Drilling unit with selective carbon dioxide and hydrocarbon drilling fluid venting system

A mud gas venting system, comprising:

• a first valve connected to a mud return line of a well drilling mud circulation system, the first valve connected to a hydrocarbon mud gas separator, a vent of the hydrocarbon
• mud gas separator connected to a first vent line having an outlet disposed at a greater elevation than any part of a drilling rig occupied by personnel, the first vent line in fluid communication with a first hydrocarbon gas sensor and a first carbon dioxide (CO2) sensor;
• a second valve connected to the mud return line, the second valve connected to at least a first CO2 mud gas separator, a vent of the CO2 mud gas separator connected to at least a second vent line having an outlet disposed at a lower elevation than any part of the drilling rig occupied by personnel, the second vent line in fluid communication with a second hydrocarbon gas sensor and a second CO2 sensor;
• and
• a controller in signal communication with the first valve, the second valve, the first and second hydrocarbon gas sensors and the first and second CO2 sensos, the controller operable to open the first valve and close the second valve when a detected CO2 concentration drops below a selected threshold, the controller operable to close the first valve and open the second valve when the detected CO2 concentration exceeds the selected threshold.