Pétron et al. (2012) have recently observed and analyzed alkane concentrations in air in Colorado's Weld County and used them to estimate the volume of methane vented from oil and gas operations in the Denver-Julesburg Basin. They conclude that "the emissions of the species we measured are most likely underestimated in current inventories", often by large factors. However, their estimates of methane venting, and hence of other alkane emissions, rely on unfounded assumptions about the composition of vented natural gas. We show that relaxing those assumptions results in much greater uncertainty. We then exploit previously unused observations reported in Pétron et al. (2012) to constrain methane emissions without making assumptions about the composition of vented gas. This results in a new set of estimates that are consistent with current inventories but inconsistent with the estimates in Pétron et al. (2012).The analysis also demonstrates the value of the mobile air sampling method employed in Pétron et al. (2012).