Microseepage of methane to the atmosphere from the Dawanqi oil-gas field, Tarim Basin, China

The microseepage of natural gas from subsurface hydrocarbon reservoirs is a widespread
process in petroleum basins. On a global scale, microseepage represents an important natural source of atmospheric methane (CH4). To date, microseepage CH4 flux data have been obtained from ~20 petroleum systems in North America, Europe, and Asia. While the seasonal variations of gas flux due to soil methanotrophic activity are known, the role of geological factors in controlling gas fluxes has been poorly investigated. Here we present new microseepage data from the Dawanqi oil-gas field located within the Tarim Basin (China), a petroleum system characterized by intense faulting and shallow (<700 m) reservoirs. We measured CH4 fluxes from the ground at 51 sites along three transects by using a closed-chamber connected to a portable gas sensor using off-axis integrated cavity output spectroscopy. Our results indicate that the highest CH4 fluxes occur over faults and/or shallow reservoirs, especially those that were not developed and that have higher fluid pressures. Microseeping CH4 is thermogenic, like that occurring within the Dawanqi reservoirs, as demonstrated by 13C enrichment (δ13C from 46.3‰to 30.7‰) in the chamber. Mean and range microseepage values (17 mg m 2d 1; from 1.4 to 330 mg m 2d 1) are similar to those reported for other petroleum fields with active tectonics. Our results confirm that dry soil over petroleum fields can be a net source of atmospheric CH4 and its flux is primarily controlled by faulting, and reservoir depth and pressure. These factors shall be considered in global bottom-up seepage emission estimates.