In-reservoir mixing of mantle-derived CO2 and metasedimentary CH4–N2 fluids – Noble gas and stable isotope study of two multistacked fields (Pannonian Basin System, W-Hungary)

Several multistacked gas fields, located in very different parts of Hungary, display a large variation in their gas composition. The shallow position of the mantle beneath the basement of the PBS is responsible for the high heat flux. It facilitates that the CO2 – intensely released from the ascending upper-mantle derived melts – could reach the basin system. In this work the results of a geochemical study of the main components and noble gases of the multistacked Répcelak (CO2–HC–N2) and Mihályi (CO2) fields are presented and discussed. The vertical variations of gas composition in the Répcelak and Mihályi fields fit well with in-reservoir mixing of two end-members, a mantle-related magmatic CO2 fluid and a N2-rich HC-gas fluid of crustal origin. However, it is very likely that other processes modified to some extent the gas composition of the two fields. Carbon isotopic composition of the CO2 of the samples studied agrees with both the carbonate decomposition and mantle degassing origins. It is very likely that the marked difference in helium isotopic composition between the neighbouring Répcelak and Mihályi fields also reflects this heterogeneity.