intrusion of heat flow diaper bodies are confirm to play the predominant roles on the CO2 source for forming dawsonite.The fluid evolution can be divided into three stages: pre-CO2 (illite&smectite mixed layer and micrite calcite step, kaolinite and quartz overgrowth step, late calcite and illite step ), dual infilling by CO2 & hydrocarbon and nowadays fluid. Once the formation water injected into CO2, its geochemical content would be change, such as appearing as high salinity and rich in Na+ and HCO3-. So the whole evolution in Changling depression- Huazijing step is a complex process accompanying continuous fluid infilling. The occurrence of dawsonite can be divided into two types, continuing growth and termination, depending on whether there is aqueous fluid in dawsonite-bearing areas. The former type means that under the dawsonite accompanying with fluid background, with the proper condition (CO2 pressure, pH, Na+,Al3+and so on), dawsonite will keep on growing; while in termination type, the amount of dawsonite can not be increased as there is no aqueous fluid.These previous studies opened a new field on basin fluid and made an advanced research on the migration of the materials in the deep-shallow of the earth or hydrocarbon source rock- reservoir. Through revealing the mechanism for the interaction between complex fluids and sandstones, the basic law for dynamic process of CO2 and oil & gas can be explored.