Petrology and isotopic geochemistry of dawsonite-bearing sandstones in Hailaer basin, northeastern China

摘要

The CO2 gas reservoir sandstones in the Hailaer Basin contain abundant dawsonite and provide an ideal laboratory to Study whether any genetic relationship exists between dawsonite and the modern gas phase of CO2. The origins of dawsonite and CO2 in these sandstones were studied by petrographic and isotopic analysis. According to the paragenetic sequence of the sandstones, dawsonite grew later than CO2 charging at 110-85 Ma. The dawsonite delta O-18 value is 7.4%. (SMOW), and the calculated delta O-18 values of the water present during dawsonite growth are from -11.4 parts per thousand to -9.2 parts per thousand (SMOW). This, combined with the NaHCO3-dominated water linked to dawsonite growth, suggests meteoric water being responsible for dawsonite growth. The delta C-13 values of gas phase CO2 and the ratios of He-3/He-4 of the associated He suggest a mantle magmatic origin of CO2-rich natural gas in Hailaer basin. Dawsonite delta C-13 values are -5.3,76 to -1.5 parts per thousand (average -3.4 parts per thousand), and the calculated delta C-13 values of CO2 gas in isotopic equilibrium with dawsonite are -11.4 parts per thousand to -73 parts per thousand. These C isotopic values are ambiguous for the dawsonite C source. From the geological context, the timing of events, together with formation water conditions for dawsonite growth, dawsonite possibly grew in meteoric-derived water, atmospherically-derived Co-2 maybe, or at least the dominant, C source for dawsonite. It seems that there are few relationships between dawsonite and the modern gas phase of CO2 in the Hailaer basin.