B737-700 FLT CREW DEVIATES FROM ASSIGNED SID AT SEA.

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AT THE GATE; I OBTAINED THE CLRNC (MOUNTAIN 5) AND SET UP THE FMC EXPECTING A RWY 16L TKOF. WE PUSHED ON TIME IN VMC CONDITIONS AND GND CLRED US TO RWY 16R WHERE THE TWR CLRED US FOR TKOF. THE FMC POS WAS UPDATED AND OUR TKOF UNEVENTFUL. AT APPROX 1000 FT; I SELECTED 'HEADING' AND CLEANED UP ON SCHEDULE. AT WHAT I BELIEVED TO BE 5 DME AND AT APPROX 2000 FT; I SELECTED LNAV; THE 'B' AUTOPLT; AND SOON THE ACFT BEGAN A L TURN. THE CAPT QUICKLY CAUGHT MY ERROR AND IMMEDIATELY DESELECTED LNAV AND BEGAN TO ROLL BACK TO THE CORRECT HEADING WHEN THE CTLR CALLED US ON THE RADIO. THE ACFT HAD TURNED TO A HEADING OF APPROX 130 DEGS (APPROX 30 DEGS OFF COURSE) WHEN CAPT CAUGHT MY MISTAKE. DEP GAVE US A STERN COMMENT AND A NEW HEADING. I AM NOT SURE AS TO HOW I MISINTERPED MY POS FROM THE FIELD AS I HAD THE VOR FREQ SELECTED IN THE #2 NAV HEAD AS A BACK UP. I THINK MY SCREW-UP WAS IN NOT VERBALIZING THAT I WAS ON THE MOUNTAIN 5 DEP INSTEAD OF THE BLUIT AS WE USUALLY RECEIVE. ALSO; I DON'T XCHK WHAT THE FMC WAS DOING (WHICH WAS WHAT I ASKED IT TO DO) WITH THE DME THAT I HAD AS A BACK UP. I APPARENTLY BECAME COMPLACENT BECAUSE I KNEW THAT THE FMC DOES A GREAT JOB AND I DIDN'T XCHK WHAT I ASKED IT TO DO. FORTUNATELY; THE CAPT QUICKLY CAUGHT MY ERROR ALTHOUGH; NOT BEFORE I HAD BLUNDERED INTO A POSSIBLY SERIOUS SIT. SUPPLEMENTAL INFO FROM ACN 581276: I BELIEVE OUR SITUATIONAL AWARENESS WOULD HAVE BEEN MUCH HIGHER HAD WE SELECTED THE LEGS PAGE ON THE CDU IMMEDIATELY AFTER LNAV SELECTION. IDEALLY WE WOULD HAVE SEEN BLUIT AS THE FIRST FIX AND REALIZED THE ACFT WAS NOT CORRECTING COURSE; BUT TURNING TO THE FIRST FIX. IF SO; WE WOULD HAVE DISENGAGED LNAV BEFORE ANY SIGNIFICANT COURSE DEV TOOK PLACE. IN RETROSPECT; I WAS WAY TOO SLOW IN REACTING WHEN I FELT SOMETHING WAS AMISS. I HAD LET MY GUARD DOWN. I WAS PAIRED WITH A VERY EXPERIENCED; VERY COMPETENT FO; THE WX WAS NICE; AND WE HAD JUST FINISHED CLEANING UP THE ACFT AFTER TKOF. I SHOULD HAVE IMMEDIATELY ASKED THE FO WHERE THE ACFT WAS TURNING; SIMULTANEOUSLY DISENGAGED THE AUTOPLT; AND HAND FLOWN THE SID UNTIL MY SITUATIONAL AWARENESS IMPROVED.