Thermal Convection Diagenesis Of Reservoir Sandstone Of Magmatic CO₂ Pool

Thermal convection diagenesis is a geological process between traditional burial diagenesis and traditional contact metamorphism. It belongs to the scientific field of basin fluids and diagenesis in sedimentary petrology, and is one of topics of geological frontier. The CO₂ gas reservoir sandstone of Wuerxun depression in Hailaer basin, abundant in dawsonite, is an ideal rock type to study the thermal convection diagenesis. By integrating the multi-discipline methods of petrology, stable isotopic geochemistry, radioactive isotopic geochemistry and fluid inclusion geology, on the basis of research on petrology characteristics, diagenetic fluid and its evolvement of dawsonite-bearing sandstone, more attention has been paid to study the coupling relationships among magma emplacement, CO₂ thermal fluid activity and sandstone diagenesis in the way of component and timing. The purpose of the study is to search the substance contribution and long-distance control action of deep geological process or magma activity on sandstone diagenesis, and to provide significant theoretical evidence for reservoir evaluation and anisotropic prediction of sandstone in magmatic CO₂ pool.The research on petrology characteristics indicates that the dawsonite-bearing sandstone of Wuerxun depression in Hailaer basin comprise of arkose and lithic arkose. Their diagenetic paragenesis includes six discrete diagenetic processes. From earliest to latest, the diagenetic processes are:(1) clay mineral coatings; (2) quartz overgrowth, authigenic quartz, authigenic kaolinite; (3) petroleum charging;(4) CO₂ charging; (5) dawsonite; (6) ankerite. The dawsonite and ankerite formed after CO₂ charging. Based on the diagenetic paragenesis above, according to the tracing characteristic of dawsonite, palaeofluid information from fluid inclusions in cements or authigenic minerals and geochemical characteristics of present groundwater, diagenetic fluid evolvement of dawsonite-bearing sandstone is classified into four stages including (1) alkali diagenetic fluid before petroleum charging, (2) acid diagenetic fluid during petroleum charging; (3) diagenetic fluid varying from acid to alkaline during CO₂ charging and (4) alkali diagenetic fluid from CO₂ charging to the present. The CO₂ charging played a very important role in changing the chemical composition of groundwater, which is characterized by its extremely high degree of mineralization with absolute proportion of Na+ and HCO3-, also with higher amount of CO32-.Dawsoniteδ13C values range from -5.3 to -1.5‰PDB. The calculatedδ13C values of CO₂ gas in isotope equilibrium with dawsonite range from -11.4 to -7.3‰PDB, in coincidence withδ13C CO₂ values(-11.36～-8.2‰PDB) from CO₂ gas pool in Hailaer Basin. These values are mainly within the range of inorganic CO₂. Dawsoniteδ18O values range from 5.8 to 10.8‰SMOW, in coincidence with the range of dawsoniteδ18O values with magmatic origin reported both at home and abroad, also show the characteristics of carbonate oxygen isotope with magmatic origin. According to the fact that 3He/4He values are from 1.68×10–6 to 2.08×10–6, R/Ra values are 1.20 to 1.49 of associated helium with CO₂ gas, and that both dawsonite and CO₂ gas pool lie in the vicinity of magmatic rocks and the deep fault, CO₂ both in gas pool and dawsonite almost derived from the mantle-magmatic origin. However, the calculated initial 87Sr/86Sr values of dawsonite and ankerite are respectively 0.708407～0.712720 and 0.706337～0.710808, indicating sialic crust origin. Considering that the trace element of dawsonite and ankerite implied crust origin, the REE contents of dawsonite and ankerite are similar with mantle-magmatic carbonate, while the chondrite-normalized REE patterns are consistent with crust rock, it is concluded that dawsonite and the paragenetic mineral ankerite from Wuerxun depression in Hailaer basin are the results of interaction of mantle-magmatic CO₂ and sandstone in crust.Research on fluid inclusions and K-Ar ages of authigenic illite indicates that petroleum charging in dawsonite-bearing sandstone began at about 120Ma, reached climax during 100 to 90Ma. CO₂ charging was later than petroleum charging in large scale, so later than 90Ma, in accord with the age of allgovite (46.2Ma). Therefore, there exists perfect coupling relationship among magma emplacement, CO₂ fluid activity and sandstone diagenesis (or dawsonite forming) not only in components but also in the time. Dawsonite occurring in Wuerxun depression, Hailaer basin, is the tracer mineral on migration, accumulation, and leakage of mantle-magmatic CO₂. In addition, it was for the first time that petroleum inclusions were found in dawsonite cement, and CO₂ gas bottom was discovered in the wells with abundant dawsonite such as Su16, Su3 and Su2 etc. These may prove that the oil displacement by CO₂ occurred in Wuerxun depression, and the dawsonite-bearing sandstone preserved the evidence of oil displaced by CO₂.Integrated study on reflectance of vitrinite of the dawsonite-bearing stratum, homogeneous temperature of fluid inclusions in ankerite, and apatite fission track shows that the CO₂ gas causing dawsonite precipitation acted as thermal fluid. The dawsonite-bearing sandstone diagenesis was characterized by vertically forced thermal convection. Based on research above, forced thermal convection diagenetic pattern of reservoir sandstone of magmatic CO₂ pool is established. The CO₂ thermal fluid from magmatic degasification migrated into Nantun Formation sandstone along the Wuxi fracture and its branches. One part of magmatic CO₂ displaced the previous petroleum and then preserved in sandstone as gas phase; another part of magmatic CO₂ participated in the diagenesis of sandstone. Firstly, CO₂ dissolves in water and reacts with water by gas-water boundary to produce weak carbonic acid. Then carbonic acid dissociates rapidly into bicarbonate ion and hydrogen ion. The increasing acidity induces dissolution of many unstable silicate minerals (mostly feldspar) in the primary host rock. With the transformation of pore fluid, the dissolved bicarbonate ion reacts with such cations as Na+ and Al3+ from dissolved feldspar to precipitate dawsonite, and the bicarbonate ion reacts with Ca2+ and Mg2+ to precipitate ankerite. Therefore, CO₂ is solidified in sandstone as secondary carbonate minerals.In theory, research on the thermal convection diagenesis and its pattern of reservoir sandstone of magmatic CO₂ pool has added to investigation of mass transformation relationship of deep geological process and shallow geological process. Moreover, it has revealed the interactional rules between deep fluid and shallow sandstone, has widen new research field of basin fluids and diagenesis. In fact, although CO₂ charging may result in dissolution of some framework grains in sandstones, CO₂ charging leads to porosity decreasing as a whole due to the new mineral formation. Therefore, it is of great significance to research on the thermal convection diagenesis and establishing its pattern of reservoir sandstone of magmatic CO₂ pool in Wuerxun depression, Hailaer basin. Conclusions drawn in this paper will play an important role in predicting anisotropic distribution of reservoir and farther directing reservoir quality forecast and evaluation.