Study on the origin of so-called mantle-derived gas in Songliao Basin--Taking the Changling fault depression as an example

• 1、School of Management Science & Engineering,Baise University,Baise Guangxi,533000

Abstract：There is abundant natural gas in the lower Cretaceous of Mesozoic in Changling County fault depression, southern Songliao Basin. In some wells, the carbon isotope of alkane is very heavy (for example, δ13CCH4 is between-20.8‰ ~-23.41‰) , the content of carbon dioxide is high (between 14.0% ~ 98.5%) and the carbon isotope is very heavy (δ13CCO2 is between-1.8‰ ~-11.4‰ , most of them are more than-8.0‰) .These abnormal natural gas has been considered as "MANTLE-DERIVED gas" . We considered that there is neither theoretical basis nor evidence for the view of "mantle-derived gas" , especially illogical to speculate the origin of carbon dioxide and hydrocarbon by using helium. By means of field geological survey, core observation, geophysics and geochemistry analysis, We found that a large number of intrusive rocks developed in the source rocks (including dark mudstone and carbonate rock) of the Upper Paleozoic in the study area, the thermal metamorphism of source rocks near intrusive rocks is strong and oil and gas show. Based on the theory of magmatic evolution, isotope fractionation and thermal simulation experiments, We determined that the so-called "mantle-derived gas" was mainly generated by thermal metamorphism of organic matter and carbonate minerals in the Upper Paleozoic source rocks caused by high temperature of intrusive rocks, and migrated along the fault to the Mesozoic.?????

Keywords： Songliao Basin mantle-derived Gas Carbon Isotope magmatic activity