Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Situational Awareness
Ground Excursion Taxiway
Flight Crew Became Reoriented
Airport; Human Factors
After pushback and upon turning out to the parallel taxiway we entered a dirt non-movement area.  Because we didn't cross any ground marking we did not identify the deviation. Unable to identify ramp area boundary due to the lack of required ground markings or edge lights. Also; poor overall ramp lighting. Proper markings and lighting to identify non movement area [are needed].
Captain reported taxiing off the hard surface into the dirt; and cited poor lighting and markings at SGU airport as contributing factors.