The chemical and isotopic compositions of volatiles in magmatic hydrothermal fluids beneath the Songliao Basin, northeastern China

Abstract

It is controversial whether deep fluids can make contributions to natural gas accumulations in the Songliao Basin, NE China. The occurrence of magmatic hydrothermal quartz veins in crystalline basement of the basin provides a way to understand the characteristics of deep fluids and their significance for natural gas accumulation. Three types of fluid inclusions were identified in hydrothermal quartz: aqueous inclusions, H2O-CO2-rich inclusions and H2O-CO2-CH4-rich inclusions. The primary inclusions show high homogenization temperatures (280–433°C) which are typical of magmatic hydrothermal fluids. The oxygen and hydrogen isotopic compositions (δ18O and δD) of water are 2.0‰ to 4.6‰ and −91.6‰ to −75.7‰, respectively. The OH isotopic compositions suggest that hydrothermal fluids associated with the quartz veins were derived from the primary magmatic system and mixed by small amounts of meteoric water.Two types of magmatic hydrothermal volatiles were identified in the study. Both types of volatiles are dominated by CO2. CO2 in type I volatiles shows more negative δ13C values (−13.8‰ to −9.7‰) than typical magmatic CO2, which likely resulted from carbon isotopic fractionation during magma degassing. Type II volatiles have higher δ13C values (−5.5‰ to −3.3‰) and are similar to typical magmatic CO2. Small amounts of hydrocarbons were observed in both types of magmatic hydrothermal fluids. Hydrocarbons in type I volatiles have relatively high δ13CCH4 values (−30.6‰ to −24.1‰) and reversed carbon isotopic trends while hydrocarbons in type II volatiles have more negative δ13CCH4 values (−49.7‰ to −37.6‰) and orderly carbon isotopic trends. CO2 in hydrothermal fluids is a potential source for CO2 gas accumulations. Hydrocarbons in hydrothermal fluids, however, can hardly make significant contributions to the commercial gas reservoirs due to their low concentration.