PLT OF ATX SMT ACFT INADVERTENTLY TAXIED OFF THE RWY INTO AN UNIMPROVED ARPT AREA AFTER LNDG DUE TO SNOW COVERING MOST IDENTIFIABLE TXWY MARKINGS.

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AFTER A MAJOR SNOW STORM THE NIGHT BEFORE; MOST TXWYS AT BOS ARPT WERE STILL COVERED WITH LIGHT SNOW. UPON LNDG ON RWY 4R TWR ADVISED ME TO EXIT THE RWY AT THE NEXT TXWY C. SINCE AIRLINER TFC WAS BEHIND ME APPROX 4 MI OUT ON FINAL SINCE AIRLINER TFC WAS BEHIND ME APPROX 4 MI OUT ON FINAL (FOR THE SAME RWY); AND I WAS SLIGHTLY BEHIND SCHEDULE I WANTED TO EXIT QUICKLY. BEING THAT ALL GRASS AND PAVEMENT SURFACES WERE COVERED WITH SNOW IT WAS HARD TO TELL WHERE THE TXWY WAS LOCATED. I TURNED OFF THE RWY AT A POINT I THOUGHT WAS TXWY C. HOWEVER I WOUND UP TURNING OFF IN THE GRASS AREA JUST SHORT OF C. TWR ASKED ME IF I WAS STUCK AND HE DIVERTED THE AIRLINER TFC TO LAND ON RWY 4L. I ADVISED TWR THAT I WAS NOT STUCK AND COULD REJOIN TXWY X WHICH WAS ONLY 100 FT AHEAD. AFTER REJOINING C THE REST OF THE TAXI WAS NORMAL. LUCKILY FOR ME THE ACFT DID NOT GET STUCK IN THE GRASS AND THE ACFT WAS NOT DAMAGED IN ANY WAY. CONTRIBUTING FACTORS TO THIS INCIDENT WERE: BEING IN A HURRY; NOT ASKING TWR FOR ASSISTANCE IN FINDING THE TXWY; THE POOR SURFACE CONDITIONS (SNOW ACROSS ALL TXWYS AND ARPT SURFACE AREAS); AND THE TXWY LAYOUT AT BOS WHERE THERE ARE MANY DIFFERENT TXWYS THAT INTERSECT THIS PARTICULAR RWY AND IT MAY NOT ALWAYS BE EASY TO PICK OUT WHICH SETS OF BLUE TAXI LIGHTS CORRESPOND TO WHICH SINCE THEY ARE NOT WELL MARKED. IT WAS A COMBINATION OF ALL THE ABOVE WHICH LED TO THIS INCIDENT. TO PREVENT A RECURRENCE FROM HAPPENING; IF IN DOUBT; ALWAYS ASK TWR FOR ASSISTANCE; PRIOR TO LNDG CONSULT AN ARPT DIAGRAM CHART TO CHK FOR TXWYS IN PLANNING YOUR RWY EXIT; AND SLOW DOWN TO PREVENT FEWER MISTAKES FROM HASTY DECISIONS.