Small transport aircraft pilot with a trainee reported disorientation and confusion during taxi at SMO. The reporter stated the lighting and taxiway markings are poor in this area of the airport and they taxied onto an unmarked ramp at night.

Narrative

Departing Santa Monica I inadvertently taxied past the parallel taxiway into the empty asphalt area between the taxiway and the runway. The taxiway near the FBO is totally unlit and begins immediately after the ramp; it is also narrower than most. I began to taxi forward using my taxi light as only reference and looking for the taxiway markings; I somehow taxied right over them; not expecting the taxiway to be so close and then saw no markings ahead. As I was stopping; Ground called me up and told us to make an immediate left; and that we had left the taxiway. Upon making the left turn; I saw the taxiway ahead and rejoined. Ground didn't give me a phone number to call or anything but I know if I had of gone much farther then they probably would have.Contributing factors likely included:1. I had been to Santa Monica a number of times; but do not remember departing during the night before this. As such I was in an somewhat unfamiliar environment.2. I had a brand new right seater with me who I was attempting to teach; which likely divided my attention during this critical phase. 3. Santa Monica's airport environment is non typical; in that it is simply a large endless bit of asphalt. The taxiway; although marked; is unlit for a large portion. 4. I was definitely fatigued; having slept poorly the night before and the hour being late for my internal clock. Corrective actions 1. In the future I will more thoroughly brief the taxi before leaving the ramp.2. I will confirm with my crew members that they see the same thing I do and encourage them to speak up if something looks amiss. 3. In the future if I do not immediately see where I am intended to go I will contact ground for clarification instead of taxing ahead assuming my taxi light will illuminate what I need.4. I will take note of when my fatigue level gets high and take extra care in situations such as this to avoid potential runway incursions. 5. I will review airport environment ground markings before my next flight. All in all a regrettable mistake; fortunately not the cause of a near miss or runway incursion but a poor showing anyway. The blame all lies with me as pilot in command; but I also do believe that relying only on paint markings and not lights in such a nonstandard environment invites this sort of mistake and is something SMO could look at as well. At the end of the day; totally my fault.