Seasonal Variation of Methane Microseepage in the Dawanqi Oilfield (China): A Possible Climatic Control

Natural gas microseepage in petroleum-bearing sedimentary basins is an important
complement to geophysical methods in oil-gas exploration and a natural source of methane (CH4) for
the atmosphere. Microseepage, typically occurring in correspondence with petroleum fields throughout
the world, is generally lower in summer, due to temperature-driven methanotrophic consumption, and
higher in winter. The global estimates of microseepage methane emission have, however, relatively
high uncertainties because of limited amounts of flux data, leading to poor knowledge of the spatial
distribution and temporal variability of the gas emission factors. We studied the seasonal variation of
microseepage flux to the atmosphere from a petroleum field in China (the Dawanqi oilfield), through
methane flux measurements performed in summer 2014, winter 2015, and summer 2019. Winter data
refer to frozen soil conditions, with snow cover and ice thickness in the soil exceeding 60 cm. Gas
concentration (CH4, CO2, C2+ alkanes) and stable C isotopic composition of CH4 and CO2 in shallow (4 m
deep) boreholes confirmed the existence of thermogenic gas seepage. Methane microseepage is higher
in summer and lower or nil in winter. This seasonal trend is opposite to what was observed in areas
where winter soil is not or poorly frozen. Our data suggest that seasonal microseepage variation may not
be univocal worldwide, being strongly dependent on the presence of ice and snow cover in winter. The
regional increase of temperature due to climate change, already demonstrated for the Tarim Basin over
the last 50 years, could, in the future, reduce winter ice and enhance annual methane emission to the
atmosphere.