RAMP EXCURSION WHILE UNDER MARSHALLER'S GUIDANCE DURING A NIGHT OP; DC9 CARGO ACFT TAXIED INTO GRASS AREA ON AN UNMARKED AND UNLIGHTED FBO RAMP AT SAT.

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REPOSITIONING THE ACFT FROM THE N CARGO RAMP TO THE FBO FOR OVERNIGHT PARKING. WE TAXIED THE ACFT FROM TXWY V TO THE FBO RAMP. THE MARSHALLER TURNED ON HIS WANDS AND I TURNED ONTO HIS RAMP. THE MARSHALLER TURNED US TO HIS L AND I THEN STARTED MOVING TO MY R. WE CONTINUED FOLLOWING HIS INSTRUCTIONS VERY CAREFULLY. THE RAMP WAS NOT LIT AND HAD NO SIGNS OR STRIPES TO SHOW PAVEMENT TO GRASS. MY COPLT SAID HE THOUGHT HE SAW A CURB. I STOPPED THE ACFT AND REALIZED THE MARSHALLER JUST PUT US IN THE GRASS. THE GRASS AREA LOOKED JUST LIKE THE RAMP IN THE RAIN AT XA00. LOOKING TO THE R; THE MARSHALLER WAS STILL WANDING US TO THE L. HE CAME OVER TO THE ACFT WHEN WE STOPPED AND FOUND US IN THE WET GRASS. THE ACFT WAS REMOVED FROM THE MUD THE NEXT MORNING WITH NO DAMAGE. AFTER TALKING TO THE LINE MGR AT FBO; HE EXPLAINED THIS HAPPENS ON MANY NIGHTS. THIS PATCH OF GRASS WAS TO BE PAVED; BUT DUE TO BUDGET CUTS; IT REMAINS UNPAVED. I FEEL IT SHOULD AT LEAST HAVE A STRIPE OF REFLECTIVE PAINT TO DEMARK THE GRASS AREA.