Methane emissions from oil and gas sites are often characterized by mixed plumes from multiple sources in close proximity. This presents a challenge for screening methods that rely on emissions quantification to direct and prioritize follow-up inspections. Here, we present results from experiments evaluating mixed-source quantifications using the University of Calgary Portable Methane Leak Observatory (PoMELO) conducted at the Colorado State University Methane Emissions Technology Evaluation Center (CSU METEC). PoMELO is a vehicle-based screening system that is designed for operator-led surveys of methane emissions at upstream oil and gas sites, producing detections, localizations, and emissions quantifications while on site. Mock upstream pads were configured with 1-6 emissions points and the PoMELO system was used to quantify emissions rates at the equipment scale for each piece of equipment. Over 5 days of testing in a wide diversity of conditions, 88 individual experiment pads were surveyed at the equipment scale, with 1-6 emitting equipment per survey (total surveyed equipment = 209). The uncalibrated model was effective at measuring differences in rates: compared against real releases there was a linear calibration factor of 6.77 (r2 = 0.71). Results were more accurate in conditions with stable flow. Experiments with measurements further downwind were more accurate, and results improved when considering pooled data on each pad (linear model fit r2 = 0.84), reflecting errors in the model attributable to disambiguating methane in mixed plumes. Results suggest PoMELO has practical utility for understanding upstream methane emissions at the equipment and total pad scale.