The mantel may be too close to the firebox opening, causing the heating of the wood. When I cleaned up a white marble mantel with soap and water, it continued to smell like heating oil. So, dampen the marble with paint thinner, and sprinkle baking soda over it, at least half an inch thick. Sweep it up and throw it away. Repeat as necessary.
Repetition is the secret to success.
And rest assured it will work. The handyman spilled some diesel oil in the trunk of his car years ago during the energy crisis. But he wiped up as much as he could, then gave everything the baking soda treatment. Other absorbents you can use are Speedy Dry and cat litter. Both are absorbent clays.