Methane seepage in an urban development area (Bacau, Romania): origin, extent, and hazard

摘要

The paper describes a case of a natural emission of methane from soil in an urban development area, generating a significant risk for the local population and buildings, due to gas explosiveness and asphyxiation potential. The site is located on the south-western margin of the East-European Platform in eastern Romania, in a hydrocarbon-prone area crossed by the Pericarpathian lineament and regional faults. Molecular composition of gas and stable isotopic analyses of methane (CH_4 > 90%, δ~(13)C_1: -49.4‰, δD_1: -173.4‰) indicate a dominant thermogenic origin, with significant amounts of C_2-C_5 alkanes (～5%), likely migrating through faults from a deep reservoir. Possible candidates are the Saucesti and Secuieni gas fields, located in the same petroleum system. Two surface geochemical surveys, based on closed-chamber flux measurements, were performed to assess the degassing intensity and the extent of the affected area. Methane fluxes from soil reach orders of 10~(4) mg m~(-2) day~(-1). Gas seepage mainly occurs in one zone 30 000 m~2 wide, and it is likely controlled by channeling along a fault and gas accumulation in permeable sediments and shallow subsoil. The estimated total CH_4 emission is about 40 t year~(-1) CH_4, of which 8-9 t year~(-1) are naturally released from soil and 30-35 t year~(-1) are emitted from shallow boreholes. These wells have likely channeled the gas accumulated in shallow alluvial sediment but gas flux from soil is still high and mitigation measures are needed to reduce the risk for humans and buildings.