I think it is probably graboids.
The floor of the Gulf of Mexico is covered in Methyl-Hydrate, thousands of feet deep from gas that escapes from cracks in the ocean floor. I have wondered if that is a by-product of that impact.
I have forgotten is the statistic I read claimed there was enough to power the world, or just the US for 300 years just laying there, but it is very deep and nobody has figured a way to extract it.