HOUSTON (Reuters) - Exxon Mobil Corp signaled in a regulatory filing 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. The largest U.S. oil producer has posted losses in the first three quarters of 2020 on an ill-timed spending increase that collided with a downturn in fuel demand and prices. It faces a proxy fight next year by an activist investor calling for deeper cuts, new directors and a refocusing on cleaner fuels. The filing after the market closed on Wednesday showed Exxon expects higher prices will sequentially lift its oil and gas operating results by between $200 million and $1 billion. That business suffered a $383 million operating loss in the third quarter. The filing also signaled another operating loss in refining, but higher chemicals margins drove operating profit in that unit by between $200 million and $400 million. In the prior quarter, refining posted a $231 million loss while chemicals turned a $661 million profit. The write down of mostly natural gas properties was previously estimated to be between $17 billion and $20 billion and the filing narrowed the range of the impairment charge. Exxon last year began providing a snapshot of its key businesses after the end of each quarter to give investors insight into operations. Many of those prior updates led Wall Street to lower profit forecasts. Official results are scheduled to be released Feb. 2. During the final quarter, the company outlined plans for deeper cost cuts that will chop capital spending by at least $10 billion this year and next, and the first stages of a 15% workforce reduction that will continue through next December. Adjusted fourth-quarter loss could hit $3.47 billion, or 61 cents per share, according to Refinitiv IBES data, compared with a year-earlier profit of $5.69 billion, or $1.33 per share. Exxon posted the details after the market close. Its shares finished up 33 cents at $41.60 but are down 41% year to date.