Activities of hydrocarbon bearing hydrothermal fluids in South Yellow Sea Basin,China:Evidence from fluid inclusions

The anomalies of geothermal and geochemical parameters of Palaeozoic formations at Wunansha uplift in South Yellow Sea Basin were concerned by many geologists and geochcmists.We have carefully studied the fluid inclusions hosted in quartz veins of Permian limestone from Qixia group in well CZ35-2-1 by combination of micro-thermometry and Raman spectroscopy in this paper.The fluid inclusions were divided into three types including six subtypes according to their petrological characteristics,phase state at room temperature and compositions.The results of Raman spectroscopy give strong information of methane and hydrocarbon in fluid inclusions.It powerfully proves that hydrocarbon bearing ever acted in researched area.The homogenization temperatures of fluid inclusions are much higher than the temperature of normal basin,which indicates that the hydrothermal fluids were trapped in fluid inclusions.There might be three times of activities of hydrocarbon bearing hydrothermal fluids which were distinguished by petrological characteristics and homogenization temperatures.The concentration of methane in the fluid inclusion increased with the homogenization temperature,from 0.1347mol/L at 214℃,0.1722mol/L at 265℃,to 0.3370mol/L at 304.5℃ respectively.The experimental results give reasonable explanation for the abnormality of heat and geochemical parameters in researched area.It means that all anomalies of geochemistry were caused by hydrocarbon bearing hydrothermal fluids.