* Q4 net loss 6.48 bln baht vs forecast 6.6 bln baht
* Q4 sales 88.5 bln baht vs 107 bln baht a year earlier
* Gross margins at $8 a barrel vs $4.9 year earlier (Adds details)
BANGKOK, Feb 12 (Reuters) - Thai Oil PCL, the country's top oil refiner, on Thursday reported a net loss for the fourth quarter due to a huge fall in inventory values as global oil prices dropped and weaker margins for its paraxylene petrochemical business.
Thai Oil, partly-owned by PTT PCL, the country's top energy firm, had stronger gross refining margins last year, but crude oil price hit a six-year low in December 2014, causing stock loss, it said in a statement.
Thai Oil's net loss was 6.48 billion baht ($198.29 million) for the October-December quarter, lower than the 6.6 billion baht loss forecast by seven analysts polled by Reuters. This compared with a revised profit of 10 million baht a year earlier.
For the whole 2014, the refiner posted a net loss of 4.03 billion baht versus a profit of 9.3 billion baht the year before.
Fourth-quarter gross integrated margin, excluding stock loss, was at $8 a barrel, up from $4.9 a year earlier.
The company's petrochemical business was pressured by an excess supply of paraxylene, which caused the spread between product prices and feedstock to drop, it said. ($1 = 32.6800 Thai Baht) (Reporting by Khettiya Jittapong; Editing by Mark Potter and Jane Merriman)