HOUSTON, Dec 30 (Reuters) - Exxon Mobil Corp signaled in a regulatory filing on Wednesday that higher oil and gas prices and improved chemicals margins would aid fourth quarter results, but the gains would be overshadowed by an up to $20 billion asset write down. Exxon’s regulatory filing showed it expects higher oil and gas prices to sequentially lift its production unit’s operating profit by $200 million to $1 billion compared to its third quarter. The noncash write down of mostly natural gas properties was previously estimated to be between $17 billion and $20 billion. (Reporting by Gary McWilliams; editing by Grant McCool)