Carbon dioxide sequestration in geologic media is being investigated as a means of reducing emissions to the atmosphere. Oil and gas fields, which were in production decline, have been raised to new production levels by CO2 enhanced recovery. Currently, these fields use nearby geologic sources of CO2. Future enhanced oil recovery (EOR) projects are likely to include some industrial CO2, deriving substantial economic benefit over CO2 disposal in deep aquifers. The formation overpressuring required for substantial rates of CO2 injection may drive buoyant gases upward. Although CO2 is likely to be a supercritical fluid, CH4 will remain gaseous and, as such, is much more mobile. If CH4 is driven toward the surface, it represents a potential hazard to shallow water wells and basements. This research presents a protocol for baseline surface geochemical measurements to evaluate the potential risk. The objective is to detect a subtle signal from depth in the presence of a large amount of near surface noise. Carbon dioxide and CH4 soil gas concentrations and exchange with the atmosphere (flux) measurements are necessary. Stable and radiogenic carbon isotopic measurements are required to differentiate the sources of these gases. Examples of these measurements from a CO2 enhanced EOR project at Rangely, Colorado, USA, are presented.